Antihistamines block the effects of histamine on smooth muscle (blood vessels, gut, bronchi). The stimulant effect of histamine on gastric secretion is not blocked by antihistamines.
Antihistamines inhibit the effects of histamine which is released by liberating substances (compound 48/80, morphine, D-tubocurarine and others), which act by releasing histamine from mast cell granules. Non mast-cell histamine is not released by these liberators.
Antihistamines inhibit the development of anaphylactic shock, which is caused by release of histamine by antigen-antibody reactions. Exception: The histamine release caused by antigen-antibody reactions of fixed antibodies is influenced to a lesser degree.