2020 Dec 14. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2020 Jan–.
A hypersensitivity reaction is an inappropriate or overreactive immune response to an antigen resulting in undesirable effects. The symptoms typically appear in an individual who had at least one previous exposure to the antigen. Hypersensitivity reactions can be classified into four types.
Type I - IgE mediated immediate reaction
Type II- Antibody-mediated cytotoxic reaction (IgG or IgM antibodies)
Type III- Immune complex-mediated reaction
Type IV- Cell-mediated, delayed hypersensitivity reaction
Type III hypersensitivity reaction
In type III hypersensitivity reaction, an abnormal immune response is mediated by the formation of antigen-antibody aggregates called "immune complexes." They can precipitate in various tissues such as skin, joints, vessels, or glomeruli, and trigger the classical complement pathway. Complement activation leads to the recruitment of inflammatory cells (monocytes and neutrophils) that release lysosomal enzymes and free radicals at the site of immune complexes, causing tissue damage.
The most common diseases involving a type III hypersensitivity reaction are serum sickness, post-streptococcal glomerulonephritis, systemic lupus erythematosus, farmers' lung (hypersensitivity pneumonitis), and rheumatoid arthritis. The principle feature that separates type III reactions from other hypersensitivity reactions is that in type III reaction, the antigen-antibody complexes are pre-formed in the circulation before their deposition in tissues.