The rapid development of aT-cell proliferative response to foreign antigens is one essential determinant of the capacity of the immune system to eliminate a pathogen that has invaded an organism. The expansion of antigen-specific T cells of the CD4+ inducer phenotype is critical in the generation of delayed-type hypersensitivity responses (DTH) as a result of the ability of these cells to secrete lymphokines (induding interferon-y) that attract and activate macro phages at the site of antigen deposition. CD4+ cells are also critical for the development of cytotoxic responses to virus-infected cells, malignant cells, and allogeneic tissue, either directly through lymphotoxin release (TNF-f3), or by the lymphokine-mediated expansion of cytotoxic T lymphocytes. Finally, these T cells also provide help for antibody responses through the secretion of interleukins during interactions with antigen-specific B cells.