Basic and Clinical Sciences (BCSE) Practice Exam

Type III hypersensitivity is characterized by the formation of immune complexes that occur when antigens bind with antibodies, typically of the IgG or IgM type. These immune complexes can deposit in various tissues and lead to localized inflammation and tissue damage. This reaction is often responsible for conditions such as systemic lupus erythematosus and rheumatoid arthritis, where the presence of these complexes triggers a strong inflammatory response mediated by complement activation and subsequent recruitment of inflammatory cells.

In contrast, Type I hypersensitivity involves an immediate allergic reaction mediated by IgE antibodies, which bind to allergens and trigger mast cell degranulation. Type II hypersensitivity is mediated by antibodies directed against specific cell surface antigens, leading to cell destruction or dysfunction. Type IV hypersensitivity is a delayed-type reaction primarily mediated by T cells and is not related to the formation of immune complexes. Thus, Type III hypersensitivity is uniquely defined by the involvement of immune complexes in its pathogenic process.