Altered immune status in the elderly.

The lung has an array of immunological defenses to protect itself against potentially invasive microorganisms, which include the immunoglobulin-rich alveolar lining fluid, alveolar macrophages, T lymphocytes, and polymorphonuclear neutrophils. Immunosenescence is a major predisposing factor to the increased incidence, morbidity, and mortality of pneumonia in the elderly. The progressive involution of the thymus gland in humans plays a pivotal role in the development of the immunodeficiency state characteristic of the older individual. Age takes its greatest toll on the cell-mediated arm of the immune system. Aged T cells are impaired in their ability to activate and proliferate in response to an antigen. This is partly due to age-associated structural and functional changes within the T cell. In addition, the ability of the T cell to secrete interleukin-2 (a cytokine necessary for the recruitment of other T cells) declines with age. The impaired antibody response of the elderly to foreign antigens, including the pneumococcal polysaccharide and the influenza vaccine, appears to be secondary to a deficiency of T helper cells. The macrophage functions well even in old age, but the recruitment of macrophages by senescent T cells is diminished. There also may be a blunted inflammatory response in the older individual secondary to impaired polymorphonuclear neutrophils chemotaxis and phagocytosis.