Type III Hypersensitivity Reaction

Excerpt

A hypersensitivity reaction is an inappropriate or overreactive immune response to an antigen resulting in undesirable effects. The symptoms typically appear in individuals who had at least one previous exposure to the antigen. Hypersensitivity reactions can be classified into four types:

1. Type I - IgE mediated immediate reaction

2. Type II - Antibody-mediated cytotoxic reaction (IgG or IgM antibodies)

3. Type III - Immune complex-mediated reaction

4. Type IV - Cell-mediated, delayed hypersensitivity reaction

In type III hypersensitivity reactions, 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 reactions, the antigen-antibody complexes are pre-formed in the circulation before their deposition in tissues.