Superantigen binding to a T-cell receptor, illustration - stock illustration

Superantigen binding to a T-cell receptor, illustration. A superantigen (SAg, pink) is being presented by an antigen-presenting cell (bottom) to a T-cell (top), a type of white blood cell. Superantigens are released by some viruses and bacteria and can generate a strong immune response by inducing huge releases of cytokines in humans. The superantigen binds to the major histocompatibility complex (MHC) class II molecules (blue) of an antigen-presenting cell. This is then presented to a T-cell via a T-cell receptor (TCR, green). This activates the T-cell (non-specific activation), which results in excessive cytokine release.