Cell recognition and the Immune System

Question 1

what are pathogens

give examples of infectious pathogens

Answer 1

pathogens are disease causing microorganisms

some examples of disease causing pathogens are: bacteria, viruses, fungi

Question 2

when does disease occur

Answer 2

disease occurs when infection leads to recognisable symptoms in the host

Question 3

what is transmission

Answer 3

transmission is when a pathogen is transferred from one individual to another

Question 4

are viruses living or non living

why?

Answer 4

viruses are non living

they don’t have a nucleus , cell surface membranes and no cytoplasm

Question 5

what does a typical virus particle always contain

Answer 5

always contain: genetic material (DNA or RNA) , protein capsid and attachment proteins

Question 6

what is is the general replication of viruses process

Answer 6

1) the virus particles attach to the host cell using their attachment proteins/ glycoproteins which are complementary to receptors on the cell surface membrane of host cells

2) the virus particles inject their nucleic acid (RNA or DNA) into the host cells

3) the genetic material is used to code for more virus particles using the organelles of the host cells

4) the involves producing copies of the viral nucleic acids and proteins to form complete viruses which are often released in the lysis of cell

Question 7

describe phagocytosis

Answer 7

it’s a non specific immune response

1) the phagocyte extends and engulfs the pathogen forming a phagosome or a phagocytic vesicle

2) lysosomes fuses with the phagosome and uses its lysozymes to hydrolyse the pathogen

3) soluble digestible products are absorbed , indigestible is removed

4) phagocytes can also remove the antigens and present it on their cell surface membrane to T cells (antigen presenting cells)

Question 8

where are b cells produced and mature

Answer 8

produced : bone marrow
mature in: bone marrow

Question 9

where t cells mature

Answer 9

mature in the thymus

Question 10

what are antigens

Answer 10

antigens are proteins or glycoproteins that appear foreign to the individual and stimulate the production of antibodies by b lymphocytes

Question 11

where may antigens present

Answer 11

• surface of pathogen
• organ transplant
• abnormal body cells
• as a toxin

Question 12

describe the humoral response of b cells

Answer 12

1) the body has a large number of different types of b cells each type of capable of producing a different specific antibody

2) these b cells secrete specific antibodies onto their cell surface membrane

3) a specific antigen may attach to the complimentary antigen on b cell

4) the b cell are stimulated to divide by mitosis. this is known as a clonal selection

5) helper T cells have to activate B cells to divide

6) mitosis results in a large population of identical plasma B cells which produce the same specific antibody and secrete it into the BLOOD PLASMA

7) the antibodies secreted by plasma B cells bind specifically to antigens forming an antibody-antigen complex which stimulates processes leading to the destruction of the pathogen

8) some of b cells divide and develop into memory b cells

Question 13

what is antigenic variation

Answer 13

Some microorganisms e.g. the influenza virus have high a mutation rate and this leads to antigenic variation. Therefore, even though an individual may become ‘immune’ to one strain of the virus, this will not provide immunity to the new forms as they are not recognised by the memory B cells and antibodies previously produced are not complementary.

Question 14

what are antibodies

Answer 14

proteins produced by b cells

Question 15

where are antibodies found

Answer 15

tissue fluid , blood , breast milk

Question 16

what’s the structure of an antibody molecule

Answer 16

basic structure of an antibody molecule consists of 4 polypeptide chains: 2 heavy chains, 2 light chains which are joined by disulphide bonds

each polypeptide chain consist of a variable region and a constant region

Question 17

how many binding sites are on an antibody molecule

what is the binding site

Answer 17

there are 2 binding sites

the variable regions of the heavy and light polypeptide chains form the 2 antigen binding sites and they have a specific tertiary structure

Question 18

do antibodies directly destroy the pathogen/antigen

what does?

name 2 of the processes

Answer 18

no, however the formation of antibody-antigen complexes stimulates different processes that do cause the destruction of the pathogen

2 processes that do : agglutination of antigens and stimulation of phagocytosis

Question 19

what’s agglutination

Answer 19

Agglutination refers to the ‘clumping’ together of cells possessing the antigen against which specific antibodies (agglutinins) react. An antibody molecule can use its two antigen-binding sites to attach to the same antigen present on two different cells.

This joins the cells together. As more antibody molecules attach more cells are linked together to form an agglutinated mass (clump) of cells which are then more easily destroyed e.g. by phagocytosis.

Question 20

what happens when someone receives wrong blood group

Answer 20

Agglutination of donor red blood cells occurs when an individual receives the wrong blood group.

Question 21

what’s stimulation of phagocytosis

Answer 21

One type of antibody attaches to the antigen on the surface of a pathogen and identifies it for destruction by phagocytic white blood cells.

Phagocytes have receptors in their cell-surface membranes that recognise the antibody, and enable them to bind to, engulf and then destroy the pathogen.

Question 22

do t cells produce antibodies

what do they do

Answer 22

no

t cells have receptors in their cell surface membrane which bind to specific antigen

Question 23

explain the cellular response of t cells

Answer 23

1. The phagocyte engulfs the pathogen and phagocytoses it.
2. The phagocyte removes the antigens and embeds them in its cell-surface membrane, thus becoming an antigen-presenting cell.
3. Helper T cells (TH cells) with the complementary protein receptor will bind to the antigen.
4. This stimulates the TH cells to divide by mitosis and form a clone of genetically identical T cells all with the same receptor.
5. The cloned TH cells:

• activate cytotoxic T cells (Tc cells) - which attach to the specific antigen on the pathogen/’foreign cell’ and secretes chemicals (e.g. a protein called perforin) to destroy it.

• develop into more helper T cells (TH cells) which stimulate B lymphocytes to divide into plasma cells and secrete antibodies.

• develop into memory T cells - which remain in the blood after the infection has cleared and produce a quicker response (secondary response) if a future infection occurs with the same antigen/pathogen.

Question 24

what are the 2 types of immunity

Answer 24

passive and active

Question 25

describe passive immunity

Answer 25

This is where an individual receives pre-formed antibodies from an outside source. The individual is not exposed to the antigens and does not form antibodies or memory cells. Although antibodies will be immediately present, this only provides short-term immunity as the antibodies are not produced by the individual and are not replaced once they are broken down.

Question 26

describe natural passive immunity

Answer 26

• Natural Passive Immunity - the antibodies are obtained across the placenta and through breast milk. This provides short-term protection, as the body is not stimulated into producing its own antibodies and memory cells.

Question 27

describe artificial passive immunity

Answer 27

Artificial Passive Immunity - the pre-formed specific antibodies are injected usually following exposure to particularly infectious pathogens or toxins e.g. the rabies virus, snake venom (toxin).
Again, this provides short-term protection.

Question 28

what’s active immunity

Answer 28

This is where an individual exposed to the antigen produces antibodies and memory cells. Although it takes time to produce these antibodies, the immunity is usually long-term as the immune system has produced its own antibodies and memory cells.

Question 29

what’s natural active immunity

Answer 29

Natural Active Immunity - this results from an individual becoming infected e.g. chicken pox, and is exposed to the specific antigen. Long-term immunity develops as memory cells are formed.

Question 30

what’s artificial active immunity

Answer 30

Artificial Active Immunity - this results from an individual being vaccinated (immunisation) and the immune system produces its own antibodies and memory cells providing long-term immunity.

Question 31

what does a vaccine contain

Answer 31

A vaccine contains antigens from a specific pathogen. The antigens in the vaccine may be present in the form of the dead pathogen, a weakened strain of the pathogen or just antigens (removed from the pathogen).

Question 32

what does injection of vaccine stimulate ?

Answer 32

Injection of this vaccine stimulates an immune response with the production of plasma cells, which release specific antibodies, memory B cells and memory T cells. These memory cells provide long-term immunity.

Question 33

why is a booster injection sometimes given

Answer 33

Sometimes a ‘booster’ injection of the same antigen is given at a later date, to ensure a more effective response (secondary response) due to exposure to more antigen producing long-term - often life-long - immunity, due to the memory cells formed.

Question 34

what is herd immunity

Answer 34

The higher the percentage of the population vaccinated against a particular antigen/pathogen, the less the risk of transmission.

This is because the probability of an infected person encountering a person without immunity will be low.

A very important factor in people getting vaccinated is the perceived safety of the vaccine and its effectiveness in providing protection.

Question 35

what is process for HIV replication

Answer 35

The host cell for HIV is the helper T cell.

1. The virus attaches using their glycoprotein ‘spikes’ which ae complementary to specific protein receptor sites on the cell-surface membrane of the helper T cells.
2. The lipid envelope fuses with the cell-surface membrane and viral RNA and the enzyme reverse transcriptase are released into the helper T cell.
3. In the T cell viral DNA is formed by the enzyme reverse transcriptase using the viral RNA as a template.
4. Viral DNA enters the nucleus and attaches to host DNA
   When the helper T cells replicates the viral DNA replicates with host DNA. The viral DNA may remain inactive (latent) for a long time.
5. When activated the viral DNA controls the synthesis of viral RNA.
6. Viral RNA codes for the synthesis of viral proteins and more viral RNA.
7. HIV particles are assembled. The viral envelope is formed part of from the host cell membrane.

The helper T cell is destroyed as viruses are released.

8. These new virus particles infect other helper T cells, or may be transmitted to another person.

Question 36

what is aids caused by

Answer 36

infection of HIV

Question 37

describe HIV structure wise

Answer 37

HIV is a retrovirus containing RNA and the enzyme reverse transcriptase which produces DNA in the host cell using RNA as a template.

• The RNA and enzyme are surrounded by a capsid (protein coat).

The capsid is surrounded by a lipid envelope which contains glycoprotein ‘spikes’. Specific glycoproteins on the surface of the virus enable it to attach to its host cell, helper T cells (TH cells).

Question 38

how is HIV transmitted

Answer 38

HIV is transmitted only by the introduction of blood, semen or vaginal secretions from an infected individual into the bloodstream of another individual. This can occur by:

• Sexual transmission
• Blood products / transfusions of infected blood
• Sharing of needles e.g. drug users.
• Mother to baby (via placenta, during child birth or via breast milk)

Question 39

how may hiv infection lead to aids

Answer 39

HIV infection need not lead to AlDs - the infection can be controlled (but not cured) by anti-viral medication.

If untreated, viral replication destroys host cells. This reduces the number of helper T cells, which are essential in activating B lymphocytes and coordinating the immune response.

As the immune system collapses an individual suffers from a range of infections and / or tumours that eventually lead to death.

Question 40

what’s the timeline of hiv infection and development of aids
(4 phases)

Answer 40

There are usually 4 phases:

1. The body produces HIV antibodies and there may be a short flu-like illness.
2. The antibody-positive phase (HIV positive phase) - the period between infection and the onset of clinical signs.
3. The AIDS-related complex (ARC) resulting from a variety of microbial infections as the number of T helper cells decreases.
4. Death

Question 41

why viruses not affected by antibiotics

Answer 41

Antibiotics affect cellular structures of bacteria such as the cell wall and ribosomes. As viruses do not possess cellular structures, antibiotics are completely ineffective against viruses.

Question 42

what are monoclonal antibodies how are they produced

Answer 42

These are identical antibodies, having the same antigen-binding site.

These can be produced in large
amounts from a single clone of antibody-producing B cells.

Question 43

what are the uses of monoclonal antibodes

Answer 43

1. To target medication to specific cell types by attaching a therapeutic drug to an antibody. e.g. cancer cells display different antigens to healthy cells, so they can be targeted by monoclonal antibodies with a toxic drug attached. This means cancer cells are killed and healthy cells are unaffected.
2. For medical diagnosis.

For example:
• to detect the presence of specific antigens (i.e. pathogens) in body fluids to diagnose if a person is infected with a particular disease

• to detect the presence of specific antibodies produced by a person against an antigen (i.e. pathogen)

Question 44

describe the primary response and secondary response

Answer 44

The first, ‘primary response is relatively slow, requiring up to 72 hours to produce a significant concentration of antibody. During this time the microorganisms reproduce and disease symptoms may arise.

If the same antigen (pathogen) is subsequently encountered the memory B-cells divide and develop into plasma cells. These plasma cells secrete antibodies more quickly and at a higher concentration (secondary response) than in the primary response. This provides immunity as the pathogens are destroyed before disease symptoms develop.