Histidine decarboxylase is present in

Histamine is formed from histidine by decarboxylation, catalyzed by histidine decarboxylase. The effects of histamine are summarised in Table 16.3. Smooth muscle contraction, enhanced vascular permeability, increased acid secretion are the impoant actions. So histamine causes fall in blood pressure. The major cells producing histamine are platelets, mast cells, and basophils. Ceain antigens such as penicillin will elicit IgE antibodies that are fixed on the mast cells. When the next dose of penicillin is injected, it reacts with the antibodies; and degranulation of mast cells takes place. Histamine and slow reacting substance (SRS) are released. This leads to peripheral vasodilatation, fall in blood pressure and anaphylaxis.