Histamine is an important protein involved in many allergic reactions. In many cases, the lymphocyte triggered to respond is a mast cell. Antigen binding to the mast cell-attached IgE then triggers the mast cell to respond. It contributes to an inflammatory response. It causes constriction of smooth muscle.
Histamine can cause inflammation directly as well as indirectly.
Upon release of histamine by an antigen activated mast cell, permeability of vessels near the site is increased. That is why histamine is known as being vasoactive. Gaps between the cells in vascular tissue are created by this phosphorylation, allowing blood fluids to seep out into extracellular space. Indirectly, histamine contributes to inflammation by affecting the functions of other leukocytes in the area. These chemicals in turn increases the inflammatory response.