False-colour transmission electron micrograph (TEM) of a single, activated cytotoxic (inskillerins) T-lymphocyte (left, a class of white blood cell) approaching its target cell. T-lymphocytes are involved in cell-mediated response to invasion of body by foreign microorganisms, notably viruses and some bacteria, which are protected from antibodies (secreted by B-lymphocytes) by their host cells. Unlike B-lymphocytes, T-cells do not produce antibodies; instead they recognize selective surface antigens on infected cells. Killer T-cells proceed to destroy infected target cells by direct contact.