Superantigens (SAgs) are microbial products that have the ability to promote massive activation of immune cells, leading to the release of inflammatory mediators that can ultimately result in hypotension, shock, organ failure and death. They achieve this by simultaneously binding and activating major histocompatibility complex class II molecules on antigen-presenting cells and T-cell receptors on T lymphocytes bearing susceptible V regions. Why SAgs function in this manner is still not fully understood although it is thought that the resulting Th1 response may divert the immune system from effective microbial clearance and/or result in the cytokine-mediated suppression and deletion of activated T cells. Many bacterial and viral species have adapted unrelated SAgs and this article will highlight some of the features of this diverse group of molecules, from the ways in which they interact with the host to the association of various SAgs with acute and chronic diseases.

Key Concepts:

• Superantigens are toxins produced by many pathogenic bacteria and viruses.
  
• Superantigens are defined as molecules that bind to the major histocompatibility complex (MHC) class II and active T cells bearing a particular T-cell receptor beta chain variable (V) domain.
  
• Superantigens are implicated in inducing autoimmunity through the activation of autoreactive T cells that express particular V elements.
  
• Superantigens are responsible for staphylococcal and streptococcal toxic shock.
  
• The superantigens of Staphylococcus aureus are responsible for staphylococcal food poisoning.
  
• The endogenous superantigen of the mouse mammary tumour virus (MMTV) causes deletion of T cells bearing a particular V element, amplification of infected B cells and transmission of virus to the offspring.