Elderly persons are more susceptible to bacterial and virus infections and neoplasias than young adults. This is related to an impaired immune response. Lymphocytes of the elderly show a decreased proliferation after induction with mitogens. The decreased proliferation is correlated to a decreased release of interleukin (IL)-2 and soluble IL-2 receptor (sIL-2R). However, IL-2R expression on the cell surface is normal. Interferon (IFN)-gamma as the main T-helper-1 (TH1) cytokine is produced less by lymphocytes of the elderly, whereas the TH2 cytokines IL-4 and IL-10 are produced in higher amounts as compared to stimulated lymphocytes of young donors. The decreased production of IFN-gamma is correlated to a decreased number of CD45RO+/CD8+ T cells. Therefore in the elderly there seems to be a dysregulation in the TH1/TH2-system which is predominated by TH2-functions. Monocyte function seems to be increased in the elderly. Leukocytes of elderly persons produce higher amounts of IL-1, IL-6, IL-8 and tumor necrosis factor (TNF)-alpha after induction with lipopolysaccharide (LPS) than leukocytes from young donors. In contrast, in vitro induction of IFN-alpha by viruses is decreased in the elderly compared to the young. In conclusion, there are cellular defects and dysfunctions in the elderly resulting in an altered immune response.