Cell-mediated immunity is often impaired in cancer. This may be partly due to increased amounts of prostaglandin E2 (PGE2) and histamine in the blood of cancer patients, since PGE2 and histamine possess inhibitory effects on cellular immunity. These effects are mediated by cyclic AMP (cAMP), which is increased in leukocytes by PGE2 through EP2 and by histamine through H2 receptors and also by epinephrine through beta 2-adrenergic receptors. Increased cAMP activates protein kinase A, which inhibits the formation of interleukin 2 (IL-2) in T cells. The formation of interferon gamma is concomitantly decreased, and cellular immunity is attenuated. In monocyte/macrophages the formation of IL-1 beta, IL-12 and tumor necrosis factor alpha is decreased by cAMP or through the increased formation of IL-10, which is up-regulated by cAMP. This attenuates cellular immunity. In monocytes histamine may decrease the formation of oxygen intermediates, which can induce apoptosis of natural killer cells and thus inhibit immunity. The superoxide anion is a potent inducer of the cyclooxygenase-2 enzyme, which is upregulated in colorectal cancer. Cyclooxygenase-2 catalyzes the formation of PGE2, e.g. in cancer cells. Thus the inhibition of cellular immunity in cancer may be at least partly mediated by cAMP and oxygen intermediates. This may offer new options for cancer immunotherapy.