Lymphocytes are small cells with a high N:C ratio; some (e.g. natural killer cells) have prominent cytoplasmic granules. Two principal types: B and T lymphocyte. B-cells express monoclonal surface (not cytoplasmic) IgM and often IgD. B-cell stimulation through cross linkage of surface Ig molecules or via effector T cells causes their differentiation into plasma cells. Predominant role is humoral immunity via Ig secretion.

T cells are derived from stem cells that undergo maturation in thymus and express T-cell receptor molecule (CD3) on cell surface. Responsible for cell-mediated immunity e.g. delayed hypersensitivity, graft rejection, contact allergy and cytotoxic reactions against other cells.

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