Classification of hypersensitivity reactions.

As the primary defense against pathogens, the immune system uses numerous strategies to ensure optimal protection for the host. When immune responses go awry, however, they can cause great damage. "Hypersensitivity" is a broad term used to describe an excessive and/or pathogenic immune response to either foreign or self antigens. Gell and Coombs were the first to categorize hypersensitivity reactions into 4 types according to pathophysiology, but more recent insights into the mechanisms of these disorders have since modified the original classification system. This review describes the immune mechanisms involved in each of the modern Gell-Coombs categories.