B-lymphocytes and antibody

Question 1

Describe the structure of an antibody.

Answer 1

Consists of 4 chains: 2 heavy, 2 light
FAB region consists of heavy and light chains. It is the antigen binding site.
Fc region is the alternate binding site for other components of the immune system e.g. B cell.
Heavy chains held together by disulphide bridges

Question 2

What is the epitope of an antigen?

Answer 2

It is the specific site at which the antigen binds an antibody. The basis of this binding is physio-chemical affinity.

Question 3

Consider the antibody isotopes. Describe immunoglobulin-M

Answer 3

First antibody produced in immune response
Relatively low affinity for antigen
Immature
Forms pentamer to increase affinity by increasing SA.
Pentamer involved 5 antibodies around a J-protein

Question 4

Consider the antibody isotopes. Describe immunoglobulin-E

Answer 4

No known function

Monomer

Question 5

Consider the antibody isotopes. Describe immunoglobulin-A

Answer 5

Found in breast milk
Can cross mucous membranes so found in gut, tears, saliva and other secretions.
For dimers with J proteins with s chain that protects it against secretory elements.

Question 6

Consider the antibody isotopes. Describe immunoglobulin-G

Answer 6

Main mature antibody

Monomer

Question 7

Consider the antibody isotopes. Describe immunoglobulin-D

Answer 7

No known function

Immature

Question 8

How do antibodies help us?

Answer 8

1. Binding directly (toxin neutralisation and receptor blocking)
2. Binding then interacting with another element of immune system e.g. phagocytes, complement, mast cell activation, antibody-dependent cellular cytotoxicity
3. Act as B cell receptor when bound to B cell

Question 9

How do phagocytic cells increase efficiency of phagocytosis?

Answer 9

They present receptors for the Fc portion of the antibody to decrease repulsion between the two negatively charged membranes.

Question 10

What is opsonisation?

Answer 10

Process of coating bacteria to enhance phagocytosis.

Question 11

Describe mast cell activation

Answer 11

Have surface Fc receptors
Become coated with IgE from circulation
When antigen binds to IgE and cross links them, the mast cell degrades
Degradation of mast cell releases histamine
Important in parasitic infections
Inappropriately triggered in allergies

Question 12

Describe antibody-dependent cellular cytoxicity.

Answer 12

A NK cell expressed a receptor that is complementary to the Fc portion of an antibody of an antibody-coated bacteria.

Leads to destruction by non-phagocytic means

Question 13

How is the variable region of an antibody made?

Answer 13

By somatic recombination.

This involves selection of different gene segments thought each DNA region and combing them.

Question 14

Suggest 4 advantages for somatic recombination

Answer 14

Huge diversity
Able to make a large number of receptors from a small are of DNA
Unique repertoire- résilient in different environment
Gene segments are inherited so there is some evolutionary benefit

Question 15

Suggest 2 disadvantages for somatic recombination

Answer 15

Randomness means some combinations don’t work because they 1. biochemically don’t fit 2. bind to our own proteins (self antigens)

B cells with dysfunction receptors mainly destroyed- this is energy intensive

Question 16

Affinity maturation occurs once an antigen has been met. This occurs by class switch and somatic hypermutation. Describe each

Answer 16

Class switch- IgM in primary response switches to IgG. Variable region of the antibody remains the same.

Somatic hypermutation- random mutations introduced into variable region so daughter cells produce ‘better fit’ antibodies. THIS IS INEFFICIENT

Question 17

What are antibody isotopes?

Answer 17

Antibodies with different heavy chain constant regions and so have different properties.