Figure 2. The strength of the immune response declines with age.

Multiple age-related changes can affect the composition and function of lymphocytes in secondary lymphoid tissues. CD4⁺ Th cells exhibit activation defects and increased differentiation into Th17 cells. CD8⁺ T cells undergo an oligoclonal expansion and loss of CD28 in humans and exhibit impaired function. The number of B cells that respond to influenza is reduced, and antibody avidity in response to carbohydrate antigens is diminished. In addition, the tissue environment includes an increased concentration of inflammatory cytokines, which may be produced by stromal elements, dendritic cells, or aging B and T cells. The increased number of memory cells that occupy tissue niches and the inflammatory milieu in turn may compromise the ability of naive B and T cells migrating from the bone marrow and thymus to lodge in the tissue. Together, these changes result in diminished immune function in the elderly.