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(cluster of differentiation) referring to cell surface molecules that are used to identify stages of maturity of immune cells, for example, CD4+ T cells.

CD8+ T lymphocyte

immune cell that carries the "cluster of differentiation 8" (CD8) marker. CD8 T cells may be cytotoxic T lymphocytes or suppressor T cells.

cell-mediated immunity (cellular immunity)

the immune response coordinated by helper T cells and CTLs. This branch of the immune system targets cells infected with microorganisms such as viruses, fungi, and certain bacteria.


in vaccine experiments, the deliberate exposure of an immunized animal or person to the infectious agent.

CTL (cytotoxic T lymphocyte)

immune system cell that can destroy cancer cells and cells infected with viruses, fungi, or certain bacteria. CTLs, also known as killer T cells, carry the CD8 marker. CTLs kill infected cells, whereas antibodies generally target free-floating organisms in the blood.


the living matter within a cell (excluding the nucleus) that is responsible for the function of the cell (for example, protein synthesis).


degree to which a substance is poisonous to cells.