Lymphocytes are a type of white blood cell that play an important role in the body’s immune system. There are two main types of lymphocytes: T cells and B cells.
T cells are responsible for directly killing infected cells, producing chemicals that help destroy viruses and bacteria, and helping to activate other immune cells. B cells produce antibodies, which recognize and attach to foreign particles in the body, such as bacteria and viruses, marking them for destruction.
The majority of lymphocytes circulate in the bloodstream, but some lymphocytes reside in lymph nodes, spleen, tonsils, thymus gland, bone marrow or gut-associated lymphoid tissue (GALT). Lymphocytes originate from hematopoietic stem cells in the bone marrow.
T-lymphocytes develop from precursors in the thymus gland located behind the breastbone above the heart. The thymus is where T-cells mature and “learn” to become either CD4+ helper T-cells or CD8+ cytotoxic T-cells. Mature T-cells leave the thymus and enter circulation through blood vessels or efferent lymphatic vessels.
Helper T-cells (CD4+) provide signals that regulate or amplify both innate and adaptive immune responses by secreting cytokines (small signaling proteins). Cytotoxic T-cells (CD8+) directly kill virally infected host cells or cancerous/transformed cells by releasing toxic granules containing enzymes that cause cell death. Regulatory T-cells (Tregs) modulate immune responses to prevent autoimmunity while maintaining immunity against pathogens; they can also suppress tumor growth. Memory T-cells remain after an initial infection has resolved; if reexposed to the same pathogen they can quickly mount an enhanced protective response resulting in faster clearance of infection with less tissue damage .