Fig. 1. Histamine bound to carbonic anhydrase II
Permission to use requested (See Protein Data Bank)
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. Thus, blood fluids (including leukocytes, which participate in immune responses) enter the area causing swelling. This is accomplished due to histamine’s ability to induce phosphorylation of an intercellular adhesion protein (called (VE)-cadherin) found on vascular endothelial cells (Andriopoulou et al 1999). 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. It has been suggested by Marone et al that histamine release triggers the release of cytokines and inflammatory mediator by some neighboring leukocytes (1999). These chemicals in turn increases the inflammatory response.
Histamine's second type of allergic response is one of the major causes for asthma. In response to an allergen (a substance that triggers an allergic reaction), histamine, along with other chemicals, causes the contraction of smooth muscle (Schmidt et al 1999). Consequently, the muscles surrounding the airways constrict causing shortness of breath and possibly complete trachial-closure, an obviously life-threatening condition. If the effects of histamine during an allergic reaction are inhibited, the life of an allergic person can be eased (in the case of inflammation) or even saved by preventing or shortening asthma attacks. Thankfully, many effective drugs have been developed to hinder histamine's allergic response activities.
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