lymphocyte

Lymphocytes are a type of white blood cell that are central to the adaptive immune system. They originate from hematopoietic stem cells in the bone marrow and differentiate into B cells, T cells, and natural killer (NK) cells. B cells mature in the bone marrow and express surface immunoglobulins as their antigen receptors. T cells mature in the thymus and express T cell receptors; major subsets include helper CD4+ T cells, which coordinate immune responses, and cytotoxic CD8+ T cells, which destroy infected or malignant cells. Regulatory T cells help maintain immune tolerance and control.

NK cells arise from lymphoid progenitors and provide rapid cytotoxic responses as part of innate immunity. They target stressed or transformed cells and can kill without prior antigen exposure, using perforin and granzymes and by modulating other immune cells.

Lymphocytes circulate in blood and lymph and reside in secondary lymphoid organs such as lymph nodes and the spleen, as well as in mucosa-associated lymphoid tissue. The diversity of lymphocyte antigen receptors results from gene rearrangements during development (V(D)J recombination). Upon antigen recognition, B and T cells undergo clonal expansion and differentiate into effector and memory cells. B cells produce antibodies via plasma cells; T cells mediate cellular immunity and help B cells through cytokine signaling.

Absolute lymphocyte counts vary with age and immune status. Lymphocytes are typically smaller than other white cells, with a large nucleus and relatively little cytoplasm. Abnormal lymphocyte levels can indicate infections, immune deficiencies, or hematologic disorders.