T lymphocytes, the cells of cell-mediated immunity, are
further subdivided into functionally distinct populations,
the best defined of which one helper T cells and cytotoxic
T cells do not produce antibody molecules.
Helper and cytotoxic T lymphocytes (CTLs) have a
restricted specificity for antigens. CTLs lyse cells that
produce foreign antigens, such as cells infected by
viruses and other intracellular microbes.