The cellular immune response is mediated by T lymphocytes that release cytokines and lyse target cells expressing foreign antigens. It generally occurs in parallel with the humoral (antibody) response, although the two can be separated in certain circumstances. Infection with viruses usually evokes both arms of the immune response, which broadly differ in their function: The cellular immune response controls the infection and the humoral response prevents further infection with the same agent. Protection of infants by transfer of maternal antibody is an important component of immune protection against infection in children (reviewed in Zinkernagel, 1996).

Cytotoxic T lymphocytes (CTLs) are major contributors to the antiviral T cell immune response. This T cell population carries the CD8 cell surface glycoprotein and recognizes peptide antigens presented by class I major histocompatibility (MHC) molecules of the immune system (Zinkermagel and Doherty, 1974, 1975). When these cells make contact with antigen through their specific T cell receptor (TCR), provided this is accompanied by certain important cosignals, the T cell is activated to divide, differentiate, and mediate lysis of infected cells. The lytic process is caused both by release of perforin and through fas ligand triggering programmed cell death in fas expressing cells (reviewed in Kagi et al., 1995a). CTLs can also release tumor necrosis factor alpha (TNF-a), which can potentiate killing, and interferon-y (IFN-y), which has activities against a number of pathogens. The CTLs that initially expand on antigen contact can persist as memory cells: The number of memory cells specific for a particular antigen is usually higher than that found in unexposed animals. CD4-positive T cells, the T helper (Th) cells, also have antipathogen activities. Th cells can be broadly grouped into three categories depending on the cytokines they are programmed to release: Th1 cells produce IFN-y and