Understanding Hypersensitivity Hypersensitivity is an exaggerated immune response causing harm to the host. Normally, immunity defends against foreign particles like bacteria or viruses without harming self-cells. However, in hypersensitivity, this defense mechanism becomes excessive and damages host tissues while targeting invaders.
Classification of Hypersensitivities There are four types of hypersensitivities classified by Coombs and Gell: Type I (allergy), Type II (cytotoxic reactions), Type III (immune complex-mediated reactions), and Type IV (delayed-type). Types I-III involve B cells producing antibodies; whereas type IV involves T-cell mediated responses with no antibody formation.
Mechanism Behind Allergic Reactions - Type I Type I hypersensitivity occurs when allergens trigger IgE production that binds to mast cells. Upon re-exposure to the allergen, these sensitized mast cells release granules containing histamine and other mediators leading to symptoms such as bronchoconstriction or edema. Examples include asthma, hay fever, eczema.
'Cytotoxic' Damage in Host Cells - Type II 'In cytotoxicity-based type-II', antigens on target cell surfaces stimulate plasma-derived IgG/IgM-antibodies forming complexes activating complement proteins resulting phagocytosis destroying healthy tissue examples blood-transfusion-reaction autoimmune diseases e.g Graves disease Myasthenia Gravis etc..