Ch 5. Lymphatic System

Question 1

COMPONENTS OF THE LYMPHATIC SYSTEM

Answer 1

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Question 2

What does the lymphatic system consist of?

Answer 2

• Lymphatic vessels: (which contain a fluid called lymph)
• Lymphatic organs:
  Lymph nodes, Tonsils, Spleen, and Thymus Gland.
• Lymphocytes: which include B cells and T cells.

Question 3

FUNCTIONS OF THE LYMPHATIC SYSTEM

Answer 3

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Question 4

What are the two major functions?

Answer 4

• It drains the tissues of excess interstitial fluid
• it participates in immunity

Note: Recall that some water moves from the blood into the interstitium as cells undergo gas, nutrient, and waste exchange with the capillaries. The water in the interstitial fluid can diffuse into body cells if needed. However, if there is too much water in the interstitium, the excess interstitial fluid drains into a nearby lymphatic capillary.

NOTE: The excess interstitial fluid will eventually make it back to the blood because lymphatic capillaries give rise to larger lymphatic vessels that ultimately empty into veins. Any pathogens (viruses, bacteria, etc.) that happen to be in the interstitial fluid trying to invade body cells will also be swept into the lymphatic capillaries as a component of lymph, just like a swimmer may be swept away from the shore by a tidal wave.

NOTE: This function is achieved by the leukocytes or white blood cells of the body that fight off the pathogens that invade the body. Because of its role in immunity, parts of the lymphatic system can also function as the immune system.

Question 5

What is edema? How does edema occur?

Answer 5

• Edema = swelling of the body’s tissues

- Occurs by excess interstitial fluid remaining in the interstitium

Question 6

What is lymph

Answer 6

• Lymph is any excess interstitial fluid found within the lymphatic
  vessels of the body.

Once inside of a lymphatic capillary, the interstitial fluid is called lymph

Question 7

Types of pathogens that can invade the body?

Answer 7

• Bacteria and Viruses

Question 8

Bacteria DETAILS

Answer 8

• Bacteria components: A plasma membrane. Cytosol, DNA, Ribosomes, Cell wall, Capsule, Pili, Flagellum

• Plasma membrane surrounds the cell and serves as a barrier that regulates what goes into and out of the bacterial cell.
• Cytosol is the fluid interior of the cell.
• The DNA of a bacterial cell contains the cell’s genes. Note that the DNA is freely floating in the cytosol and not located within a nucleus. A bacterial cell does not have a nucleus. In other words, there is no black dot in the center of the cell. So, the DNA of a bacterial cell is essentially “naked” without anything to cover it. Because a bacterial cell does not have a nucleus, a bacterial cell is referred to as a prokaryote. The prefix “pro” means “before” and the suffix “karyote” means “nucleus”. In other words, bacterial cells were among the first cells to exist before cells started to have a nucleus. The cells of the human body contain a nucleus that surrounds and covers the DNA. Since a human body cell contains a nucleus, it is referred to as a eukaryote. The prefix “eu” means true and the suffix “karyote” means “nucleus”. This means that a eukaryote is a cell that has a nucleus. In addition to having “naked DNA”, the cytosol of a bacterial cell also has ribosomes.
• Ribosomes are structures where proteins are synthesized.
• Cell wall surrounds the plasma membrane. The cell wall gives the bacterial cell additional protection.
• Capsule is around the cell wall of the bacteria. Note that pili (singular is pilus) extend from the capsule. The capsule and the pili allow the bacterial cell to attach to surfaces, such as a door knob, your cell phone, or a person’s body. A final component of a bacterial cell is the flagellum, which the bacterial cell uses to move around.

Question 9

What Are the 3 Types of Bacterial Cell Shapes?

Answer 9

• Bacillus, Coccus, and Spirochete.
• Bacillus: rod shape.
• Coccus: spherical or circular shape.
• Spirochete: spiral or corkscrew shape.

Question 10

What the Components of a Virus?

Answer 10

• Capsid, Genetic material, Enzymes, Membrane Envelope, Glycoproteins
• The CAPSID of a virus is a hollow shell or protein coat that is polyhedral (in other words, it has many sides). Within the capsid is the genetic material of the virus.
• In some viruses, the GENETIC MATERIAL is DNA. In other viruses, the genetic material is RNA. Also within the capsid are enzymes that allow the virus to make copies of itself.
• Around the capsid is a MEMBRANE ENVELOPE. The membrane envelope is a plasma membrane that comes from the previous cell that the virus infected. In other words, the virus does not make the membrane envelope. Instead, it takes a piece of plasma membrane from the cell that it just infected and then puts the membrane around it as the membrane envelope. Then the virus uses the membrane to trick other cells into letting it enter because body cells are more likely to allow the virus to enter if the virus resembles the cell by having a membrane around it. So, the virus uses the membrane envelope as a trick to enter other body cells. In other words, the membrane envelope is like a Trojan horse.
• Extending from the virus are GLYCOPROTEINS. Glycoproteins are also used by the virus to help the virus enter into a body cell.

NOTE: Note that even though all viruses have the same general structure that I just described, there are many different types of viruses that exist and viruses differ from one another based on the number of sides that the polyhedral capsid has. Other things that can vary from virus to virus include the type of genetic material that the virus has, the collection of enzymes found within the virus, and the shapes of the glycoproteins.

Question 11

LYMPHATIC VESSELS

Answer 11

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Question 12

What are the Types of Lymphatic Vessels?

Answer 12

• The lymphatic system begins with lymphatic capillaries. Lymphatic capillaries are the smallest lymphatic vessels. There are no larger structures that turn into lymphatic capillaries. Instead, lymphatic capillaries just periodically begin in various locations throughout body tissues. Note that lymphatic capillaries are always close to blood capillaries. This allow the water that leaves a blood capillary to enter the interstitium and become interstitial fluid and then enter a nearby lymphatic capillary if the interstitial fluid is in excess. In this figure you can again see that lymphatic capillaries are close to blood capillaries.
• Like a blood capillary, a lymphatic capillary consists of endothelial cells. *However, a lymphatic capillary differs from a blood capillary in two major ways. (1) A lymphatic capillary lacks a basement membrane. (2) The endothelial cells of a lymphatic capillary overlap. The overlap of the endothelial cells of a lymphatic capillary allows for unidirectional flow of fluid into the lymphatic capillary. As excess interstitial fluid approaches a lymphatic capillary, the endothelial cells spread apart, forming large spaces between each other. These spaces are larger than the pores in a regular blood capillary and allow small molecules (like water) and relatively large substances (such as proteins, bacterial cells, viruses, and debris) to move from the interstitium into the lumen of the lymphatic capillary. Thus, a lymphatic capillary is far more permeable than a blood capillary. If the lymph tries to move out of the lymphatic capillary back into the interstitium, the endothelial cells come back together and overlap with one another, which closes off the spaces and essentially traps the lymph within the lymphatic vessel.
• Lymphatic capillaries converge to form larger lymphatic VESSELS. In this figure, you can see three lymphatic capillaries that merge to form a larger lymphatic vessel. Note that most of the relatively large lymphatic vessels do not have specific names. Instead, they are just referred to as larger lymphatic vessels because they are much larger than the lymphatic capillaries. If you look at this figure, you can also see several larger lymphatic vessels. Again, each larger lymphatic vessel forms from the union of several small lymphatic capillaries. A larger lymphatic vessel resembles a vein in structure, but has a thinner wall and more valves. As the larger lymphatic vessels course through the body, they give rise to lymph nodes, where lymph is filtered of any pathogens and debris. For example, here is a larger lymphatic vessel. Here is a lymph node. Then another larger lymphatic vessel in between. Another lymph node. Another larger lymphatic vessel. Another lymph node. And so forth. The larger lymphatic vessels empty into the largest lymphatic vessels, which are the right lymphatic duct and the thoracic duct, which is also known as the left lymphatic duct. The right lymphatic duct drains lymph from lymphatic vessels coming from the right upper quadrant of the body. In other words, the right upper limb, the right side of the chest, the right side of the neck, and the right side of the head. The thoracic duct, which is also known as the left lymphatic duct, drains lymph from lymphatic vessels coming from the rest of the body. From the right lymphatic duct and the thoracic duct, lymph empties into the venous circulation. This is due to the fact that each lymphatic duct connects to the venous circulation at the junction of the internal jugular vein and the subclavian vein. As you can see here, the right lymphatic duct empties at the junction of the right internal jugular vein and the right subclavian vein. The thoracic duct empties at the junction of the left internal jugular vein and the left subclavian vein. You should realize that although the lymphatic system drains excess interstitial fluid, it does not get rid of the excess interstitial fluid from the body. Instead, the excess interstitial fluid becomes lymph once it enters the lymphatic system and then eventually gets into the venous circulation due to the right and left lymphatic ducts draining into the subclavian veins. So, the main thing that happens to excess interstitial fluid in the lymphatic system is that pathogens and debris are filtered from this fluid as lymph goes from one lymphatic vessel through a lymph node and then to another lymphatic vessel, and so forth. Thus, the lymph that enters the venous circulation is interstitial fluid that has been cleansed. If the body really wants to get rid of the excess interstitial fluid, then once lymph enters the bloodstream and becomes part of blood, the kidneys can filter out the excess water from the blood and then excrete the water from the body via urine.

Question 13

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Answer 13

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Question 14

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Answer 14

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Question 15

LYMPHATIC ORGANS

Answer 15

• Lymph nodes, Tonsils, Spleen, Thymus gland.

Most lymphatic organs (namely, the lymph nodes, tonsils, and spleen) contain reticular connective tissue.

Reticular connective tissue consists of reticular fibers and white blood cells. The reticular fibers are thin collagen fibers that interact together to form a net. The white blood cells present in reticular connective tissue include the following. Macrophages. Lymphocytes (both B cells and T cells). The PURPOSE of reticular connective tissue is to filter foreign substances, such as pathogens and debris. As materials move through the reticular connective tissue of lymph nodes, tonsils, and the spleen, the netlike organization of the reticular connective tissue traps pathogens and debris, preventing these substances from moving any farther.

Then the white blood cells interact with and destroy the pathogens and debris. Note that white blood cells initially are in the bloodstream. From time to time, they leave the blood and then enter the reticular connective tissues of the lymph nodes, tonsils, and spleen and patrol these organs for foreign substances. Then the white blood cells leave these organs and then go back into the blood, and later on repeat the same cycle.

Realize that thymus gland is the only lymphatic organ that does not contain reticular connective tissue. That’s because the thymus gland does not have a role in filtering foreign substances.

Question 16

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Answer 16

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Question 17

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Answer 17

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Question 18

IMMUNITY

Answer 18

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Question 19

Two types of immunity:

Answer 19

Nonspecific immunity & Specific immunity.

Question 20

Non specific details

Answer 20

• Nonspecific immunity is the ability to protect the body from any foreign substance in a general, nonspecific way. Recall that there are several types of white blood cells involved in nonspecific immunity. Neutrophils. Basophils. Eosinophils. Monocytes, which turn into macrophages. Neutrophils, eosinophils, and macrophages function as phagocytes. Basophils promote inflammation. Specific immunity is the ability to protect the body from any foreign substance in a way that involves specificity and memory. Recall that specific immunity is achieved through the activities of lymphocytes. These activities include the release of antibodies that destroy the foreign substance or poking holes in the membrane of the foreign substance, causing it to explode. The two types of lymphocytes are B lymphocytes, which are also called B cells, and T lymphocytes, which are also called T cells. The difference between nonspecific immunity and specific immunity is based on specificity and memory.

Question 21

Non-specific immunity DETAILS

Answer 21

• Nonspecific immunity: the ability to protect the body from any foreign substance in a general, nonspecific way.

Recall that there are several types of white blood cells involved in nonspecific immunity. Neutrophils. Basophils. Eosinophils. Monocytes, which turn into macrophages. Neutrophils, eosinophils, and macrophages function as phagocytes.
Basophils promote inflammation.

• Specific immunity is the ability to protect the body from any foreign substance in a way that involves specificity and memory. Recall that specific immunity is achieved through the activities of lymphocytes. These activities include the release of antibodies that destroy the foreign substance or poking holes in the membrane of the foreign substance, causing it to explode. The two types of lymphocytes are B lymphocytes, which are also called B cells, and T lymphocytes, which are also called T cells. The difference between nonspecific immunity and specific immunity is based on specificity and memory.

Question 22

Specific immunity DETAILS

Answer 22

• Specific immunity is the ability to protect the body from any foreign substance in a way that involves specificity and memory. Recall that specific immunity is achieved through the activities of lymphocytes.

These activities include the release of antibodies that destroy the foreign substance or poking holes in the membrane of the foreign substance, causing it to explode. The two types of lymphocytes are B lymphocytes, which are also called B cells, and T lymphocytes, which are also called T cells. The difference between nonspecific immunity and specific immunity is based on specificity and memory.

Question 23

NOTES:

Answer 23

• Let’s talk about specificity in more detail. Specific immunity targets a specific pathogen. For example, the bacterium Escherichia coli vs the influenza virus. The more specific the immune response is, the easier it is to kill the invading pathogen. Nonspecific immunity is more general and, therefore, can target any type of pathogen (for example, any type of bacterium, virus, etc). A major disadvantage to this generalized approach is that it is harder to kill a pathogen without being able to specifically target it. Now let’s talk about memory in more detail. Specific immune responses involve memory, whereas there is no memory associated with nonspecific immunity. A person often becomes ill upon the first exposure to a particular pathogen. This is because it usually takes time for nonspecific immunity and for a specific community to become effective. Because specific immunity involves memory, the person does not get sick due to a subsequent exposure to the same pathogen because the specific immunity response acts quicker this time around. Since there is no memory associated with nonspecific immunity, the nonspecific immunity response will still occur at the same slow pace as before and the person runs the risk of still becoming sick until the nonspecific immunity response can become effective. Now let’s take a closer look at specific immunity. In order to describe specific immunity in more detail, you must first understand the concept of an antigen. An antigen is any substance that the body recognizes as being foreign (non-self) and is therefore immunogenic (or promotes a specific immune response). Antigens are usually proteins or carbohydrates that have strange shapes that the body does not recognize. Certain components of pathogens are antigens. Examples include the cell wall, capsule, and flagellum of a bacterial cell and capsid and glycoproteins of a virus. These components contain either proteins and/or carbohydrates that are immunogenic. Another example of an antigen is pollen. During their reproductive cycles, many plants release pollen, which is the plant’s sperm. The cell membranes of the cells in pollen contain proteins that are immunogenic in many people. For some people, certain foods such as shellfish and peanuts can be antigens. That’s because shellfish and peanuts contain proteins and carbohydrates that the body of these individuals cannot tolerate, resulting in an immune response. Components of foreign human cells are also antigens. Examples include the following. A or B antigens in the cell membranes of the red blood cells of a person who has a blood type different than yours. MHC proteins found in cells of tissues and organs of people not related to you. Strange proteins found in cell membranes of cancer cells that form in the body. Once antigens are introduced into the body, B cells and T cells will find them and destroy them. Note that most plastics are not immunogenic. Consequently, they can be used to replace damaged heart valves or damaged areas of the hip or knee without fear of rejection from the body. Now let’s examine MHC proteins in more detail. Our cells contain a variety of macromolecules. The majority of lipids, nucleic acids, carbohydrates, and proteins are the same from person to person and, consequently, are not immunogenic to other people. However, there is a group of proteins called major histocompatibility complex (or MHC) proteins that is unique from individual to individual, and consequently, causes an immune response when introduced into other people. MHC proteins are found in the plasma membranes of nucleated cells (in other words, in cells that have a nucleus). So, here is an MHC protein. Here’s another copy. Another copy. And so forth. MHC proteins are determined by your genes and, therefore, are unique from individual to individual. Because MHC proteins are determined by genes, a close relative (such as a parent, brother, sister, and so forth) will have MHC proteins similar to yours. And if you have an identical twin, the MHC proteins will be the exact same in the other twin. MHC proteins serve as cellular “identity tags” or self-antigens. Self-antigens are proteins that belong in one person and nobody else. So, in person X, all nucleated cells have MHC proteins with this particular shape. The MHC proteins tell the immune system of person X that this cell belongs in person X’s body. In person Q, who is not related to person X, all nucleated cells have MHC proteins with this particular shape. These MHC proteins tell the immune system of person Q that this cell belongs in person Q’s body. The MHC proteins are the basis of tissue rejections during tissue or organ transplantations. So, if person X tries to donate an organ to person Q, because person X and person Q have very different MHC proteins, person Q’s body will reject the organ from person X. However, the MHC proteins in the cells of one of your close relatives are very similar to your own MHC proteins. Therefore, a close relative can donate an organ to you without there being a severe immunogenic response in your body. Note that MHC proteins are not found in red blood cells because red blood cells lack a nucleus. Nevertheless, red blood cells do contain their own self-antigens, namely the antigens of the ABO blood group and those of the Rh blood group.

Question 24

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Answer 24

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Question 25

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Answer 25

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Question 26

PRODUCTION OF B & T CELLS

Answer 26

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Question 27

B & T cells details

Answer 27

• B cells and T cells are produced in red bone marrow from hemocytoblasts via the process of hematopoiesis. Once they are produced, B cells remain in the red bone marrow for a while to undergo maturation.
• T cells, however, do not mature in red bone marrow. Once produced, T cells migrate via the blood to the thymus gland to mature there with the help of thymic hormones called thymosins.
• During the maturation process, B cells and T cells become immunocompetent, which is the process by which B cells and T cells develop specific antigen-binding receptors in their plasma membranes. The process of forming antigen receptors in the plasma membranes of B cells and T cells is a random process.
• This means that a B cell or T cell forms antigen receptors for antigens that they have never come in contact with. So, let’s say that you come in contact with a particular antigen twenty years from now. Well, you already have a B cell or a T cell that just so happens to have the correct antigen receptor to bind to that antigen due to the random process by which antigen receptors are formed in B cells and T cells during the maturation or immunocompetence process.
• Once the maturation process has been completed, the immunocompetent B cells and T cells leave the red bone marrow and thymus gland, respectively, and migrate into the bloodstream. From the bloodstream, the B cells and T cells can enter into the reticular connective tissue of lymphatic organs and patrol for foreign substances.

Question 28

DIVERSITY OF B & T CELLS

Answer 28

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Question 29

Diversity details

Answer 29

• There are millions of different antigens in the environment that could potentially cause a person to become sick. Fortunately, the body already contains a specific B cell or T cell that can destroy each one of these antigens before you even encounter them. Hence, there are millions of different types of B cells and millions of different types of T cells in the body.
• Each of these B cells and T cells contains a specific antigen-binding receptor in its plasma membrane. For example, B cell 1 has an antigen receptor that specifically binds to antigen L. T cell 2 has an antigen receptor that specifically binds to antigen C.

Question 30

CLONAL SELECTION

Answer 30

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Question 31

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Answer 31

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Question 32

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Answer 32

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Question 33

AQUIRED IMMUNITY

Answer 33

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Question 34

Acquired Specific Immunity DETAILS

Answer 34

Let’s begin with some general information about acquiring specific immunity. Immunity can be acquired either actively or passively. Immunity is acquired actively if the person’s own immune system develops antibodies and immune system cells (such as plasma cells, cytotoxic T cells, helper T cells, and B and T memory cells) against a specific antigen. Immunity is acquired passively if a person receives antibodies from another person or animal. Because there is no memory component involved, passive immunity is only temporary and lasts until the antibodies degrade. Immunity can also be acquired naturally or artificially. Immunity is acquired naturally if the person develops immunity as a result of a natural event, such as exposure to an antigen by chance. Immunity can be acquired artificially if the person develops immunity as a result of artificial means, such as deliberate exposure to an antigen through vaccination. Based on all of these factors, there are 4 ways to acquire specific immunity. Active natural immunity. Active artificial immunity. Passive natural immunity. Passive artificial immunity. Active natural immunity develops when a person is exposed to an antigen by chance, becomes ill, and then produces antibody-secreting plasma cells, cytotoxic T cells, helper T cells, and B and T memory cells. This is the most common method of acquiring specific immunity. Active artificial immunity develops due to deliberate exposure to an antigen through vaccination. A vaccine is an attenuated (or weakened) antigen. The antigen, which is usually a microbe or portions of a microbe, has been pretreated to be immunogenic but not pathogenic, meaning that it will trigger an immune response but not cause significant illness. The antigens introduced through vaccination stimulate the immune system to produce antibodies and immune system cells, including memory cells. Passive natural immunity develops when antibodies are passed from mother to fetus through the placenta or from mother to infant via breast milk. Neither the fetus nor the infant has a well-developed immune system. Both are susceptible to illness. The antibodies introduced through passive natural immunity help the fetus and infant fight antigens that enter their bodies. However, these antibodies do not last forever. They eventually degrade and the infant will have to rely on his or her own developing immune system to provide protection. Passive artificial immunity occurs when a person receives serum containing antibodies from another person or an animal (such as a horse, rabbit, or goat) that has already been vaccinated against an antigen. This type of immunity provides immediate protection against an antigen and is the preferred type of specific immunity when there is not enough time for a person to develop his or her own antibodies and immune system cells. Such is the case when there is an outbreak or epidemic of a lethal pathogen. The protection from passive artificial immunity is only temporary. The antibodies will eventually degrade and the person will have to develop his or her own immunity against the antigen.

Question 35

Acquired Specific Immunity DETAILS

Answer 35

Let’s begin with some general information about acquiring specific immunity. Immunity can be acquired either actively or passively. Immunity is acquired actively if the person’s own immune system develops antibodies and immune system cells (such as plasma cells, cytotoxic T cells, helper T cells, and B and T memory cells) against a specific antigen. Immunity is acquired passively if a person receives antibodies from another person or animal. Because there is no memory component involved, passive immunity is only temporary and lasts until the antibodies degrade. Immunity can also be acquired naturally or artificially. Immunity is acquired naturally if the person develops immunity as a result of a natural event, such as exposure to an antigen by chance. Immunity can be acquired artificially if the person develops immunity as a result of artificial means, such as deliberate exposure to an antigen through vaccination.

• Based on all of these factors, there are 4 ways to acquire specific immunity. Active natural immunity. Active artificial immunity. Passive natural immunity. Passive artificial immunity. Active natural immunity develops when a person is exposed to an antigen by chance, becomes ill, and then produces antibody-secreting plasma cells, cytotoxic T cells, helper T cells, and B and T memory cells. This is the most common method of acquiring specific immunity. Active artificial immunity develops due to deliberate exposure to an antigen through vaccination. A vaccine is an attenuated (or weakened) antigen. The antigen, which is usually a microbe or portions of a microbe, has been pretreated to be immunogenic but not pathogenic, meaning that it will trigger an immune response but not cause significant illness. The antigens introduced through vaccination stimulate the immune system to produce antibodies and immune system cells, including memory cells.
• Passive natural immunity develops when antibodies are passed from mother to fetus through the placenta or from mother to infant via breast milk. Neither the fetus nor the infant has a well-developed immune system. Both are susceptible to illness. The antibodies introduced through passive natural immunity help the fetus and infant fight antigens that enter their bodies.
• However, these antibodies do not last forever. They eventually degrade and the infant will have to rely on his or her own developing immune system to provide protection.
• Passive artificial immunity occurs when a person receives serum containing antibodies from another person or an animal (such as a horse, rabbit, or goat) that has already been vaccinated against an antigen. This type of immunity provides immediate protection against an antigen and is the preferred type of specific immunity when there is not enough time for a person to develop his or her own antibodies and immune system cells. Such is the case when there is an outbreak or epidemic of a lethal pathogen. The protection from passive artificial immunity is only temporary. The antibodies will eventually degrade and the person will have to develop his or her own immunity against the antigen.

Question 36

AQUIRED IMMUNITY

Answer 36

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Question 37

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Answer 37

This video is about the ways to acquire specific immunity. Let’s begin with some general information about acquiring specific immunity. Immunity can be acquired either actively or passively. Immunity is acquired actively if the person’s own immune system develops antibodies and immune system cells (such as plasma cells, cytotoxic T cells, helper T cells, and B and T memory cells) against a specific antigen. Immunity is acquired passively if a person receives antibodies from another person or animal. Because there is no memory component involved, passive immunity is only temporary and lasts until the antibodies degrade. Immunity can also be acquired naturally or artificially. Immunity is acquired naturally if the person develops immunity as a result of a natural event, such as exposure to an antigen by chance. Immunity can be acquired artificially if the person develops immunity as a result of artificial means, such as deliberate exposure to an antigen through vaccination. Based on all of these factors, there are 4 ways to acquire specific immunity. Active natural immunity. Active artificial immunity. Passive natural immunity. Passive artificial immunity. Active natural immunity develops when a person is exposed to an antigen by chance, becomes ill, and then produces antibody-secreting plasma cells, cytotoxic T cells, helper T cells, and B and T memory cells. This is the most common method of acquiring specific immunity. Active artificial immunity develops due to deliberate exposure to an antigen through vaccination. A vaccine is an attenuated (or weakened) antigen. The antigen, which is usually a microbe or portions of a microbe, has been pretreated to be immunogenic but not pathogenic, meaning that it will trigger an immune response but not cause significant illness. The antigens introduced through vaccination stimulate the immune system to produce antibodies and immune system cells, including memory cells. Passive natural immunity develops when antibodies are passed from mother to fetus through the placenta or from mother to infant via breast milk. Neither the fetus nor the infant has a well-developed immune system. Both are susceptible to illness. The antibodies introduced through passive natural immunity help the fetus and infant fight antigens that enter their bodies. However, these antibodies do not last forever. They eventually degrade and the infant will have to rely on his or her own developing immune system to provide protection. Passive artificial immunity occurs when a person receives serum containing antibodies from another person or an animal (such as a horse, rabbit, or goat) that has already been vaccinated against an antigen. This type of immunity provides immediate protection against an antigen and is the preferred type of specific immunity when there is not enough time for a person to develop his or her own antibodies and immune system cells. Such is the case when there is an outbreak or epidemic of a lethal pathogen. The protection from passive artificial immunity is only temporary. The antibodies will eventually degrade and the person will have to develop his or her own immunity against the antigen.

Question 38

CLONAL SELECTION

Answer 38

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Question 39

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Answer 39

This video is about clonal selection. As you have already learned, there are millions of different types of B cells and millions of different types of T cells in the body. However, there are only a few copies of each of these different types of B cells and T cells before the initial exposure to antigens. Such a small army consisting of only a few copies of each of these lymphocytes is not enough to fight a massive invasion of pathogens. As a solution to this problem, when a B cell or T cell binds to an antigen, it undergoes clonal selection. Clonal selection is the process by which a B cell or T cell divides into a clone of cells that can bind to the same antigen. Clonal selection results in the production of more B cells or T cells (often thousands of them) that can be used to destroy an antigen. In addition, the cells of the clone become differentiated. Although the differentiated cells of the clone bind to the same antigen, they function differently in the specific immune response that is about to occur. Let’s first examine B cell clonal selection. An antigen invades the body. The receptor on the appropriate B cell binds to the antigen. The B cell is then activated by cytokines released from a helper T cell. The B cell subsequently undergoes clonal selection, resulting in the production of many plasma cells and memory B cells. Both of these cell types bind to the same antigen as the original B cell. In other words, they have the same receptor type. Plasma cells secrete antibodies. An antibody, also called an immunoglobulin, is a protein that binds to and subsequently destroys an antigen. The antibodies secreted by plasma cell are specific for the antigen that was recognized by the original B cell. In fact, the antibody has the same shape as the antigen receptor that is in the plasma membrane of the plasma cell. Antibodies are found in many types of body fluids, including blood, saliva, lymph, tears, mucus, and breast milk. Memory B cells remember the antigen that caused the original B cell to undergo clonal selection. Should the same antigen invade the body again, memory B cells immediately produce more plasma cells and more memory B cells that possess the same antigen specificity. Consequently, there is a rapid production of antibodies produced by the plasma cells, which results in the quick destruction of the pathogen. This response is so fast that the person typically does not exhibit any signs of being ill. Memory B cells stay around in the body for decades. Now let’s talk about T cells clonal selection. An antigen invades the body. The receptor on the appropriate T cell binds to that antigen. The T cell is then activated by cytokines released from a helper T cell. The T cell subsequently undergoes clonal selection resulting in the production of many cytotoxic T cells, helper T cells, suppressor T cells, and memory T cells. All of these cell types within the clone bind to the same antigen as the original T cell. In other words, all of the cells have the same receptor type. Cytotoxic T cells, also called killer T cells, function by poking holes in the cell membranes of their target antigens, which results in cell lysis and cell death. Helper T cells activate both B cells and T cells that need to undergo clonal selection. Consequently, a specific immune response cannot be achieved without the helper T cells. Helper T cells activate B cells and T cells via the secretion of chemicals called cytokines. An example is interleukin. HIV, the virus that causes AIDS, kills helper T cells. When the helper T cell count becomes very low, the immune system ceases to function. Suppressor T cells reduce the activity of B cells and T cells once the pathogen has been destroyed. Hence, these cells only become active once the battle is over and victory has been declared. Memory T cells remember the antigen that caused the original T cell to undergo clonal selection. Should the same antigen invade the body again, the memory T cells immediately produce more cytotoxic T cells, helper T cells, suppressor T cells, and memory T cells. All of these cells will have the same antigen specificity. Consequently, the pathogen is quickly killed by the huge number of cytotoxic T cells. This response is so rapid that the person typically does not exhibit any signs of being ill. Like memory B cells, memory T cells also stay around in the body for decades.

Question 40

DIVERSITY OF B AND T CELLS

Answer 40

This video is about the diversity of B cells and T cells. There are millions of different antigens in the environment that could potentially cause a person to become sick. Fortunately, the body already contains a specific B cell or T cell that can destroy each one of these antigens before you even encounter them. Hence, there are millions of different types of B cells and millions of different types of T cells in the body. Each of these B cells and T cells contains a specific antigen-binding receptor in its plasma membrane. For example, B cell 1 has an antigen receptor that specifically binds to antigen L. T cell 2 has an antigen receptor that specifically binds to antigen C.

Question 41

PRODUCTION OF B AND T CELLS

Answer 41

This video is about the production of B cells and T cells. B cells and T cells are produced in red bone marrow from hemocytoblasts via the process of hematopoiesis. Once they are produced, B cells remain in the red bone marrow for a while to undergo maturation. T cells, however, do not mature in red bone marrow. Once produced, T cells migrate via the blood to the thymus gland to mature there with the help of thymic hormones called thymosins. During the maturation process, B cells and T cells become immunocompetent, which is the process by which B cells and T cells develop specific antigen-binding receptors in their plasma membranes. The process of forming antigen receptors in the plasma membranes of B cells and T cells is a random process. This means that a B cell or T cell forms antigen receptors for antigens that they have never come in contact with. So, let’s say that you come in contact with a particular antigen twenty years from now. Well, you already have a B cell or a T cell that just so happens to have the correct antigen receptor to bind to that antigen due to the random process by which antigen receptors are formed in B cells and T cells during the maturation or immunocompetence process. Once the maturation process has been completed, the immunocompetent B cells and T cells leave the red bone marrow and thymus gland, respectively, and migrate into the bloodstream. From the bloodstream, the B cells and T cells can enter into the reticular connective tissue of lymphatic organs and patrol for foreign substances.

Question 42

IMMUNITY

Answer 42

This video is about immunity. Immunity is resistance to disease. There are two major types of immunity. Nonspecific immunity. Specific immunity. Nonspecific immunity is the ability to protect the body from any foreign substance in a general, nonspecific way. Recall that there are several types of white blood cells involved in nonspecific immunity. Neutrophils. Basophils. Eosinophils. Monocytes, which turn into macrophages. Neutrophils, eosinophils, and macrophages function as phagocytes. Basophils promote inflammation. Specific immunity is the ability to protect the body from any foreign substance in a way that involves specificity and memory. Recall that specific immunity is achieved through the activities of lymphocytes. These activities include the release of antibodies that destroy the foreign substance or poking holes in the membrane of the foreign substance, causing it to explode. The two types of lymphocytes are B lymphocytes, which are also called B cells, and T lymphocytes, which are also called T cells. The difference between nonspecific immunity and specific immunity is based on specificity and memory. Let’s talk about specificity in more detail. Specific immunity targets a specific pathogen. For example, the bacterium Escherichia coli vs the influenza virus. The more specific the immune response is, the easier it is to kill the invading pathogen. Nonspecific immunity is more general and, therefore, can target any type of pathogen (for example, any type of bacterium, virus, etc). A major disadvantage to this generalized approach is that it is harder to kill a pathogen without being able to specifically target it. Now let’s talk about memory in more detail. Specific immune responses involve memory, whereas there is no memory associated with nonspecific immunity. A person often becomes ill upon the first exposure to a particular pathogen. This is because it usually takes time for nonspecific immunity and for a specific community to become effective. Because specific immunity involves memory, the person does not get sick due to a subsequent exposure to the same pathogen because the specific immunity response acts quicker this time around. Since there is no memory associated with nonspecific immunity, the nonspecific immunity response will still occur at the same slow pace as before and the person runs the risk of still becoming sick until the nonspecific immunity response can become effective. Now let’s take a closer look at specific immunity. In order to describe specific immunity in more detail, you must first understand the concept of an antigen. An antigen is any substance that the body recognizes as being foreign (non-self) and is therefore immunogenic (or promotes a specific immune response). Antigens are usually proteins or carbohydrates that have strange shapes that the body does not recognize. Certain components of pathogens are antigens. Examples include the cell wall, capsule, and flagellum of a bacterial cell and capsid and glycoproteins of a virus. These components contain either proteins and/or carbohydrates that are immunogenic. Another example of an antigen is pollen. During their reproductive cycles, many plants release pollen, which is the plant’s sperm. The cell membranes of the cells in pollen contain proteins that are immunogenic in many people. For some people, certain foods such as shellfish and peanuts can be antigens. That’s because shellfish and peanuts contain proteins and carbohydrates that the body of these individuals cannot tolerate, resulting in an immune response. Components of foreign human cells are also antigens. Examples include the following. A or B antigens in the cell membranes of the red blood cells of a person who has a blood type different than yours. MHC proteins found in cells of tissues and organs of people not related to you. Strange proteins found in cell membranes of cancer cells that form in the body. Once antigens are introduced into the body, B cells and T cells will find them and destroy them. Note that most plastics are not immunogenic. Consequently, they can be used to replace damaged heart valves or damaged areas of the hip or knee without fear of rejection from the body. Now let’s examine MHC proteins in more detail. Our cells contain a variety of macromolecules. The majority of lipids, nucleic acids, carbohydrates, and proteins are the same from person to person and, consequently, are not immunogenic to other people. However, there is a group of proteins called major histocompatibility complex (or MHC) proteins that is unique from individual to individual, and consequently, causes an immune response when introduced into other people. MHC proteins are found in the plasma membranes of nucleated cells (in other words, in cells that have a nucleus). So, here is an MHC protein. Here’s another copy. Another copy. And so forth. MHC proteins are determined by your genes and, therefore, are unique from individual to individual. Because MHC proteins are determined by genes, a close relative (such as a parent, brother, sister, and so forth) will have MHC proteins similar to yours. And if you have an identical twin, the MHC proteins will be the exact same in the other twin. MHC proteins serve as cellular “identity tags” or self-antigens. Self-antigens are proteins that belong in one person and nobody else. So, in person X, all nucleated cells have MHC proteins with this particular shape. The MHC proteins tell the immune system of person X that this cell belongs in person X’s body. In person Q, who is not related to person X, all nucleated cells have MHC proteins with this particular shape. These MHC proteins tell the immune system of person Q that this cell belongs in person Q’s body. The MHC proteins are the basis of tissue rejections during tissue or organ transplantations. So, if person X tries to donate an organ to person Q, because person X and person Q have very different MHC proteins, person Q’s body will reject the organ from person X. However, the MHC proteins in the cells of one of your close relatives are very similar to your own MHC proteins. Therefore, a close relative can donate an organ to you without there being a severe immunogenic response in your body. Note that MHC proteins are not found in red blood cells because red blood cells lack a nucleus. Nevertheless, red blood cells do contain their own self-antigens, namely the antigens of the ABO blood group and those of the Rh blood group.

Question 43

LYMPHATIC ORGANS

Answer 43

This video is about lymphatic organs. The lymphatic organs include the following. Lymph nodes. Tonsils. Spleen. Thymus gland. Most lymphatic organs (namely, the lymph nodes, tonsils, and spleen) contain reticular connective tissue. Reticular connective tissue consists of reticular fibers and white blood cells. The reticular fibers are thin collagen fibers that interact together to form a net. The white blood cells present in reticular connective tissue include the following. Macrophages. Lymphocytes (both B cells and T cells). The purpose of reticular connective tissue is to filter foreign substances, such as pathogens and debris. As materials move through the reticular connective tissue of lymph nodes, tonsils, and the spleen, the netlike organization of the reticular connective tissue traps pathogens and debris, preventing these substances from moving any farther. Then the white blood cells interact with and destroy the pathogens and debris. Note that white blood cells initially are in the bloodstream. From time to time, they leave the blood and then enter the reticular connective tissues of the lymph nodes, tonsils, and spleen and patrol these organs for foreign substances. Then the white blood cells leave these organs and then go back into the blood, and later on repeat the same cycle. Realize that thymus gland is the only lymphatic organ that does not contain reticular connective tissue. That’s because the thymus gland does not have a role in filtering foreign substances. Now let’s talk about each type of lymphatic organ in more detail. Let’s begin with the lymph nodes. Lymph nodes are small, bean-shaped masses that are located between lymphatic vessels. So, here is a lymph node. A lymphatic vessel in between. Another lymph node. Another lymphatic vessel. Lymph nodes often exist in groups or clusters. This is especially the case with the lymph nodes in the neck, armpits, and groin. These lymph nodes are called as follows. Cervical nodes. Axillary nodes. Inguinal nodes. Respectively. If you look at the internal organization of a lymph node, you can see that it contains reticular connective tissue and germinal centers. The reticular connective tissue gives the interior of the lymph node a netlike organization. The germinal centers are sites where white blood cells divide during immune responses. Note that more than one lymphatic vessel goes to and from a given lymph node. Lymphatic vessels that carry lymph to a lymph node are referred to as afferent lymphatic vessels. The lymphatic vessels that carry lymph away from a lymph node (after the lymph has been filtered) are called efferent lymphatic vessels. You should realize that the terms afferent and efferent are relative. Although these vessels here are efferent lymphatic vessels for this lymph node, these same vessels serve as the afferent lymphatic vessels for the next lymph node farther down the line. The function of lymph nodes is to filter lymph of foreign substances, such as pathogens and debris. This occurs in the following way. Afferent lymphatic vessels bring lymph to the lymph node. As lymph travels through the lymph node, any foreign substances in the lymph are trapped in the netlike reticular fibers, which then allows the white blood cells (macrophages and lymphocytes) within the lymph node to destroy them. Then the efferent lymphatic vessels carry the filtered lymph away from the lymph node. So, again, a lymph node filters lymph of foreign substances. You should realize that no one lymph node filters all foreign substances. That’s because some foreign substances by chance don’t get caught in the net and can pass through some of the spaces within the reticular connective tissue. However, any foreign substances that don’t get filtered by a given lymph node will eventually get filtered by another lymph node farther along the lymphatic pathway. Now let’s talk about the tonsils. The tonsils are located in the pharynx (or throat) and the oral cavity. There are 3 types of tonsils. Pharyngeal tonsil. Palatine tonsils. Lingual tonsils. The pharyngeal tonsil is an unpaired tonsil that is located in the wall of the nasopharynx. Note that the pharynx (or throat) has three portions. Nasopharynx. Oropharynx. Laryngopharynx. The nasopharynx is the upper portion of the pharynx that is continuous with the nasal cavity. The oropharynx is the portion of the pharynx that is continuous with the oral cavity. The laryngopharynx is the portion of the pharynx that is continuous with the larynx. Again, the pharyngeal tonsil is located in the wall of the nasopharynx. Another name for the pharyngeal tonsil is the adenoid. The palatine tonsils are paired tonsils located in the posterior wall of the oral cavity. The palatine tonsils are most susceptible to infection and may have to be removed, a procedure known as a tonsillectomy. The lingual tonsils are paired tonsils located at the base of the tongue. The tonsils are isolated masses that consist of reticular connective tissue. Periodically, the tonsils have invaginations or folds that give rise to valleys called tonsillar crypts. The function of the tonsils is to filter air, food, and beverages of foreign substances, such as pathogens and debris. The location of the tonsils in the oral cavity and pharynx allows the tonsils to achieve this function. As pathogens in air, food, or a beverage interact with the tonsils, the pathogens become trapped in the tonsillar crypts. By chance, the pathogens then move deeper into the reticular connective tissue within the tonsils, where they become trapped and then macrophages and lymphocytes destroy them. Note that the tonsils do not filter lymph or blood because they are not connected to lymphatic vessels or blood vessels. Again, tonsils are strategically positioned to filter air, food, and beverages. Now let’s talk about the spleen. The spleen is the largest lymphatic organ. It is located on the left side of the body between the diaphragm and the stomach. The spleen consists of reticular connective tissue that is organized into regions called white pulp and red pulp. The white pulp and red pulp are named as such because the white pulp is lighter in color then the red pulp. However, both white pulp and red pulp are regions of the reticular connective tissue located within the spleen. In addition, the spleen is heavily vascularized. The spleen artery provides blood to the spleen. The splenic vein drains blood from the spleen. Since the spleen is so heavily vascularized, trauma to the spleen can cause severe bleeding and even death. If this happens, the spleen must be removed (a procedure known as a splenectomy) to stop the bleeding. The spleen has two major functions. One function of the spleen is that it filters the blood of foreign substances. The splenic artery brings blood to the spleen. From the splenic artery, blood eventually moves into the reticular connective tissue that forms the white pulp and the red pulp. As blood moves through the white pulp and red pulp, lymphocytes and macrophages remove and destroy any pathogens that get caught in the reticular fibers. The blood then moves from the white pulp and red pulp into the splenic vein, which takes the filtered blood away from the spleen. Another function of the spleen is that it destroys worn out red blood cells. As blood is filtered through the spleen, the macrophages can remove and destroy any worn out red blood cells via phagocytosis. Now let’s talk about the thymus gland. The thymus gland is a bilobed gland that partially covers the superior or upper portion of the heart. Unlike the other lymphatic organs, the thymus gland does not contain reticular connective tissue. Instead, the thymus gland contains T lymphocytes (or T cells). The function of the thymus gland is to promote the maturation of T cells. This occurs in the following way. Immature T cells are initially produced in red bone marrow. Afterwards, these immature T cells are released into the blood and then migrate to the thymus gland. The thymus gland produces hormones called thymosins that mature the T cells. The mature T cells then migrate from the thymus gland back into the blood. Note that B cells are produced and matured in the red bone marrow and, therefore, do not have migrate to the thymus gland. Once B cells and T cells are mature, they enter the bloodstream. From the bloodstream, the B cells and T cells go to lymphatic organs and patrol these areas for pathogens, go back to blood, then back to lymphatic organs, and so forth.

Question 44

LYMPHATIC VESSELS

Answer 44

This video is about the different types of lymphatic vessels. The lymphatic system begins with lymphatic capillaries. Lymphatic capillaries are the smallest lymphatic vessels. There are no larger structures that turn into lymphatic capillaries. Instead, lymphatic capillaries just periodically begin in various locations throughout body tissues. Note that lymphatic capillaries are always close to blood capillaries. This allow the water that leaves a blood capillary to enter the interstitium and become interstitial fluid and then enter a nearby lymphatic capillary if the interstitial fluid is in excess. In this figure you can again see that lymphatic capillaries are close to blood capillaries. Like a blood capillary, a lymphatic capillary consists of endothelial cells. However, a lymphatic capillary differs from a blood capillary in two major ways. (1) A lymphatic capillary lacks a basement membrane. (2) The endothelial cells of a lymphatic capillary overlap. The overlap of the endothelial cells of a lymphatic capillary allows for unidirectional flow of fluid into the lymphatic capillary. As excess interstitial fluid approaches a lymphatic capillary, the endothelial cells spread apart, forming large spaces between each other. These spaces are larger than the pores in a regular blood capillary and allow small molecules (like water) and relatively large substances (such as proteins, bacterial cells, viruses, and debris) to move from the interstitium into the lumen of the lymphatic capillary. Thus, a lymphatic capillary is far more permeable than a blood capillary. If the lymph tries to move out of the lymphatic capillary back into the interstitium, the endothelial cells come back together and overlap with one another, which closes off the spaces and essentially traps the lymph within the lymphatic vessel. Lymphatic capillaries converge to form larger lymphatic vessels. In this figure, you can see three lymphatic capillaries that merge to form a larger lymphatic vessel. Note that most of the relatively large lymphatic vessels do not have specific names. Instead, they are just referred to as larger lymphatic vessels because they are much larger than the lymphatic capillaries. If you look at this figure, you can also see several larger lymphatic vessels. Again, each larger lymphatic vessel forms from the union of several small lymphatic capillaries. A larger lymphatic vessel resembles a vein in structure, but has a thinner wall and more valves. As the larger lymphatic vessels course through the body, they give rise to lymph nodes, where lymph is filtered of any pathogens and debris. For example, here is a larger lymphatic vessel. Here is a lymph node. Then another larger lymphatic vessel in between. Another lymph node. Another larger lymphatic vessel. Another lymph node. And so forth. The larger lymphatic vessels empty into the largest lymphatic vessels, which are the right lymphatic duct and the thoracic duct, which is also known as the left lymphatic duct. The right lymphatic duct drains lymph from lymphatic vessels coming from the right upper quadrant of the body. In other words, the right upper limb, the right side of the chest, the right side of the neck, and the right side of the head. The thoracic duct, which is also known as the left lymphatic duct, drains lymph from lymphatic vessels coming from the rest of the body. From the right lymphatic duct and the thoracic duct, lymph empties into the venous circulation. This is due to the fact that each lymphatic duct connects to the venous circulation at the junction of the internal jugular vein and the subclavian vein. As you can see here, the right lymphatic duct empties at the junction of the right internal jugular vein and the right subclavian vein. The thoracic duct empties at the junction of the left internal jugular vein and the left subclavian vein. You should realize that although the lymphatic system drains excess interstitial fluid, it does not get rid of the excess interstitial fluid from the body. Instead, the excess interstitial fluid becomes lymph once it enters the lymphatic system and then eventually gets into the venous circulation due to the right and left lymphatic ducts draining into the subclavian veins. So, the main thing that happens to excess interstitial fluid in the lymphatic system is that pathogens and debris are filtered from this fluid as lymph goes from one lymphatic vessel through a lymph node and then to another lymphatic vessel, and so forth. Thus, the lymph that enters the venous circulation is interstitial fluid that has been cleansed. If the body really wants to get rid of the excess interstitial fluid, then once lymph enters the bloodstream and becomes part of blood, the kidneys can filter out the excess water from the blood and then excrete the water from the body via urine.