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A lymphocyte is a type of leukocyte (i.e., white blood cell), which are a major component of the immune system. The group broadly breaks into B-lymphocytes and T-lymphocytes. [1]

The lymphocyte count is "the number of lymphocytes per unit volume of blood."[2]


These are cells involved in the production of antibiodies (i.e., the humoral immune response. They have no relationships to the B- or Beta-cells of the pancreas.


Lymphocytes of these types directly destroy appropriate target cells, or help generate cells that do.

T4 Helper Cells

T-lymphocytes with the T4 protein on their surface which recognizes the antigenic peptide while the CD4 molecule recognizes the major histocompatibility complex (MHC-II) molecule. These "helper T-lymphocytes" cause the production of more cells for cell-mediated immunity, but they first must be activated by cytokines, such as interleukin-1 (Il-I).[3]

As well as being invoked by cytokines, they generate cytokines:

T8 Killer cells

CD8-protein containing lymphocytes, also called T8-lymphocytes, are a subset of circulating "killer cells". All CD8-cells are killers, but other killer cells may be monocytes, macrophages (derived from monocytes) or polynuclear neutrophils. The key is that a killer cell attacks material labeled with the B-lymphocyte generated antibody.

These cytotoxic lymphocytes may be generated in vitro in mixed lymphocyte cultures (MLC), in vivo during a graft-versus-host (GVH) reaction, or after immunization with an transplantation#allograft, tumor cell or virally transformed or chemically modified target cell. The manner in which these cells destroy targets is sometimes called cell-mediated lympholysis (CML).


  1. Anonymous (2023), Lymphocyte (English). Medical Subject Headings. U.S. National Library of Medicine.
  2. Anonymous (2023), Lymphocyte count (English). Medical Subject Headings. U.S. National Library of Medicine.
  3. Kaiser, Gary E., T4 -Lymphocytes (T4-Cells; T4-Helper Cells; CD4+ Cells), "The adaptive immune system: I. Introduction, B. Major cells and key cell-surface molecules involved in adaptive immune responses", Doc Kaiser's Microbiology Home Page