Bcells

B cells, or B lymphocytes, are a type of white blood cell that mediates humoral immunity as part of the adaptive immune system. They originate in the bone marrow, where they rearrange immunoglobulin genes to form B cell receptors (BCRs) and then mature into naive B cells that express membrane-bound immunoglobulins as their antigen receptors.

BCR engagement by antigen, along with signals from helper T cells, activates B cells. Most responses occur in germinal centers within lymphoid tissue, where B cells undergo somatic hypermutation and class switch recombination, increasing antibody affinity and changing isotypes. Some responses are extrafollicular and faster.

Activated B cells differentiate into antibody-secreting plasma cells and memory B cells. Plasma cells produce antibodies that neutralize or mark pathogens; memory B cells persist for years and respond rapidly upon re-exposure.

B cells also present antigen to T cells via MHC class II and secrete cytokines that shape the immune response. Naive B cells initially express IgM and IgD on their surface; upon activation they can switch from IgM to other isotypes (IgG, IgA, IgE) while retaining antigen specificity.

B cell subtypes include conventional B2 cells, B1 cells found in body cavities, and marginal zone B cells that respond to blood-borne pathogens. Development in the bone marrow proceeds from pro-B to pre-B to immature B cells, which then exit to populate peripheral lymphoid organs.

Dysfunction or depletion of B cells underlies humoral immunodeficiencies and certain cancers. Treatments such as anti-CD20 antibodies (for example rituximab) target B cells. Vaccines rely on B cell responses to generate protective antibodies and memory.