Bcells

B cells originate from hematopoietic stem cells in the bone marrow, where they undergo V(D)J recombination of immunoglobulin genes to generate a diverse B-cell receptor (BCR) repertoire. Central tolerance eliminates strongly self-reactive clones (negative selection) in the marrow. Mature naive B cells (IgM+/IgD+) circulate and reside in lymphoid follicles of secondary lymphoid organs (spleen, lymph nodes, MALT).

Activation occurs through two main pathways: 1. T-cell dependent activation: Naive B cells bind antigen via BCR, internalize and process it, then present peptide on MHC II to helper T cells (Th2 predominantly). CD40L on T cells binds CD40 on B cells, along with cytokine signals (IL-4, IL-5, IL-6), driving germinal center formation. Within germinal centers, B cells undergo somatic hypermutation (affinity maturation) and class-switch recombination (isotype switching from IgM to IgG, IgA, or IgE) to enhance and diversify antibody response, differentiating into plasma cells and memory B cells. 2. T-cell independent activation: Occurs with polysaccharide/lipopolysaccharide antigens that cross-link multiple BCRs (Type 2 TI antigens) without T-cell help, producing lower affinity, mainly IgM antibody without memory formation (important in defense against encapsulated bacteria; explains poor response in young children whose T-independent pathways are immature, and why conjugate vaccines link polysaccharides to protein carriers).

Surface markers: CD19, CD20 (target of rituximab), CD21 (EBV receptor, also complement receptor CR2), MHC II, and surface Ig (BCR).

Clinical correlations: - X-linked agammaglobulinemia (Bruton's): BTK gene mutation, absent B cells, recurrent bacterial infections after maternal antibody wanes. - Common variable immunodeficiency: impaired differentiation into plasma cells, low immunoglobulins. - Hyper-IgM syndrome: CD40L deficiency, impaired class switching, elevated IgM with low IgG/IgA/IgE. - Selective IgA deficiency: most common primary immunodeficiency, risk of anaphylaxis with blood transfusion (anti-IgA antibodies). - Multiple myeloma and lymphomas arise from malignant transformation of B-cell lineage. - Rituximab (anti-CD20) used in B-cell lymphomas, rheumatoid arthritis, and certain autoimmune diseases by depleting B cells.

Understanding B-cell biology is essential for vaccine design (conjugate vs. polysaccharide vaccines), immunodeficiency diagnosis, and targeted immunotherapies.