Immune mechanism, aging, season and diseases: modulatory role of melatonin

Abstract

Immune mechanism of the body plays an important role in arresting neoplastic growth and in controlling infectious diseases. The innate immunity, adaptive immunity comprising of cellular and humoral immunity have distinct roles in fighting against cancer and infectious diseases. The role of neutrophils, monocytes-macrophages, T helper (Th)-1 and Th-2 lymphocytes, B-lymphocytes and cytokines in arresting neoplastic growth and in combating infections and the complex interrelationship among themselves and with neuro-endocrine network in the body has gained much impetus with the discovery of number of receptors and binding surfaces on these cells. The increased incidence of neoplastic and infectious diseases seen in the elderly is attributed primarily to decreased immune function of the body, and termed as immunosenescence. Alteration in circadian rhythmicity of various subsets of lymphocyte population has been documented in the elderly. Similarly recent studies on cancer patients reveal that there exists two distinct types of lymphocytes with some cells exhibiting acrophase during morning and others in the night and circadian variation of lymphocyte population in cancer patients suggest impaired integration of nervous, endocrine and immune responses in neoplastic disease. Seasonal outbreak of some infectious diseases seen in some parts of the world has supported the photoperiodic regulation of immune function with enhancement during short photoperiods and inhibition during long photoperiods. Although the evidences for this are largely derived from animal studies, its application to human studies is still in a preliminary stage. However, the neurohormone melatonin which was shown to have an immunomodulatory role may stimulate immune mechanisms and in this way, melatonin could be a very useful resource for inhibiting neoplastic growth. Melatonin stimulates natural killer cells which are known to attack and destroy cancerous cells by their immunosurveillance mechanism. In addition, Th-1 cells, B-lymphocytes, release of cytokines from immunoregulatory cells are influenced by melatonin. The synthesis of melatonin by lymphocytes and thymus supports an immunomodulatory role for melatonin and its application in the control of infectious and neoplastic diseases.