Immunology 5 - B lymphocytes Flashcards
What is an epitope?
The site on an antigen where an antibody binds
Where do B cells originate from?
Derived from stem cells in the bone marrow
How are mature B cells specific?
They have different B cell receptors
Describe the process of clonal selection
- Each lymphocyte bears a single, unique receptor
- Interaction between a foreign molecule and that receptor leads to activation
- Differentiated effector cells of that lineage will bear the same receptor
- Self specific receptors are deleted early in development
Describe the structure and distribution of the B Cell Receptors.
- Consists of Iga and Igb subunits.
- mIg is present - tail is too sort to signal
- Iga and Igb tails are long enough to interact with intracellular signalling molecules
- Is present in thousands of identical copies on the surface of the B lymphocyte
How is antibody receptor diversity generated?
- Each BCR receptor chain) is encoded by separate multigene families on different chromosomes, which are rearranged and bought together.
- They are rearranged by splicing.
How are segments joined in generation of antibodies?
During joining of gene segments unused DNA is looped out and removed by Rag/recombinase genes. Lack of Rag1 / Rag2 results in scid
Describe the stages of B cell development
Pro B cells (early to late) -> Pre B cells (large to small) -> naive B cells (immature to mature)
How are naive B cells activated?
- Naive B cells cannot be activated by binding to the antigen alone.
- An accessory signal is required (either directly from microbial constituents or from a T helper cell)
What is the difference between the two pathways of antibody production of B cells?
- T helper cells (thymus dependent) involves all Ig classes and has memory
- Microbial constituents (thymus independent) only uses IgM and has no memory
How does thymus independent activation of B cells occur?
- B cell receptors bind to bacterial components/polysaccharide as it must be a repetitive structure
- Second signal is provided by the microbial constituent or an accessory cell.
How does thymus dependent activation of B cells occur?
- The membrane bound BCR recognises antigen
- The receptor-bound antigen is internalised and degraded into peptides
- Peptides associate with self molecules and are expressed at the cell surface
- This complex is recognised by matched CD4 T helper cell
- B cell activated to express cytokine receptors, which T cell derived cytokines bind to causing the B cells to proliferate and differentiate
What is the consequence of thymus dependent activation of B cells?
- T helper cells secrete lymphokines to direct the immune system
- B cells form plasma cells (secrete soluble BCRs/antibodies) and memory cells
What is the Ig class switch?
- T cells release cytokines which cause the DNA of the B cell to change, and this results in production of the heavy chain
- This changes the class of the antibody - IgM/IgD to IgG/IgA/IgE
How does immunological memory form in relation to B cellsand wat is the purpose?
- Occurs following clonal selection/once the immune system has recognised and responded to an antigen
- Antigen-specific lymphocytes (B + T) are the cellular basis
- Memory responses are characterised by a more rapid and heightened immune reaction that serves to eliminate pathogens fast and prevent diseases.
