Many changes occur in the aging immune system, which culminate in its functional decline, but the underlying mechanisms have proved difficult to define. Wick and Grubeck-Loebenstein have recently classified the changes in function as either primary ‘those that occur in perfectly healthy older individuals’ as the result of ‘an age dependent intrinsic decline of immune responsiveness’ or secondary ‘those that are due to underlying disease or various environmental factors including diet, drug intake, physical activity, etc’. Both types of alteration are important when considering immune dysfunction in older people which seems to be centred on the loss of cell-mediated immunity, in particular the activity of CD4 helper T lymphocytes.