Bcellmediated

B cell–mediated immunity, commonly known as humoral immunity, is a central component of the adaptive immune system. It is driven by B lymphocytes and the antibodies they secrete, providing targeted defense against extracellular pathogens, toxins, and viruses in the bloodstream and tissues.

Activation and differentiation: Naive B cells recognize antigens through their B cell receptor (BCR). Antigen binding, often with help from CD4+ T follicular helper cells, delivers signals that drive activation, clonal expansion, and differentiation. In germinal centers of secondary lymphoid organs, B cells undergo somatic hypermutation and class switch recombination, increasing antibody affinity and changing isotypes (for example, from IgM to IgG, IgA, or IgE). The end products are plasma cells that secrete large quantities of antibodies and memory B cells that confer long-term protection.

Functions of antibodies: antibodies neutralize pathogens and toxins, block viral entry, promote opsonization to enhance phagocytosis by macrophages, and activate the classical complement pathway. In addition to antibody production, B cells can act as antigen-presenting cells, presenting processed antigen on MHC II to CD4+ T cells, reinforcing the adaptive response.

Development and clinical relevance: B cells mature in the bone marrow and traffic to lymphoid tissues. Vaccination aims to stimulate B cell responses and establish memory. B cell deficiencies or dysfunctions can lead to recurrent infections or immunodeficiency, while autoreactive B cells contribute to autoimmune diseases. B cell–targeted therapies (e.g., monoclonal antibodies such as rituximab) are used to treat certain autoimmune conditions and B cell malignancies.