Question

What is the difference between primary and secondary pharmacological mediators in the type I hypersensitivity response? Name two of each.

Answer 1

Type 1 hypersensitivity response is an immediate type of hypersensitivity reaction which occurs within seconds to minutes after the exposure and re-exposure to specific allergens. Type 1 hypersensitivity reaction is mediated by IgE immunoglobulin and mast cells and its mediators. Examples of type I hypersensitivity response are :- anaphylaxis reaction, hay fever, urticaria, angioedema, allergic asthma, allergy to some specific food items.

Mediators of type I hypersensitivity response are of two types namely primary mediators and secondary mediators.

Primary cellular mediators are those molecules which are performed and present in the granules of mast cells and basophils. These are histamine, serotonin, lysosomal enzymes, neuropeptides like substance P and neurokinin A.

Histamine is the main primary mediator and it causes vasodilation, increased vascular permeability, smooth muscles contraction (bronchoconstriction).  

Secondary mediators are newly synthesized mediators. These are not present inside the granules within the mast cells and basophils but are later synthesized during the course of hypersensitivity reaction. Examples are nitric oxide, cytokines, chemokines, platelet activating factor, arachidonic acid metabolites like prostaglandins and leukotrienes.

Nitric oxide causes vasodilation, relaxation of smooth muscles, inhibits platelet aggregation.