Linfòcits

Linfòcits are a type of white blood cell that play a central role in the adaptive immune system. They circulate in the blood and are abundant in lymphoid tissues such as lymph nodes, the spleen, and mucosal-associated lymphoid tissue. The main linfòcits are B cells, T cells, and natural killer (NK) cells, each with distinct functions in recognizing and responding to pathogens.

Linfòcits B are responsible for humoral immunity. They produce antibodies after activation, and some differentiate into plasma cells that secrete antibodies while others become memory B cells that confer lasting protection. B cells express B cell receptors (BCRs) that bind specific antigens.

Linfòcits T mediate cellular immunity. They include helper T cells (CD4+), which coordinate immune responses by signaling other cells, and cytotoxic T cells (CD8+), which destroy infected or abnormal cells. Regulatory T cells help maintain tolerance and prevent autoimmunity. T cells recognize peptide antigens presented by major histocompatibility complex (MHC) molecules on other cells and undergo differentiation and clonal expansion upon activation. Thymic maturation is essential for T cell development.

Linfòcits NK are part of the innate immune system and provide rapid responses against virally infected or transformed cells. They kill targets without prior sensitization and contribute to early defense, often by recognizing changes in surface molecules, including reduced MHC class I expression.

Developmentally, linfòcits arise from hematopoietic stem cells in the bone marrow; T cells migrate to the thymus to mature, B cells mature in the bone marrow, and NK cells originate from common lymphoid progenitors. Normal counts vary, but lymphocytes typically comprise about 20–40% of white blood cells in adults. Abnormal levels can indicate infection, immune disorders, or hematologic diseases.