What is a Superantigen?

Superantigens are proteins that cause the T-cells of the immune system to over-react to infection. They are produced by certain infectious bacteria and viruses. The immune system over-reaction to the antigen causes a group of diseases that manifest in fever and shock, such as food poisoning, toxic shock syndrome and Kawasaki disease.

Common bacterial species that may use a superantigen as part of their virulence strategy are staphylococci and streptococci. These bacteria usually live harmlessly on the body but can cause infections in certain circumstances. The superantigens of each species are, like antigens, molecules the immune system recognizes as foreign.

Superantigens cause symptoms of illness by tricking the T-cells of the immune system into over-reacting to these molecules. Parts of a bacterium or a virus are usually recognized by the macrophage cells of the immune system. The macrophage ingests the foreign invaders and breaks them down. Then the macrophage takes parts of the broken-down invader or other molecules that it ingested and posts the fragments on the outside of the cell using a major histocompatibility complex (MHC) to hold the fragment.

A T-cell comes along and uses its T-cell receptor molecule on the outside of its own cell to bind to the fragment. Once the T-cell recognizes the fragment, it begins an immune response. Each T-cell recognizes certain MHCs and certain fragments and usually responds only to those particular stimuli. Superantigens bind to the MHC presenter molecule differently than regular antigens, so more T-cells are tricked into recognizing the superantigen fragment than would recognize a regular fragment.

A normal antigen causes from 0.001 percent to 0.0001 percent of T-cells to produce an immune response. A superantigen causes from 2 percent to 20 percent of T-cells to produce a response. When a T-cell is exposed to a normal antigen, it releases molecules with immunological action and these molecules lead to the normal inflammatory pathway, which is designed to help rid the body of infection. A superantigen activates many more T-cells than regular antigens, so this inflammatory response is exaggerated, and fever, rash, low blood pressure and shock can occur.

Superantigens are medium-size proteins that are highly resistant to heat and to protein-degrading enzymes. These characteristics help the proteins to survive cooking and digestive enzymes, making superantigens important virulence factors in food-borne disease. They also play a role in autoimmune diseases, toxic shock syndrome, diabetes and Kawasaki disease, a leading cause of acquired heart disease in children.