Lymphocyte

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.

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

B-lymphocyte

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.

T-lymphocyte

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 I (Il-I).

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

• Interleukin-2 (Il2)
• Interleukin-3 (Il3)
• Tumor necrosis factor-alpha (TNFa)
• B-cell growth factor (BCGF)
• B-cell differentiation factor (BCDF)

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).