Week 3 Practise Questions L7-8

Question 1

How do T cell receptors differ from B cell (antibody) receptors? How does this reflect their roles in the immune response?

Answer 1

T cells (at least αβ T cells) can only recognise processed antigen presented by MHC proteins, whereas B cells can recognise “free”, unprocessed (native) antigens. T cells must be able to recognise antigen that was originally internal e.g. from infected cells, whereas B cells must make antibodies that can bind to free pathogens.

Question 2

Why don’t T cell receptor genes undergo somatic mutation?

Answer 2

1. May be too risky (mutation may generate receptors that recognise “self”)-fine for B cells as they are controlled by T cells anyway most of the time
2. May lose the ability to recognise self-MHC
3. Don’t really need to be high affinity (unlike antibodies) as only recognise cell-associated antigen.

Question 3

What are the main differences between MHCI and MHCII?

Answer 3

MHCI presents endogenous antigen to cytotoxic CD8 T cells, MHCII presents exogenous antigen to helper CD4 T cells.

Question 4

Why are the MHC genes so polymorphic?

Answer 4

Polymorphism in the population allows the binding of a greater range of peptides from pathogens. Increases likelihood of some individuals in the population being able to respond.

Question 5

Why is IgM useful in the primary adaptive response?

Answer 5

High valency means it’s good at cross-linking antigen (hinders pathogen movement, may help prevent adhesion to host cells), efficient activator of complement (induces inflammation, recruits leukocytes, mediates opsonisation, direct killing of bacterial cells).

Question 6

Give examples of two consequences of IgG binding to Fc receptors on leukocytes.

Answer 6

Opsonisation, “frustrated” phagocytosis, ADCC

Question 7

What is the main function of secretory IgA?

Answer 7

Appears to act mainly to prevent pathogens/foreign material adhering to host cells without inducing inflammation.

Question 8

What are the differences between thymus-dependent and -independent antigens?

Answer 8

Thymus-dependent antigens are typically proteins that require T cell help for signalling to B cells (must be processed and presented to CD4 cells via MHCII), whereas thymus-independent antigens typically have repeated epitopes that can cross-link B cell receptors to induce signalling independently of T cells.

Question 9

Describe in 4 steps how thymus independent activation of B cells can occur and an example of a TI (thymus independent antigen) which does this

Answer 9

1. B Cell Receptor (BCR): B cells express a membrane-bound form of immunoglobulin known as the B cell receptor (BCR), which is specific to a particular antigen. Each BCR has two antigen-binding sites.
2. Cross-Linking: Cross-linking occurs when multiple BCRs on the surface of a B cell bind to multiple epitopes on a single antigen or multiple antigens that are physically linked. This causes the BCRs to cluster together, or “cross-link.”
3. Thymus-Independent Antigens: TI antigens can activate B cells without T cell help. They are typically characterized by repetitive epitope structures that can effectively cross-link the BCRs on B cells. Because of these repetitive structures, a single TI antigen can bind to and cross-link multiple BCRs.
4. BCR Signaling: Cross-linking of BCRs is a critical first step in B cell activation. It triggers signaling cascades within the B cell that lead to B cell activation, proliferation, and differentiation. For T cell-dependent (TD) antigens, this signal is typically not enough, and additional help from T cells (in the form of CD40 ligand and cytokines) is needed to fully activate the B cell.

> Examples of TI Antigens: Common examples of TI antigens include polysaccharides on bacterial capsules and lipopolysaccharides on the surface of bacteria. These antigens can induce the B cell to produce antibodies without the involvement of helper T cells, hence the term “thymus-independent.”