Mechanism and Types of Allergic Reactions

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Chapter: Essential pharmacology : Adverse Drug Effects

Type I (anaphylactic) reactions Reaginic antibodies (IgE) are produced which get fixed to the mast cells. On exposure to the drug, AG: AB reaction takes place on the mast cell surface .....


MECHANISM AND TYPES OF ALLERGIC REACTIONS

 

A. Humoral

 

Type I (anaphylactic) reactions Reaginic antibodies (IgE) are produced which get fixed to the mast cells. On exposure to the drug, AG: AB reaction takes place on the mast cell surface (see Fig. 11.2) releasing mediators like histamine, 5HT, leukotrienes especially LTC4 and D4, prostaglandins, PAF, etc. resulting in urticaria, itching, angioedema, bronchospasm, rhinitis or anaphylactic shock. The manifestations occur quickly after challenge and are called immediate hypersensitivity. Antihistaminic drugs are beneficial in some of these reactions.

 

Type II (cytolytic) reactions Drug + component of a specific tissue cell act as AG. The resulting antibodies (IgG, IgM) bind to the target cells; on reexposure AG: AB reaction takes place on the surface of these cells, complement is activated and cytolysis occurs, e.g. thrombocytopenia, agranulocytosis, aplastic anaemia, haemolysis, organ damage (liver, kidney, muscle), systemic lupus erythematosus.

 

Type III (retarded, Arthus) reactions These are mediated by circulating antibodies (predominantly IgG, mopping AB). AG: AB complexes bind complement and precipitate on vascular endothelium giving rise to a destructive inflammatory response. Manifestations are rashes, serum sickness (fever, arthralgia, lymphadenopathy), polyarteritis nodosa, Stevens Johnson syndrome (erythema multiforme, arthritis, nephritis, myocarditis, mental symptoms). The reaction usually subsides in 1–2 weeks.

 

B. Cell mediated

 

TypeIV (delayed hypersensitivity) reactions These are mediated through production of sensi tized Tlymphocytes carrying receptors for the AG. On contact with the AG these T cells produce lymphokines which attract granulocytes and generate an inflammatory response, e.g. contact dermatitis, some rashes, fever, photosensitization. The reaction generally takes > 12 hours to develop.

 

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