Circulatory, lymphatic, and immune systems

Question 1

Circulatory system

Answer 1

a system of closed tubes. Circulation occurs in two large groups: the pulmonary circulation and the systemic circulation. In both cases, arteries carry blood from the heart to capillary beds, where exchange occurs. Veins return blood to the heart.

Question 2

Pulmonary circulation

Answer 2

carries blood between the heart and the lungs for gas exchange.

Question 3

Systemic circulation

Answer 3

Carries blood between the heart and the rest of the body’s tissues.

Question 4

The heart

Answer 4

A muscular double pump whose contractions push blood through the circulatory system. It is located in the thoracic cavity behind and slightly to the left of the sternum, between the lungs. It is surrounded by a thin membranous sac, the pericardium, which supports and lubricates the heart during contraction.

Question 5

Epicardium

Answer 5

The outer layer of the heart consists of epithelial cells and underlying fibrous connective tissue. The coronary arteries, which supply oxygen to the heart, are embedded in this layer.

Question 6

Myocardium

Answer 6

The middle layer of the heart is composed of cardiac muscle, whose cells make extensive contacts with adjacent cells to allow electrical activities to spread easily from one cell to the next.

Question 7

Endocardium

Answer 7

The innermost layer of the heart is made up of endothelial cells, modified epithelium that is continuous with the endothelium lining the blood vessels that enter and exit the heart’s chambers.

Question 8

The four chambers of the heart

Answer 8

The left and right atria, and the left and right ventricles. The atrial septum divides the two atria, and the interventricular septum divides the two ventricles.

Question 9

The four valves of the heart

Answer 9

These prevent the backflow of blood. The right atrioventricular (AV) valve, also called the tricuspid valve, separates the right atrium and the right ventricle, and the pulmonary semilunar (pulmonic) valve separates the right ventricle from the pulmonary arteries. The left atrioventricular (AV) valve, also called the bicuspid valve or mitral valve, separates the left atrium from the left ventricle, and the aortic semilunar valve separates the left ventricle from the aorta.

Question 10

Circulation through the heart

Answer 10

De oxygenated blood from the systemic circulation collects in the superior and inferior venae cavae, which empty into the right atrium. Contraction of the right atrium forces blood through the tricuspid valve into the right ventricle. Contraction of the right ventricle forces blood out through the pulmonary semilunar valve, through the pulmonary trunk, and the left and right pulmonary arteries. Blood then travels to the lungs, picking up oxygen and releasing carbon dioxide in the capillaries surrounding the alveoli. Returning to the heart via the left and right pulmonary veins, oxygenated blood enters the left atrium. Contraction of the left atrium forces blood through the mitral valve into the left ventricle. Contraction of the left ventricle forces blood through the aortic semilunar valve into the aorta. the aorta rises up from the top of the heart before turning and descending through the thoracic and abdominal cavities of the major arteries in the body branch directly from the aorta. Arteries branch further into arterioles, which lead to capillary beds within the tissues, where the blood releases its oxygen. Blood becomes deoxygenated as it passes through the capillaries and then enters venules, which link to form veins. Major veins empty into the venae cavae, which return blood to the right atrium, completing the cycle.

Question 11

Simple outline of circulation of the heart

Answer 11

• Deoxygenated blood enters the right atrium: from the body through the vena cava.
• Blood moves to the right ventricle: through the tricuspid valve.
• Right ventricle pumps blood to the lungs: via the pulmonary artery.
• Oxygenated blood returns to the heart through the pulmonary veins: entering the left atrium.
• Blood moves to the left ventricle: through the mitral valve.
• Left ventricle pumps oxygenated blood to the body: through the aorta.
• The right side of the heart handles deoxygenated blood, sending it to the lungs.
• The left side of the heart receives oxygenated blood and pumps it to the body.

Question 12

Contraction of the heart and blood pressure

Answer 12

Each heartbeat cycle includes a contraction and a relaxation of each chamber. The contraction, called systole, develops pressure, and forces blood through the system. The relaxation, called diastole, allows the chamber to fill again. The two atria contract together, as do the two ventricles. Atrial systole occurs slightly before ventricular systole and is not as forceful. The familiar “lubb-dupp” sound of the heartbeat is actually the sound of the valves closing - the “lubb” is the closing of the two AV valves at the start of ventricular systole, and the “dupp” is the closing of the two semi lunar valves as arterial backpressure forces them shut at the start of ventricular diastole

Blood in the circulatory system is under pressure, even during ventricular diastole. Blood pressure, measured by a sphygmomanometer, is the measure of the force of blood on the arterial walls. Blood pressure is given as the ratio of ventricular systole to diastole. Blood pressure differs markedly at different points in the circulatory system, and for this reason, it is always measured from the brachial artery at the upper arm.

Question 13

Layers of blood vessels

Answer 13

In general, blood vessels, except for capillaries, have three discrete layers surrounding the lumen, or the space in which blood flows. The tunica adventitia, or outer layer, is composed of connective tissue; the tunica media, or middle layer, is made of smooth muscle; and the tunica intima, or inner layer, is composed of a single layer of endothelial cells.

Question 14

Arteries

Answer 14

Arteries carry blood away from the heart. They have a thick muscular wall that can expand when blood is pumped into them and then contract to maintain flow and pressure during diastole. Arteries are located deeper than veins, but can be found by feeling for a pulse. They branch into smaller arterioles, which ultimately branch to form capillaries.

A myocardial infarction (MI or heart attack) occurs when the heart receives in adequate blood supply through the coronary arteries. This often occurs when the arteries become clogged with built up atherosclerotic plaques. The resulting lack of oxygen, termed ischemia, causes damage to the muscle. The damage, cardiac muscle releases a number of proteins into the circulation, which can be used for diagnosis to determine the timing of the MI.

Question 15

Test used to diagnose a myocardial infarction

Answer 15

• CK total: creatine kinase total
• CK-MB fraction: creatine kinase MB fraction
• Myoglobin
• Troponin T (TnT)
• Troponin I

Question 16

Capillaries

Answer 16

Composed only of the tunica intima, a single layer of endothelial cells. This allows rapid diffusion of gases and nutrients between tissues and blood across the capillary membrane. Capillaries form meshworks, called capillary beds, which permeate the tissues. On average, no cell is farther than a few cells from a capillary. In its chemical composition, capillary composition is more similar to arterial blood than to venous blood, especially in warmed tissue, where blood flow is rapid.

Question 17

Veins

Answer 17

Carry blood back toward the heart. Capillary blood enters venules, the smallest veins. Venules joint to form larger veins. Veins have thinner walls and less muscle than arteries do, because they do not experience large fluctuations in blood pressure. To help prevent backflow of blood, veins have valves within them at various points along their length that are pushed closed when blood flows back against them. Veins are closer to the surface than are arteries. Most blood tests are performed on venous blood because of the easier access and because venipuncture is safer than arterial puncture.

Question 18

The antecubital fossa

Answer 18

the area just distal to the elbow joint, where blood is usually drawn. This area is usually accessible and contains several prominent veins that are usually located a safe distance from nerves and arteries, making it an ideal location for venipuncture.

Question 19

Circulatory anatomy of the antecubital fossa

Answer 19

The capillary beds of the hand and drain into a network of veins that pass into the forearm. These collect to form several major veins. On the anterior surface (where blood is drawn), the most prominent of these are the cephalic vein, the median cubital vein, and the basilic vein (less commonly drawn). The median cubital vein splits just below the elbow, sending one branch to the basilic vein and one branch to the cephalic vein. Thus these veins form a rough M in this region. Blood is typically drawn from one of the veins forming part of this M. In other patients, the veins will resemble the H.

The exact anatomy of this region may vary considerably from person to person. Veins may branch multiple times, some smaller veins may be absent, or they may be located in unusual places. This rarely causes problems, as long as a prominent vein can be found for drawing blood. Problems can arise, however, from the location of other structures in the antecubital fossa. The brachial artery passes through the elbow, splitting into the radial and ulnar arteries. These are located deeper than the veins, though, and the skilled phlebotomist rarely has any trouble avoiding them. A more common complication (although still rare) arises from the position of two nerves that also pass through this busy intersection. The external cutaneous nerve passes close to the cephalic vein, and the internal cutaneous nerve passes close to the basic vein. In many patients, the nerves are no deeper than the veins, and in some patients, the nerves may pass over, rather than under, the veins. Contacting the nerve with the needle causes an intense, sharp pain and may lead to long term nerve damage.

Question 20

Blood

Answer 20

An average adult has 5 to 6 L of blood. Blood is composed of plasma - the fluid portion - and cellular components, called the formed elements.

Question 21

Plasma

Answer 21

Constitutes 55% of the volume of blood. It is 90% water, with the rest made up of dissolved proteins, amino acids, gases, electrolytes, sugars, hormones, lipids, and vitamins, plus waste products, such as urea, designed for excretion.

The most significant elements of plasma are albumin, a plasma, protein, responsible for osmotic, pressure and transport of many types of molecules; the immunoglobulins, or antibodies, which are important parts of the immune system; and fibrinogen, responsible for blood clotting. Other proteins include hormones, such as insulin, and transport proteins, such as transferrin, which carries iron. Plasma also contains complement, a group of immune system proteins that, when activated, destroy target sells by puncturing their membranes. Electrolytes include the major ions of plasma - sodium and chloride, plus potassium, calcium, magnesium, carbonate, phosphate, and sulfate ions.

A plasma sample is collected in a tube containing an anticoagulant, a chemical that keeps blood from clotting. The sample is spun at high speed in a centrifuge, which separates the plasma from the cells. The plasma is the straw-to-yellow color fluid that floats on top of the cells in the sample. Plasma samples are collected in tubes with a top that is one of these colors: light blue, royal blue, pink, pearl, gray, green, light, green, or tan. Plasma samples are used for short turnaround time (stat) chemistry tests, because when results are needed immediately, there is no time to allow coagulation. The samples are collected in a green-top tube. They are also used for coagulation tests, which are collected in a blue-top tube.

Question 22

Serum

Answer 22

Plasma without its clotting factors. Serum is formed when a blood sample is collected in a glass or a plastic container that has no additives and is induced to clot (serum separator tube is used for this purpose). The conversion of fibrinogen to fibrin forms strands that trap all of the cellular elements. After clotting, the serum is separated from the clotted material by centrifugation. A serum sample is used for many types of tests, because clotting would interfere with the results. Serum samples are collected in tubes with a top that is one of these colors: orange, royal blue, red, gold, or tiger-speckled (red and gold).

Question 23

Formed elements

Answer 23

Constitute 45% of blood volume. Of these, 99% are red blood cells (RBCs), with white blood cells (WBCs) and platelets making up the rest. All blood cells are formed in the bone marrow from the division of long lived progenitors called stem cells.

Question 24

Red blood cells

Answer 24

RBCs carry hemoglobin, the iron containing oxygen transport protein that gives blood its red color. There are 5 million RBCs in a microliter of whole blood. An RBC initially contains a cell nucleus, but this is expelled shortly after formation, at which stage the RBC is known as a reticulocyte. (the reticulocyte count, a common laboratory test, provides the physician with an indirect measure of how well the bone marrow is producing RBCs.) Once released into the peripheral circulation, within a day or two, it matures into an erythrocyte. A single RBC remains in the peripheral circulation about 120 days before being removed by the liver, bone marrow, or spleen. RBC destruction, results in three major breakdown products: iron, amino acids, and bilirubin. The iron and amino acids are recycled. The only waste product, bilirubin, is transported to the liver for elimination from the body. The iron-containing heme groups are recycled to make a new hemoglobin for packing into new RBCs.

Question 25

White blood cells

Answer 25

WBCs or leukocytes protect the body against infection. WBCs are produced in the bone marrow and lymph nodes and undergo a complex maturation process, which may involve the thymus and other organs. There are 5000 to 10,000 WBCs in one microliter of whole blood. At any time, most WBCs are not in the blood, but in the peripheral tissues and lymphatic system. An important feature of all WBCs is their ability to recognize specific molecules on the surface of infectious agents, and to distinguish them from markers on the body’s cells. This molecular recognition is responsible for the extraordinary ability of the immune system to protect the body against the daily threat of attack from bacteria, viruses, and other infectious organisms.

Two major categories of leukocytes exist: granulocytes and mononuclear leukocytes. Granulocytes get their name because of the presence of visible granules in their cytoplasm. The granulocytes include neutrophils, eosinophils, and basophils. Mononuclear leukocytes have a larger, unsegmented nuclei. They include lymphocytes and monocytes. They all contain powerful chemicals that destroy foreign cells and signal other parts of the immune system.

Question 26

Neutrophils

Answer 26

Neutrophils, so called because their granules do not take up either acidic or basic dyes, make up 40% to 60% of all leukocytes in the blood. They are phagocytes, whose role is to attack and digest bacteria, and their numbers increase during a bacterial infection. They are often the first WBCs on the scene of an infection, and their rather short lifespan (approximately 3 to 4 days) is further shortened when they engulf and digest bacterial and cellular debris. Neutrophils are also called polymorphonuclear (PMN) leukocytes, or segmented neutrophils (segs), because of their highly divided and irregularly shaped nuclei.

Question 27

Eosinophils

Answer 27

*Eosinophils** take up the dye eosin, which stains their granules an orange-red. They represent 1% to 3% of all circulating leukocytes. Eosinophils are also phagocytic, and their principal targets are parasites and antibody-labeled foreign molecules rather than cells. Their numbers increase in the presence of allergies, skin infections and parasitic infections.

Question 28

Basophils

Answer 28

Basophils stain darkly with basic dyes and are deep purple in a standard blood smear. They account for less than 1% of leukocytes in the blood. Basophils are phagocytic. They also release histamine, a chemical that swells tissue and causes allergic reactions. Basophils produce heparin as well, which is an anticoagulant. Increased numbers of basophils are usually associated with some kind of leukemia.

Question 29

Lymphocytes

Answer 29

Lymphocytes make up 20% to 40% of all circulating leukocytes, but this is a small fraction of their total number, most of which reside in lymph nodes. Lymphocytes circulate between the lymphatic system and the circulatory system, and their numbers may increase during viral infection. Lymphocytes include B cells, which produce antibodies; natural killer (NK) cells, which destroy both foreign and infected cells; and T cells, which control cellular immunity.

Question 30

Monocytes

Answer 30

*Monocytes** are large phagocytic cells that make up 3% to 8% of all circulating leukocytes. Monocytes pass from the circulatory system into the peripheral tissues, where they transform into macrophages and act as roving sentries. Their signals help activate B cells to make antibodies.

Question 31

Platelets

Answer 31

Platelets are created in the bone marrow from megakaryocytes, which package and release them into the circulation. Although they are also called thrombocytes, platelets are not actually cells; they are simply membrane-bound packets of cytoplasm. Platelets play a critical role in blood coagulation. Every microliter of blood contains approximately 200,000 platelets, each of which remains in circulation for 9 to 12 days.

Question 32

Hemostasis

Answer 32

Refers to the processes by which blood vessels are repaired after injury. It occurs in a series of steps, from muscular contraction of the vessel walls, through clot formation, to removal of the clot when the vessel repairs itself.

Question 33

Vascular phase

Answer 33

Rupture of a vein or artery causes an immediate vascular spasm, or contraction of the smooth muscle lining the vessel. This reduces the vessel diameter, substantially reducing the blood loss that would otherwise occur. This contraction lasts about 30 minutes. For capillaries, this may be enough to allow the wound to seal.

Question 34

Platelet phase

Answer 34

Exposure of materials beneath the endothelial lining causes platelets to stick to the endothelial cells almost immediately, a process known as adhesion. Additional platelets then stick to these, a process known as aggregation. Aggregating platelets become activated, releasing factors that promote fiber accumulation in the next phase. The combination of the vascular phase and the platelet phase is called primary hemostasis. The next phase, coagulation, is known as secondary hemostasis.

Question 35

Coagulation phase

Answer 35

Coagulation involves a complex and highly regulated cascade of enzymes and other factors whose activation ultimately results in formation of a blood clot - a meshwork of fiber, platelets, and other blood cells that closes off the wound. The coagulation cascade begins from 30 seconds to several minutes after the injury.

Coagulation is initiated through two different pathways that feed into a single common pathway. The extrinsic pathway begins with the release of tissue factor by endothelial cells, which combines with calcium ions and coagulation factor VII from the plasma to form an active enzyme. The intrinsic pathway is initiated when other plasma coagulation factors contact the materials exposed when the blood vessel is damaged (similar to platelet adhesion). A series of reactions takes place to form an enzyme that can initiate the common pathway.

Enzymes from both pathways then into the common pathway, reacting with factors X and V to convert circulating inactive prothrombin (also called factor II) to active thrombin. Once activated, thrombin converts circulating fibrinogen to fibrin. Fibrin is a fibrous protein whose strands adhere to the platelets and endothelial cells at the wound, forming a dense fibrous network that, in turn, traps other cells. Platelets then contract and pull the edges of the wound closer together, allowing endothelia cells to grow across the wound and repairing the damage lining.

Question 36

Fibrinolysis

Answer 36

As the wound is closed and tissue repair commences, fibrin itself is broken down slowly, a process called fibrinolysis. This creates fiber degradation products (FDPs). FDPs are monitored to diagnose disseminated intravascular coagulation (DIC), a condition in which blood clots abnormally in the circulatory system. DIC is a potential complication of pregnancy, as well as other conditions.

Fibrinolysis is controlled by plasmin, an enzyme made from plasminogen by tissue plasminogen activator (t-PA). Synthetic t-PA is used to dissolve blood clots in stroke, MI, pulmonary embolism, and other conditions. Urokinase and streptokinase are also used to activate plasminogen in these conditions.

Question 37

Blood disorders

Answer 37

There are many blood disorders and tests to identify them. It is important to remember that, as a phlebotomist, you may not know the actual reason any particular test is ordered for any particular patient. You should not discuss the purpose of a test with a patient - that is the role of the doctor.

Question 38

Hemostasis, Platelets, and Clotting Disorders

Answer 38

• Hemophilia: group of inherited disorders marked by deficient clotting factor production and increased bleeding; the most common is hemophilia A, a deficiency in clotting factor VIII. Tests include activated partial thromboplastin time (aPTT) or prothrombin time (PT) and clotting factor activity (factor VIII activity).
• Disseminated intravascular coagulation (DIC): activation of the clotting system throughout the circulatory system, in response to bacterial toxins, trauma, or other stimuli; fibrin is eventually degraded, but small clots may form, damaging tissue; fibrinogen deficiency may follow. Tests include fibrin degradation product (FDP), D-Dimer, fibrinogen.
• Thrombocytopenia: decreased number of platelets. Test includes complete blood count (CBC).
• Thrombosis or deep vein thrombosis (DVT): localized activation of the clotting system, called thrombophlebitis in veins; an embolus is a piece of clot that has broken off and entered the circulation. Test includes protein C and protein S.

Question 39

Red blood cells (RBCs) and hemoglobin disorders

Answer 39

• Anemia: decrease in number of RBCs or amount of hemoglobin in the blood. Tests include reticulocyte count; iron studies: ferritin and total iron binding capacity (TIBC); vitamin B12 and folate levels; and CBC
• Polycythemia: increase in total number of blood cells, especially RBCs; treated with therapeutic phlebotomy. Test includes CBC.

-Sickle cell disease: inherited, hemoglobin disorder, resulting in sickle shaped RBCs; requires inheritance from both parents (autosomal recessive inheritance). Tests include sickle cell solubility, hemoglobin electrophoresis, and CBC.

• Thalassemia: group of inherited hemoglobin disorders, resulting in decreased production of hemoglobin and anemia. Tests include hemoglobin electrophoresis and CBC.

Question 40

White blood cells (WBCs) disorders

Answer 40

• Bacterial infection: WBCs respond to many kinds of infection and inflammations. Tests include CBC with differential, bacterial cultures.
• Human immunodeficiency virus (HIV) infection: infection of helper T cells by HIV; can lead to acquired immunodeficiency syndrome (AIDS). Tests include anti-HIV antibody, western bolt, and T cell count: CD3, CD4, and CD8.
• Leukemia: malignant neoplasm in the bone marrow, causing increased production of WBCs. Tests include CBC with differential, and cell marker studies.
• Mononucleosis: increased number of reactive lymphocytes in response to infection with the Epstein-Barr virus. Tests include CBC and monospot or heterophile antibody.

Question 41

Disorders of the immune system

Answer 41

-Rheumatoid arthritis: body produces antibodies that attack the membranes aligning the joints. Tests include rheumatoid factor and anticitrullinated peptide (anti-CCP)

• Systemic lupus erythematosus: body produces antibodies that attack a variety of tissues and organs: joints, lungs blood cells, nerves, and kidneys. Tests include antiphospholipid antibodies (APLs) and antinuclear antibodies (ANA).
• Multiple sclerosis: body produces antibodies that attack nerve cells. There is no blood test, but you can use spinal tap and magnetic resonance imaging.
• ** Severe combined immune deficiency (SCID)**: genetically inherited disease in which the immune system is severely impaired. Test includes immunoglobulin quantitation.

Question 42

Lymphatic system

Answer 42

Includes the lymphatic vessels, the lymph nodes, and several associated organs, plus the lymph fluid flowing in the lymphatic vessels and the leukocytes that reside in the lymph nodes and elsewhere. The lymphatic system returns tissue fluid to the circulatory system - in the process, screening it for signs of infection - and provides a passageway for lymphocytes patrolling the tissues.

Question 43

Lymphatic vessels

Answer 43

In contrast to arteries and veins, which form a complete circulatory loop, lymphatic vessels are closed at their distal ends. These closed end tubes, called terminal lymphatics, are slightly larger than capillaries and permeate the tissues (except for the central nervous system), although not as extensively as capillaries do. The wall of a terminal lymphatic vessel is an endothelial layer that is one cell thick. Small lymphatic vessels feed into larger ones and finally into the thoracic duct and right lymphatic duct. These empty into the venous system just distal to the superior vena cava. Along the way, valves within the lymphatic vessels keep fluid moving in one direction.

Lymphatic fluid is derived from the fluid cells, called interstitial fluid. Interstitial fluid is a combination of plasma that has been forced out of capillaries by blood pressure and cellular fluid exiting the cells through osmosis. Lymphatic fluid thus carries the chemical signature of the peripheral fluid as a whole. Most important, it contains proteins and other molecules released by infectious agents, which are carried to the lymph nodes for examination by WBC. Lymphatic fluid is also important for the transport of fat - terminal lymphatics in the intestine absorb dietary fat, keeping it out of the capillary circulation.

Question 44

Lymph organs

Answer 44

lymph nodes are chambers located along the lymphatic vessels and are populated by lymphocytes. Lymph nodes are especially common in the digestive, respiratory, and urinary tracks. The tonsils are particularly large lymph nodes. Lymphocytes in the nodes screen the lymph fluid for signs of infection. In addition, macrophages journey to the lymph nodes from infection sites, carrying molecular samples for examination by B and T cells.
The other organs of the lymphatic system include the spleen and the thymus. In the spleen, worn out RBCs are recycled by macrophages, and blood is examined by resident B and T cells. The thymus is principally involved in maturation of the immune system during development and early life.

Question 45

Lymphatic system disorders

Answer 45

Lymphedema is an accumulation of interstitial fluid in tissues as a result of a blocked lymphatic vessel. Elephantiasis is a severe form of lymphedema, caused by a mosquito borne parasite that colonizes the lymphatic vessels. Lymphoma is a tumor of a lymph gland. Hodgkin disease is a type of lymphoma. Because they provide passageways for mobile cells, lymphatic vessels provide a route for the spread of metastatic cancer cells, which may either take up residence in a lymph node, or pass out of the lymphatic system to invade surrounding tissue. For this reason, biopsy of nearby lymph nodes is often performed to determine whether a cancer has metastasized.

Question 46

Immune system

Answer 46

The immune system involves the coordinated action of many cell types, along with the circulating proteins of antibodies and complement. In addition, it includes physical barriers to the entry of infection, such as the skin and the epithelial lining of the lungs, gut, and urinary tract.

Question 47

Nonspecific immunity

Answer 47

Refers to the defense against infectious agents, independent of the specific chemical markers on their surfaces. There are several components that contribute to non-specific immunity, including physical barriers (such as intact, skin, mucous membranes, and the tears found in the eyes), non-specific chemical systems (including the complement system and interferon), and phagocytic cells that engulf and destroy foreign cells without regard to their exact identity. Monocytes and neutrophils are both phagocytes.

Inflammation is a coordinated, nonspecific defense against infection or irritation that occurs initially in response to foreign invaders. It combines increased blood flow and capillary permeability, activation of macrophages, temperature increase, and the clotting reaction to wall off the infected area. Activation of the complement system and release of interferon chemicals also play a part in inflammation. These actions also activate the mechanism of specific immunity.

Question 48

Specific immunity

Answer 48

Involves the molecular recognition of particular markers, called antigens, on the surface of a foreign agent. Recognition of these antigens in the appropriate context, triggers activation of T and B cells, while also increasing the activity and accuracy of nonspecific defenses, such as complement and macrophages.

In one form of antigen recognition, antigens are taken up by a macrophage, which displays them on its surface in a receptor complex controlled by genes in the major histocompatibility complex (MHC). The proteins in these complexes are also known as human leukocyte antigens (HLAs). The antigen complex is presented to T cells, which are activated when they recognize the HLA-antigen combination. Activated T cells influence the production of both cytotoxic T cells, which recognize antigens and destroy both foreign and infected host cells, and memory T cells, which are primed to respond more rapidly if they antigen is encountered again later in life. Other T cells, called helper T cells, are important regulators of the entire immune response. They are needed to make both antibodies and cytotoxic T cells. T cells are also known as CD4+ cells, named after one of their surface receptors; these are the cells infected by HIV. T cell-based immunity is also called cellular immunity.

Antigens can be recognized in another way. An antigen can bind to an antibody, a product of a B cell. Each B cell makes an antibody of a slightly different shape, allowing recognition of an enormous variety of antigens. If the antibody shape is complementary to the antigen shape, it will activate the B cell under the influence of helper T cells and macrophages to undergo rapid cells division. Most of the offspring are plasma cells, which produce and release large numbers of antibodies into the lymphatic system and, ultimately, into the circulation. These antibodies bind to antigens at the site of infection and elsewhere, directly inactivating them; they also act as flags for targeting by macrophages. The remaining B cell offspring become memory cells, which serve a similar function to memory T cells. This is the basis of immunization. Antibody based immunity is also called humoral immunity, after humor, the antiquated term for a body fluid.

In addition to the direct contact between cells, immune cells communicate through cytokines, chemical messengers that include interferons and interleukins. Functionally, these molecules are hormones, released by one cell to influence the behavior of another.

Question 49

Immune system disorders

Answer 49

Autoimmunity is an attack by the immune system on the body’s own tissues. Autoimmune disorders include rheumatoid arthritis, systemic lupus erythematosus, myasthenia gravis, and multiple sclerosis. Allergy is an inappropriately severe immune reaction to an otherwise harmless substance. Severe combined immune deficiency (SCID) is an inherited disorder marked by an almost total lack of B and T cells. Acquired immunodeficiency syndrome (AIDS) is caused by HIV infection.