be immune tỏ an infection implies being protected from it. The development of immunity to a particular infectious organism, however, usually requires initial exposure to it, which in turn often causes mild illness. Nonetheless, on recovery from the sickness, the individual is immune to subsequent infection and illness from that organism; the body has developed an "immunological memory" (sometimes called adaptive immunity)so that it can act more swiftly and effectively the next time it is exposed to the same invader. The development of this immunological memory occurs by one of t wo general processes, called the humoral-mediated and cell-mediated immune responses. The former ocess produces proteins called antibodies, which circulate through the blood (or bumor") and specifically bind to the foreign organism; the latter process activates white blood cells called killer T-lymphocyte cells, which directly destroy the invading organism. Lymphocytes play a critical role in the development of immunity to infections and cancer. Unfortunately, it is these lymphocytes whose function most noticeably diminishes with age. The age-related decline of immune system functioning gives rise to three general categories of illness that preferentially afflict the elderly: infections, cancer, and autoimmune disease.