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Hypersensitivity or Allergic Reactions of Drug

In this condition, the abnormal reaction occurs usually from the second or subsequent doses. The first dose (antigen) generally stimulates the body to produce some reactive substance (antibody), which sensitizes the tissues of the body. When a second dose of that drug is given the reaction (antigen—antibody reaction) occurs in the tissue called allergic reaction.

The reactions may be immediate (anaphylactic) or a delayed one (hypersensitivity). The anaphylactic reactions are—fall of blood pressure, shock, collapse, bronchospasm, cyanosis and even death. The hypersensitivity reactions are—urticaria, skin rash, asthma, drug fever, dermatitis, hepatitis, hay fever etc.

Antihistaminics, Adrenaline and cortisone preparations are used to treat these reactions.

Most drugs are of relatively low molecular weight and only become antige-nic when they are combined covalently and irreversibly with other substances of high molecular weight, usually proteins. Allergic reactions to drugs are the result if interaction of drug or metabolite or a non-drug element in the formulation with the patient, and subsequent re-exposure. The chief target organs of hypersensitivity reactions are the skin, respiratory tract, gastrointestinal tract, blood and blood vessels.

Hypersensitivity reactions may be of four types,any of which can be produced by drugs:

Type I (immediate type)- These reactions may occur within minutes of re-exposure to the drug. The drug causes formation of tissue-sensitising IgE antibodies resulting in the release of pharmacologically active substances, e.g. hista-mine, leukotrienes.

The released substances cause rashes, angioneurotic edema, bronchospasm, peripheral vasodilatation and cardiovascular collapse leading to the anaphylactic shock. They are most frequently produced by penicillin.

Type II reactions- The drug forms an antigenic complex with the surface of the cell and combination with antibody activate complement system causing cell destruction. Many allergic blood dyscrasias e.g. thrombocytopenic purpura, agranulocytosis or aplastic anemia are-due to type II reactions and may be caused by quinine, methyldopa or chlo-ramphenicol.

Type III reactions- These reactions are due to circulation antigen antibody complexes which are deposited in the walls of small blood vessels. These reactions include serum sickness, glomerulonephritis, and vacuities.

Type IV reactions (delayed or cell-mediated allergy)- Antigen-specific receptors develop on T-lymphocytes. Subsequent administration leads to local or tissue allergy. Contact dermatitis is the typical example and is due to drugs like penicillin, sulfonamides, local anesthetics, etc.