[lymph- + -cyte]
ABBR: LY A white blood cell responsible for much of the body's immune protection. Less than 1% are present in the circulating blood; the rest lie in the lymph nodes, spleen, and other lymphoid organs, where they can maximize contact with foreign antigens.
SYN: SEE: lymphoidcell
SEE: B cell; SEE: T cell; SEE: blood for illus.; SEE: natural killer cell; SEE: plasma cell; SEE: cell-mediated immunity; SEE: humoral immunity
Lymphocytes vary from 5 to 12 μm in diameter; subpopulations can be identified by unique protein groups on the cell surface called clusters of differentiation. T cells, derived from the thymus, make up approx. 75% of all lymphocytes; B cells, derived from the bone marrow, 10%. A third classification is natural killer cells. In the blood, 20% to 40% of the white cells are lymphocytes.

activated lymphocyte

A lymphocyte stimulated by exposure to a specific antigen or by macrophage processing so that it is capable of responding to a foreign antigen by neutralizing or eliminating it.

autoreactive lymphocyte

A lymphocyte that reacts with autoantigens. Most of these cells are eliminated during lymphocyte maturation or by activation-induced cell death.
SYN: SEE: self-reactive lymphocyte

B lymphocyte

A lymphocyte formed from pluripotent stem cells in the bone marrow that migrates to the spleen, lymph nodes, and other peripheral lymphoid tissue where it comes in contact with foreign antigens and becomes a mature functioning cell. Mature B cells are able to independently identify foreign antigens and differentiate into antibody-producing plasma cells or memory cells; their activity also may be stimulated by IL-2 (formerly called B-cell growth factor). Plasma cells are the only source of immunoglobulins (antibodies). Memory cells enable the body to produce antibodies quickly when it is invaded by the same organism at a later date.
SYN: SEE: B cell
SEE: humoral immunity; SEE: immune response

reactive lymphocyte

A lymphocyte that has become enlarged as a result of stimulation by antigens. Such a lymphocyte can be over 30 µm in diameter and of varying sizes and shapes. Common causes include mononucleosis and other viral infections, pertussis and cat scratch disease, and some hematologic malignancies.

self-reactive lymphocyte

SEE: Autoreactive lymphocyte.

T lymphocyte

SEE: T cell.

tumor-infiltrating lymphocyte

A lymphocyte found in solid tumors, e.g., lung cancers, melanomas, and renal cell carcinomas. Tumor-infiltrating lymphocytes include helper T cells and cytotoxic T cells; they participate in tumor recognition and, in some cases, tumor destruction.