Lymphocytes are class of immune cells that specifically recognize and respond to foreign antigens. They are mediators of humoral and cellular immunity & unique cells of adaptive immunity. They can be broadly divided into three major populations on the basis of functional and phenotypic differences: B Cells, T Cells and Natural Killer Cells.
B cells/ B lymphocytes, the cells that produce antibodies, were so called because in birds they were found to mature in an organ called the Bursa of Fabricus. In mammals, no anatomic equivalent of bursa exists, and early stages of B cell maturation occur in bone marrow. Thus, B lymphocytes now refer to bone marrow–derived lymphocytes.
B lymphocytes, are a type of white blood cell which functions in humoral immune response of adaptive immune system by secreting antibodies. They are produced in bone marrow, then migrate to spleen and other secondary lymphoid tissues where they mature and differentiate into immunocompetent B cells. These cells mediates production of antigen-specific immunoglobulin (Ig) directed against invasive pathogens (typically known as antibodies), which they bind via B cell receptors (BCR).
B cells are found in germinal centers of lymph nodes, in white pulp of the spleen, and in the MALT (Mucous associated lymphoid tissue).
Effector functions of B cells
B cells perform two important functions:
- They differentiate into plasma cells and produce antibodies.
- They can present antigen to helper T cells (Act as Antigen Presenting Cells). They are professional antigen-presenting cells (APCs).
Additionally to these major ones, B cells also secrete cytokines.
Production of numerous plasma cells is end result of B cell activation. The plasma cells in turn produce large amounts of immunoglobulins specific for the epitope of an antigens. Some activated B cells also produce memory cells, which remain in a stage of quiescence for months or years. Most memory B cells have surface IgG that acts as the antigen receptor, but some even have surface IgM. These quiescent memory cells are activated rapidly on re-exposure to antigen. Memory T cells produce interleukins that facilitate antibody production by the memory B cells. The presence of these cells is responsible for the rapid appearance of antibody in the secondary immune responses.
Many B cells mature into plasma cells that produce antibodies (proteins) necessary to fight off infections while other B cells mature into memory B cells. All of plasma cells descended from a single B cell produce same antibody which is directed against the antigen that stimulated it to mature. The same principle holds with memory B cells. Memory cells remember/memorizes the stimulus or antigens that led to their formation.
Subsets of B cells
The major subsets of B cells are follicular B cells, marginal zone B cells, and B-1 cells, each of which is found in distinct anatomic locations within lymphoid tissues. Follicular B cells express highly diverse, clonally distributed sets of antibodies that serve as cell surface antigen receptors and as the key secreted effector molecules of adaptive humoral immunity. In contrast, B-1 and marginal-zone B cells produce antibodies with very limited diversity.
A special set of B cells are also able to dampen T-cell driven immune responses, giving rise to the concept of regulatory B cells (Breg). Interleukin (IL-10) secreting B cells with suppressive functions are referred to as B10 Bregs. They reduce disease severity in animal models e.g. during experimental autoimmune encephalomyelitis (EAE), secretion of IL-10 in mice has effect of countering T-cell-mediated autoimmune disease of central nervous system.