B lymphocytes

Question 1

How do B cells & T cells differ in how they recognise antigens?

Answer 1

-B & T cells are stimulated by different components of the same antigen
-T cells recognise processed peptide associated with MHC class I or II
-B cells recognise large, conformational determinants or epitopes
-May be ‘free’ or non-specifically attached to the surface of an APC

Question 2

What are the 2 types of B cell antigen recognition?

Answer 2

1. Thymus (T cell) independent (rare)
2. Thymus (T cell) dependent (majority - 99% of antigens are T cell dependent)

Question 3

How are B cells activated by thymus-independent antigens?

Answer 3

-smIg cross-linking (all antibodies are holding onto same antigen)
-Antigens are polymers of simple repeating units
-Results in B-cell activation > maturation into plasma cells & antibody production

Question 4

How are B cells activated by (T-cell) thymus-dependent antigens?

Answer 4

-T cell ‘helps’ B cell become fully activated
-Naive B cells in the periphery (spleen, lymph nodes, mucosa) which express both IgM & IgD (binding sites of the same antigen)
-Enhanced by the ability of B cells to present antigen to Cd4 Th cells via MHCII molecules
1. B cell receptor (BCR) binds to an epitope
2. Co-stimulation (provided by T cell) - CD40L (L=ligand) expressed by CD4 cells bind to CD40 on the B cell
3. Cytokine produced by CD4 cells binds to cytokine receptors on B cells

Question 5

What happens after B cell activation?

Answer 5

1. Transform into a lymphoblast
2. Ig class switch to IgG, IgA or IgE (no longer express IgM & IgD)
3. Clonal proliferation & differentiation into plasma cells & memory cells
4. Plasma cells secrete antibody that has the same specificity as the original smIg or smIgD

Question 6

What are the steps of antibody production?

Answer 6

1. APCs & T cells interact in the T cell areas of the secondary lymphoid tissues
2. Activated T cells move into the follicles & interact with B cells in the germinal centre
3. B cells that recognise antigen with high affinity divide & mature into plasma cells (as long as they receive all 3 signals)

Question 7

What is the process of immunoglobulin class switch?

Answer 7

1. Naive B cell expressing smIgD & smIgM is activated & undergoes a class switch
2. Activated B cell selects a single class of Ig (eg smIgG)
3. Transforms to plasma cell
4. Produces antibodies (all have the same specificity)

Question 8

How do heavy chain genes result in expression of IgM & IgD on B cells?

Answer 8

1. Antibodies consist of 2 heavy chains & 2 light chains
2. There are lots of Variable region genes, Diversity region genes & Joining region genes
3. There are 5 types of heavy chain constant region genes, which correspond to each class of Ig (𝛂, 𝛆, 𝛄, 𝛅, 𝛍)

Question 9

What is affinity maturation of the B cell response?

Answer 9

As the antibody response matures, single nucleotide substitutions (‘point mutations’) in Ig genes occur (‘somatic hypermutations’)
Selection of B cells with high affinity BCRs from those with low affinity BCR as the immune response matures & the antigen concentration decreases

Question 10

Why are monoclonal antibodies important?

Answer 10

Used in:
-research
-diagnostics
-therapeutically
-allergy diagnosis
-treatment for various conditions (eg psoriasis, rheumatoid, arthritis, T & B cell lymphomas, allergies))