Lecture 4- Part 2 Adaptive Immunity

Question 1

Name the two ways B cell is activated?

Answer 1

Thymus (T) dependent and thymus (T) independent antigen

Question 2

describe the properties of T dependent antigens?

Answer 2

Dependent upon helper T cells to induce antibody production

e.g. Proteins

Question 3

Describe the properties of thymus (T) independent antigens?

Answer 3

• Does not need helper T cells to induce antibody production
• REPEATING EPITOPES

e.g. polysaccharides
Lipids

Question 4

Describe the T independent antigen response?

Answer 4

Simple and repetitive antigens
Mostly IgM- no class switching
Modest affinity
No memory
B cells are activated by direct BCR crosslinking
B cells still require a second activation signal - often via Toll like receptor engagement

Question 5

Describe the constituents of B cell receptor?

Answer 5

BCR consists of 2HC and 2 LC (membrane and secreted Ig)

Question 6

Describe the constituents of T cell receptors?

Answer 6

TCR consists of alpha and beta hetero-dimer (membrane form only)

Question 7

How do both B cell and T cell signal?

Answer 7

By associating with SIGNALLING COMPLEX in membrane: Ig alpha and Ig beta for B cells
CD3 complex for T cells

Question 8

Compare how B cell and T cell bind to an antigen?

Answer 8

B cell bind to intact protein antigen in solution

T cell bind peptides displayed on the surface of another cell-an antigen-presenting cell (macrophages, dendritic cell or B cell)
MHC I or MHC II molecules

Question 9

Describe the stages of T-dependent B cell response?

Answer 9

1. Antigen binding to BCR provides signal 1 to B cell
2. The antigen is INTERNALISED, PROCESSED and antigenic peptides are DISPLAYED on MHC for T cell recognition
3. T helper recognises antigen-MHC complex via the T cell antigen receptor (TCR) provides signal 1 to T cell
4. CD80/CD86 on B cell binding to CD28 on T cell provides signal 2 to T cell
5. T cell activation leads to up-regulation of CD40L which binds to CD40 providing signal 2 to B cell
6. Cytokine (IL-4) production by activated T cell also help to activate B cell- Signal 3

B cell PROLIFERATES and DIFFERENTIATES into antibody-secreting B cell (plasma cell) or becomes a memory B cell

Question 10

Describe the process of T independent activation of B cell?

Answer 10

• T-independent antigens usually contain REPEATING epitopes classically polysaccharides, glycolipids and nucleic acid
• cross-linking of B cell receptor to an antigen is the first signal for activation
• Second signal is from TLR engagement

-Once activated: Produces IgM