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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