Lymphocytes and antibodies

Question 1

Antibody

Answer 1

Y shaped antigen-specific immunoglobulin produced by B lymphocytes.

Question 2

Specificity of B cells

Answer 2

Each B lymphocyte only produces a unique antibody expressed on their plasm membrane.

Therefore each B cell is antigen-specific as only one antibody can match a specific antigen.

Question 3

Structure of antibodies

Answer 3

Y shaped: two of each chain- heavy and light.

Variable region: different in each antibody and contains FAB site. FAB site is the antigen binding site.

Constant region: very little variation between different antibodies. This is the Fc region.

Question 4

How does an antibody bind to an antigen?

Answer 4

The FAB region in the antibody interacts with the epitope on the antigen.

The FAB region has a physio-chemical affinity to the epitope.

Question 5

Antibody isotype

Answer 5

5 different types of antibodies. The constant regions are different but the variable region remains the same.

IgA

IgD

IgG

IgM

IgE

Question 6

IgA

Answer 6

Only type of antibody that can cross mucosal surfaces: found in saliva, gut, tears.

It does not act as a monomer, instead forms DIMERS held by a J chain.

Contains S chain (secretory component) which protects the dimer from digestive enzymes.

Found in colostrum, precursor to breast milk: can be vertically passed to newborn babies.

Question 7

IgG

Answer 7

The main mature antibody found in the blood.

IgG circulates as a monomer.

Question 8

IgM

Answer 8

The first antibody produced in an immune response.

Has a low affinity to antigens, does not exist as a monomer: forms pentamers linked by a J chain.

Question 9

IgD

Answer 9

Primarily released in an immune response like IgM.

Question 10

IgE

Answer 10

Mature antibody that circulates the blood as a monomer.

Involved in mast cell activation which is important in allergies.

Also involved in parasitic infection.

Question 11

Functions of antibodies in the immune system.

Answer 11

Neutralisation: binds to toxins making them inactive.

Blocks antigen receptors: Prevents antigen from binding to cells and causing disease.

Opsonisation: Coats bacteria and allow phagocytosis to be easier for phagocytes.

Mast cell activation: Triggers the release of histamines in inflammation.

Activates complement pathway: Opsonisation, inflammation, terminal attack pathway.

Antibody-dependent cellular toxicity: Tags bacteria so that NK cells recognize bacteria and destroy it them by releasing chemical agents.

Question 12

Somatic recombination

Answer 12

Gives rise to the variation and antigen-specificity of B cells.

Occurs during gestation:
The genes coding for variable region are in different sections of DNA.
Genes are randomly selected from each region of DNA and combined to form one gene coding for a variable region.

Question 13

Advantages of somatic recombination

Answer 13

Allows a vast number of receptors for antibodies to be formed from a small region of DNA.

Allows diversity within a population. Population will not be susceptible to the same diseases.

Provides unique DNA regions for antibodies in each individual.

Advantageous gene segments can be inherited.

Question 14

Disadvantages of somatic recombination

Answer 14

Some of the genetic combination do not produced viable variable regions: only a few B cells are successful.

It is energy extensive to destroy the non-viable B cells.

B cells that recognise self-antigens can be destroy which leads to autoimmune diseases.

Question 15

Affinity maturation

Answer 15

This occurs after clonal selection of a B cell, allowing the FAB region to be as efficient as possible.

1. Class switch: After B cell is selected, the IgM on the cells keeps its variable region but changes its constant region to IgG.
2. Somatic hyper-mutation: B cells mutate their genes coding for variable region so that it has a better affinity to antigens.

Question 16

Clonal selection

Answer 16

This occurs when the B cell with antibodies that has highest affinity to an antigen is selected.

This triggers the B cell to proliferate, through clonal expansion.

This process takes a few days as at first there is a very small number of B cells specific to the antigen.

Question 17

Mast cell activation

Answer 17

Mast cells are coated with IgE with the Fc region binding to the mast cell receptors.

When the specific antigen binds to the antibodies on the mast cells, this triggers the release of histamines.

This process can be triggered inappropriately in allergic reactions.

Question 18

Primary response of the immune system.

Answer 18

When an antigen in newly introduced, it takes a few days for the B cell containing the right antigen to be selected.

At first IgM antibodies are produced before a greater amount of IgG antibodies are produced through clonal expansion.

Some mature B cells are long-lived and stay in the blood until they become stimulated in the blood.

Question 19

Secondary response of the immune system.

Answer 19

Memory B cells in the blood recognise the repeated entry of an antigen.

This allows a greater number of antibodies to be produced at a quicker rate.

The production of IgG also occurs at a quicker rate and more is made compared to the primary response.