Infectious pathogens generally have to cope with the host's adaptive immune system, i.e., T and B lymphocytes. Common evasion mechanisms in this complex interaction are antigenic variations, the escape to immunologically priviledged sites or the use of immunosuppressive mechanisms. Many bacteria and other microorganisms eleborate soluble factors or toxins that act suppressively on cells of the immune system, such as pore-forming molecules or proteins that interfere with the function of G proteins. Gram-positive cocci and a mycoplasma have developed an extremely potent mechanism of T cell stimulation by closely mimicking recognition of specific antigen. From the functional similarity to antigen recognition and the multiclonal activation of T cells the designation "superantigens" has been suggested for these molecules.