Hypersensitivity Reaction- Type 1
Type I hypersensitivity is an allergic reaction due to prolonged or repeated exposure to antigens that involves the release of antibodies against a soluble antigen. This reaction is also known as an immediate reaction. Hypersensitivity reactions are exaggerated or inappropriate responses of the immune system. They are mostly targeted at not harmful antigens but can lead to tissue damage called inflammation. The antibodies secreted in this reaction are IgE type and IgE- FcεRI complex of innate cells cross linked with antigen and transduce signal cascade mechanism and degranulate the granulocytes to release the vasoactive amines (eg: histamines). These mediators cause vascular endothelial junctions to loosen and lead to vasodilation (increase the size of blood vessels) and promote the accumulation of fluid in blood vessels called oedema (selling in tissues). This hypersensitivity reaction taking place in the branchial tissues lead to potential fatal desorder “Astma”. Histamines also induces muscle contraction in smooth muscles in arterial walls to make the narrowing called vasoconstriction (narrowing blood vessels) which results the blood vessels of the main central organs lose fluid into the peripheral organs.
The stages of Type I hypersensitivity:
- Sensitization stage: The person encounters the antigen but doesn’t experience any symptoms. This is the first attempt of infection.
- Effect stage: The person is exposed to the antigen again. This is the second attempt of infection.
The reaction involves the following steps:
- Immunoglobulin E (IgE) antibodies are released from the plasma cells against the soluble antigen.
- Innate immune cells (mast cells, blood basophils majorly) degranulate and release histamine and other inflammatory mediators.
Clinical signs of type I hypersensitivity include:
Facial or periorbital oedema (tissue to become swollen with fluid), Urticaria (itchy welts or red bumps of skin), Cutaneous hyperemia (excess of blood flow due to vasoconstriction)in blood vessels, Generalized pruritus (general itching), Salivation, Hypotensive shock due to severe blood or fluid loss (low blood pressure), Tachypnea (excessive breath movements), Vomiting, Diarrhea, Collapse, Death.