Paper I
Question
B-lymphocytes
Answer
B lymphocytes (B cells) are the cells of the adaptive immune system responsible for humoral (antibody-mediated) immunity, deriving from bone-marrow lymphoid progenitors and maturing entirely within the bone marrow.
Key features:
- Each B cell expresses a unique surface immunoglobulin (membrane-bound IgM/IgD) acting as its antigen receptor (BCR), conferring specificity for one antigenic epitope.
- On encountering specific antigen (with T-helper-cell cooperation for most protein antigens), a B cell undergoes clonal proliferation and differentiates into:
- Plasma cells — terminally differentiated, antibody-secreting cells.
- Memory B cells — long-lived cells that mount a faster, stronger response on re-exposure to the same antigen.
- Undergo class switching, allowing the same antigen specificity to be expressed as IgM, IgG, IgA, or IgE, and somatic hypermutation/affinity maturation in germinal centres, improving antibody affinity over the course of a response.
- Surface markers used to identify B cells: CD19, CD20, CD21, and surface immunoglobulin.
- Also function as antigen-presenting cells, internalizing antigen bound by their BCR and presenting peptides on MHC class II to helper T cells.
- Make up roughly 10–20% of circulating lymphocytes.