HISTAMINE AND ANTI-HISTAMINIC AGENTS

HISTAMINE AND ANTI-HISTAMINIC AGENTS

Various sensitising substances, derived from food or environment, cause human allergy and anaphylactic manifestations. These substances, called antigens, release histamine in the body which is b-imidazolylethylamine. An allergic reaction often includes inflammed nasal membranes (rhinitis), running nose, itching in the nose and eyes, coughing, spitting, severe shortness of breath and coma. Allergy is a form of immune reaction in which the individual becomes hypersensitive to a particular substance (an allergen) that may be touched, swallowed, or carried by air into the lungs. Common allergens include pollens, mold spores, household dust, animal dandruff and industrial emissions. Foods, drugs and some toxic plants are also common allergens. When allergens touch or enter the body, they trigger the production of a reactive substance (an antibody) that attaches itself to certain types of body cells: mast cells in the body and basophils in other tissues. This reaction prompts the cells to secret histamine, which acts as an irritant that causes itching, tear formation, and other allergic symptoms.

Histamine brings about numerous physiological effects which are mediated by two sets of receptors, called H-and H, receptors. The combined effects cause dilatation and increased permeability of capillaries so that fluid and plasma proteins escape to extracellular fluid and produce oedema, bronchiolar constriction, cardiac acceleration and stimulating action on the secretion of gastric acids.

Histamine causes a wide variety of physiological responses. It decreases blood pressure by dilating the capillaries and increases the heart rate. In lungs, histamine acts on smooth muscles producing bronchiolar constriction. In the gastrointestinal tract, histamine stimulates some excretory glands, causing excess secretion of gastric juice which may produce gastric ulcers. Histamine stimulates the lacrimal (tear) and secretory glands, and takes part in the rapid growth of tissues particularly after wounds.

Antihistamines are the agents which diminish or prevent several actions of histamine in the body. They do not prevent the formation of histamine, but act through competition in the tissues between the agent and histamine. It combines with the receptive substance first before histamine can produce its side effect on the tissue. The antihistamines are used for palliative treatment of allergic reactions such as hay fever, rhinitis, urticaria, mild asthma, nasal discharge, dysentery, irradiation sickness, post operative vomiting, nausea and vomiting of pregnancy. These drugs are generally administered orally or deep intramuscular or slow intravenous injection.

The antihistamines include histamine H-receptor antagonists (amino alkyl ethers, ethylenediamines, alkylamines, phenothiazines, piperazines), histamine H-receptor (cimetidine) and some miscellaneous agents (cyproheptadine, diphenylpyraline and phenindamine