Immunoglobulins & BCRs

Question 1

Immunoglobulin fold

Answer 1

The immunoglobulin fold is a characteristic motif of anti-parallel B-strands that are commonly found in immune receptors

They give receptors great stability

Question 2

Immunoglobulin definition

Answer 2

Immunoglobulins are proteins that recognise and bind to a particular antigen with very high specificity

Question 3

Immunoglobulin structure

Answer 3

Immunoglobulins anchored to the plasma membrane are BCRs and free immunoglobulins are antibodies

The BCR and antibody have the same structure

X2 identical light chains
X2 identical heavy chains

Question 4

Antigen binding site

Answer 4

The amino terminal of each chain of immunoglobulins has sequence variability

This forms the antigen binding site also known as Fab

Question 5

Variable regions

Answer 5

The variable regions (Fv) of immunoglobulins are made up of Ig folds that contain 3 hypervariable loops between strands of B-pleated sheets

They give the immune system diversity

Question 6

Constant regions

Answer 6

The constant regions (Fc) initiate many effector functions of the immunoglobulin

These functions are mediated by binding to Fc receptors and complement activation

Question 7

Epitopes

Answer 7

Antibodies recognise epitopes rather than antigens as a whole

Epitopes can be linear or discontinuous

Greater binding affinity tends to elicit a stronger immune response

Question 8

Avidity

Answer 8

The avidity of an antibody is the total strength of binding of antibodies to antigens

It describes the binding of multiple epitopes to multiple antigen binding sites

Question 9

Affinity

Answer 9

The affinity of an antibody describes the binding strength of one epitope to one antigen

Question 10

IgM antibodies

Answer 10

Found on the surface of naive B-cells

They are the first antibody to be secreted and have not undergone class-switching

Functions = complement activation, agglutination, neutralisation

Form pentamers

Question 11

IgA antibodies

Answer 11

Found in mucous membrane secretions, including breast milk

Have undergone class-switching

Functions = agglutination, neutralisation

Form dimers

Question 12

IgE

Answer 12

Found in the serum on the surface of mast cells

Have undergone class-switching

Trigger the release of histamine from basophils and mast cells

Question 13

IgG

Answer 13

Found in the serum and intracellular fluid

Have undergone class-switching

Functions = complement activation, agglutination, opsonisation, neutralisation

Can cross the placenta

Question 14

IgD

Answer 14

Found on all B-cells

Have an unknown function

Question 15

B-cell membrane anchoring

Answer 15

The BCR must be anchored to the membrane

• Membrane-bound Ig (mIg)
• X2 Iga
• X2 IgB

Iga & IgB have signallining motifs, ITAMs, in their cytoplasmic domains and so activate the cell

This leads to the development of plasma cells or memory cells

Question 16

Genetic variation

Answer 16

B-cells have immense variation

The genes encoding the heavy and light chains (V,D &J) are rearranged at a transcriptional level to give diversity

Each B-cell has a unique BCR