Hypersensitivity

Hypersensitivity is an exaggerated or inappropriate reaction produced by the normal immune response against an innocuous or harmless environmental substance called allergen. It is an over-reaction of the immune system, and the outcome may be undesirable. There are four types of hypersensitivity reactions, of which the first three are antibody-mediated reactions, and the fourth type is due to antigen-sensitized 'T cells'. Type I or immediate hypersensitivity is characterized by allergic responses to the relevant antigen, mediated by mast cell-bound IgE antibodies arising from responses to foreign proteins that have no toxic potentials (eg, pollens, house dust mites, animal danders). Type II or cytotoxic reaction is mediated by IgG or IgM antibody directed against antigen on the surface of own cells of the body or foreign antigens (eg, transfused red blood cells). Type III or reactions mediated by immune complexes that are not adequately cleared from the circulation, and trigger inflammatory reactions through deposition in various tissues such as skin, joints, blood vessels. Type IV or delayed hypersensitivity reaction mediated by lymphokines and cytokines from effector T cells which cause unwanted responses.

Broadly, the first three types of hypersensitivity reactions usually occur within 24 hours, and are considered immediate hypersensitivity. The fourth type occurs with the recruitment of antigen-specific T cells to the site of exposure of the exogenous antigen and the development of a local inflammatory response usually over 24-72 hours, and considered a delayed type hypersensitivity (DTH) reaction. The most common diseases involving a Type I reaction includes hay fever, rhinitis, allergy, allergic asthma, conjunctivitis, dermatitis; Type II reaction includes autoimmune hemolytic anemia, hemolytic disease of the newborn; Type III reaction includes serum sickness, rheumatoid arthritis, farmers lung (pneumonitis); Type IV reaction includes contact sensitivity to chromate, rubber accelerators, nickel, poison ivy, graft rejection, skin responses of leprosy, the inflated response to measles. [Laila Noor Islam]