The function of the immune system is to maintain body integrity by defending against infections, cancers, autoimmune diseases and inflammation-related chronic diseases. The immune response is known to become defective with aging, leading to decreased longevity and appearance of age-related disease. The most important changes occur in T-cell immunity, and are manifested particularly as altered clonal expansion of cells of limited antigen specificity. The causes of these alterations are multifactorial, and include thymic involution, T-cell subset changes and signal transduction alterations. The clinical consequences of these changes are not well defined, except for their extremely important negative impact on defence against infections, especially by new pathogens, and decreased responses to vaccination. Considering the public health consequences of decreased immune competence in old age, strategies for immune response modulation are desirable to decrease the health burden for the elderly and improve their quality of life.