The memory T cells then become dormant and are only reactivated upon a subsequent exposure to that same antigen. A marked difference has been observed between young and old subjects in the subpopulations of naive and memory T cells. The elderly have almost no naive T cells at all, since as the thymus progressively deteriorates with age, fewer T cells are produced, and the naive T cell subpopulation is not replenished. In addition to the decline of certain subpopulations of T cells, important changes occur at the cell surface of all T cells. When a T cell, using T cell receptor proteins found on the cell surface, binds to an antigen, that environmental stimulus must be communicated to the interior of the T cell.
Signal transduction is a cascade of chemical reactions, each dependent upon the preceding event.
Aged T cells do not display the CD28 antigen, a molecule critical for signal transduction and T cell activation, on the cell surface. Without this protein, T cells remain quiescent and do not respond to foreign pathogens. One indication of a malfunctioning signal transduction pathway in T cells is that the presence of CD69 antigen on the cell surface is lower in elderly individuals. T cells are induced to display CD69 antigen only after antigen binds to the T cell receptor. If the antigen-binding signal is not transmitted to the interior of the T cell, CD69 antigen will not appear on the cell surface, and is an indication that in older people, less signal transduction is occurring.