Histamine suppression of lymphocyte activation.

Histamine suppresses lymphocyte transformation induced by the mitogen Con A. The effect is dose-related and is not blocked by H1 or H2 antagonists. High dose of H2 antagonists (10(-3) M) enhances the suppression induced by histamine. The proliferation of T cells alone cultured with histamine is suppressed indicating that histamine can exert its effect directly on T cell proliferation without the requirement for accessory cells. Atopic subjects are more sensitive to histamine-induced suppression than normals. The presence of histamine produced locally during a Type I response may provide a mechanism for preventing the proliferation of sensitized T cells which enter the site of antigen stimulation. In the atopic, who already has the immunological machinery to mount cell-mediated immune responses to pollens, this increased histamine sensitivity may be protective in suppressing unwanted delayed hypersensitivity reactions.