Paper I
Question
B cells.
Answer
B lymphocytes (B cells) are the cells of the adaptive immune system responsible for humoral (antibody-mediated) immunity. They derive from bone-marrow lymphoid progenitors and, unlike T cells, mature entirely within the bone marrow (the “B” originally referring to the bursa of Fabricius in birds, where they were first characterized).
Key features:
- Each B cell expresses a unique surface immunoglobulin (membrane-bound IgM/IgD) that acts 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 (do not express surface Ig; secrete large amounts of specific antibody).
- Memory B cells — long-lived cells that mount a faster, stronger response on re-exposure to the same antigen (basis of immunological memory and vaccination).
- Undergo class switching (isotype switching), allowing the same antigen specificity to be expressed as IgM, IgG, IgA, or IgE depending on the cytokine milieu, and somatic hypermutation/affinity maturation in germinal centres to improve antibody affinity.
- Surface markers used to identify B cells include CD19, CD20, CD21, and surface immunoglobulin.
- B cells also function as antigen-presenting cells, internalizing antigen bound by their BCR and presenting peptides on MHC class II to helper T cells.