Mechanism of damage caused in type 3 hypersensitivity

Priya sarda

Type III, or immune-complex, reactions are characterized by tissue damage caused by the activation of complement in response to antigen-antibody (immune) complexes that are deposited in tissues. Type III hypersensitivity reactions can be provoked by inhalation of antigens into the lungs. A number of conditions are attributed to this type of antigen exposure, including farmer’s lung, caused by fungal spores from moldy hay; pigeon fancier’s lung, resulting from proteins from powdery pigeon dung; and humidifier fever, caused by normally harmless protozoans that can grow in air-conditioning units and become dispersed in fine droplets in climate-controlled offices. In each case, the person will be sensitized to the antigen—i.e., will have IgG antibodies to the agent circulating in the blood. Inhalation of the antigen will stimulate the reaction and cause chest tightness, fever, and malaise, symptoms that usually pass in a day or two but recur when the individual is reexposed to the antigen. Permanent damage is rare unless individuals are exposed repeatedly. Some occupational diseases of workers who handle cotton, sugarcane, or coffee waste in warm countries have a similar cause, with the sensitizing antigen usually coming from fungi that grow on the waste rather than the waste itself. The effective treatment is, of course, to prevent further exposure.