What is the mechanism of type I hypersensitivity?

Type I hypersensitivity refers to the hypersensitivity that occurs within a few minutes after the sensitizer contacts the same antigen again. Its main characteristics are: ① rapid occurrence and rapid regression; ② Mainly mediated by specific IgE; (3) it usually causes physiological dysfunction of the body, and generally does not leave tissue damage; ④ There are obvious individual differences and genetic background.

According to the mechanism of type I hypersensitivity, the process can be divided into three stages: sensitization stage, excitation stage and effect stage.

Uniform sensitivity level

Refers to the first time that an allergen enters an allergic body and stimulates it to produce specific IgE antibodies. IgE binds to the IgE Fc receptor on the surface of FC segment mast cells and basophils, thus sensitizing them. Mast cells and basophils combined with IgE formed at this stage are called sensitized cells, and the body containing sensitized cells is in a sensitized state, which usually lasts for months, years or longer.

Second excitation stage

Refers to the stage when the same allergen re-enters the body, specifically combines with IgE on sensitized cells to thresh, release and synthesize active media.

Generally, only multivalent allergens combine with two or more IgE molecules on sensitized cells to crosslink the IgE receptor (Fc ε R 1) on the cell surface, and then cause a series of activation reactions in the cell, leading to cell degranulation, releasing reserve media such as histamine and kininogenase, and newly synthesizing some active media such as leukotrienes, prostaglandins and platelet activating factors.

Three-effect stage

Refers to the stage when the active medium combines with the corresponding receptor on the effector organ, causing local or systemic lesions.

Pathological changes caused by type ⅰ hypersensitivity can be divided into early reaction and late reaction. The early reaction occurs within a few seconds after exposure to allergens and can last for several hours, which is mainly caused by histamine. The delayed reaction usually occurs within 6~ 12 hours after contact with allergens and lasts for several days. It is mainly caused by LTs and PGD2, and the active medium released by PAF and eosinophils also plays a role.