Aging is a complex physiological process that involves a number of biochemical reactions, with molecular changes that are manifested in single cells as well as in the whole organism. Aging reflects the sum total of all changes that occur in living organisms with the passage of time that lead to functional impairment and increased pathology. Aging is characterized by a diminished ability to respond to stress 1. Among the many theories proposed for aging, the Oxidative Theory of Aging put forth by Harman in 1956 2 has received wide support.

Aging is associated with a decline in immune function known as immunosenescence. This situation implies increased susceptibility to infectious diseases and cancer due to a decreased capacity of the immune system to respond to antigenic stimulation 3. This results in altered cytokine microenvironment and impairment of both innate and adaptive immunity 4. It is interesting to note that many hormones that are associated with maintenance of immune function also decline with advancing age and the interrelationship between the endocrine system and the immune system is considered of crucial importance in normal human physiology and in mediating age-associated degenerative diseases 5678. The decline in the production of a number of hormones associated with aging such as growth hormone (GH), estrogen and dehydroepiandrosterone, as well as of the pineal substance melatonin, have been proposed to play a significant role in contributing to immunosenesecence 5. Among these, melatonin has been demonstrated to bear a general immunoenhancing effect in many animal species as well as in humans 9.

Melatonin is a natural antioxidant with significant anti-aging properties 10. Indeed, any search for a therapeutic agent that can improve the quality of life in the elderly implies the identification of substances that have both antioxidant and immunoenhancing qualities. In this vein, the role for melatonin has been put forth 111213 and in this paper the evidence indicating that melatonin is effective to combat age associated decline in immune function will be reviewed with the aim of advocating melatonin as a possible therapeutic agent for enhancing the quality of life in the elderly.

Immunosenescence is associated with increased incidence of cancer and of degenerative and infectious diseases. The progressive functional T cell and B cell deficits may be the main responsible factors for age-associated disorders 41415. The involution of thymus with age results in alterations of gene expression 16; indeed, immunosenescence is reflected at cellular, molecular and genetic levels 17. Individuals of the same chronological age may exhibit variations in the degree of senescence associated functional impairment 18. The role of immunity as a predictor of individual longevity in human beings has been suggested by several studies like OCTO and NONA longitudinal studies and they all reveal the existence of "immunological risk phenotype", that can predict the life span in the elderly 19.

Aging affects the innate immune system 20. In the innate immune system natural killer (NK) cells play an important role for inhibiting cancer and metastases. Longer life in centenarians has been associated with increased NK cell number, augmented interferon (IFN)-gamma production and phagocytosis 212223. The age-associated increases in NK cells (21) have been interpreted as a compensatory response to overcome the generally decreased immune function and has been considered helpful in arresting the growth of neoplastic cells. For example, in human NK cells from healthy subjects over 90 years of age, the ability to synthesize chemotactic cytokines upon stimulation by IL-12 or IL-2, or to express the corresponding chemokine receptors are maintained (24). However most investigators are of the opinion that functional competence of individual human NK cells declines with age 2125. Indeed, NK cells of aged people exhibited a diminished production of IFN gamma and chemokines in response to interleukin (IL)-2 and IL-6 25. Recently, Albright and her coworkers 26 found severe impairment in the production of mRNA transcripts representing several cytokines in NK/LAK cells of aged mouse. The cytotoxic capacity of NK cell is well preserved in peripheral blood of the centenarians 27.

Functional impairment of macrophages and granulocytes are reported in the elderly. Diminished intracellular phagocytic activity, degranulation and decrease in chemotactic and phagocytic activity have all been found in polymorphonuclear leukocytes of elderly individuals 2428. In a study in centenarians, Miyaji and his coworkers 22 found that granulocytes exhibited decreased superoxide production, irrespective of subject's health conditions. A decrease in superoxide production in elderly subjects has also been reported in other studies 293031, the decreased production of superoxide in the granulocytes being attributed to the reduction in signal transduction in granulocytes 29. The attenuation of Fc mediated superoxide generation and phagocytosis in the elderly has been suggested as the major factor for the age-related decline in neutrophil function 2832. With regard to macrophages, increased production of proinflammatory mediators like IL-1, IL-6 and IL-8 occurs in both healthy aged subjects and people showing pathological aging 3334. Macrophages are important for phagocytosis and destruction of microorganisms and also for cytokine production that regulates the functional ability of other cells of innate immunity.

Diminished IL-1 levels and diminished generation of reactive oxygen species (ROS) from monocytes of elderly subjects has been reported (reviewed by 35). IL-6 (which has been termed as a "cytokine for gerontologists", 36) increases in aged subjects 3738. The increase in IL-6 occurs in healthy individuals older than 85 years of age 39. The increase in IL-6 seen in aged subjects may contribute to age-ssociated diseases 40 and mortality 41. Plasma concentrations of soluble intercellular adhesion molecule-1 (ICAM-1) increased with age 394243. Collectively, the results suggest that it is this shift in cytokine profile that is largely responsible for triggering immunosenescence and increased morbidity and mortality in the elderly 39.

Aging results in changes in humoral immunity such as an increase in the levels of serum immunoglobulins like IgA and IgG, and decrease in the number of B and T lymphocytes 4445. A decline in organ specific autoantibodies together with an increase in non-organ specific autoantibodies have been found in the elderly 46. Reduction in CD 27+ memory B cells has been reported and this correlated with low T cell number 47. A decrease in CD5+ B cells independent of T cell decline was also reported in aging 48. Therefore, the reduced humoral responsiveness and altered antibody-mediated defense mechanisms seen in aged individuals are explained mainly by an intrinsic primary cell deficit 49. The ability of T cells to promote B-cell activation and antibody production may be compromised in elderly individuals, as suggested by studies using cytometric phenotypic analysis 50. A significant decrease in IL-2 production with aging plays a role in reducing antibody production 2349.

Aging not only causes changes in innate immunity and humoral immunity, but also causes changes in cellular immunity. A significant decrease in CD3+, CD4+, CD8+ cells and naïve T lymphocytes (CD45RA+CD4+) occurs with increase in age. An extensive review on T cell function in aging was published by Pawelec et al. 14. With aging, alterations in signal transduction may also occur. The age-associated decline in T cell function is preceded by involution of the thymus 35. The striking feature of T cell alterations in aging is the marked shift from naive to memory cells with an imbalance of virgin and memory cells being noted especially in CD8+ T lymphocytes 45. Naïve T cells, which are concerned with the mounting primary immune response, are dependent upon CD28, a co-stimulatory signal for their proliferation 45. Both the decrease in the number of naïve T cells and in their responsiveness with aging cause the decline of specific immunization response in aged individuals 51. Large increases in CD8+ T cells with receptors for single epitopes of cytomegalovirus are common in the elderly 52. Longitudinal studies (OCTO) suggest that the cluster of immune parameters like low CD4+ cells, an increase in CD8+ cells and a low IL-2 production are all predictive of mortality 535455. The decline in naïve T cells is one of the factors that cause a decreased IL-2 production56.

Melatonin (N-acetyl-5-methoxytryptamine) is formed mainly in the pineal gland of most mammals including man 57. In the pineal gland, serotonin is converted in to melatonin through a two-step enzymatic process involving N acetylation followed by O-methylation. In humans, plasma melatonin level begins to increase steadily after 1900 h to 2300 h to attain the peak values at around 0200 – 0400 h 7. The study of plasma melatonin among subjects of different age groups reveals a consistent decrease as aging progresses. With some exceptions 5859 the decline of melatonin with age has been repeatedly reported 60616263646566. The melatonin day/night rhythm has been found altered with phase advance in the elderly as compared to young women 67. Great variations in the amplitude of nocturnal melatonin secretions are found among individuals suggesting that some individuals produce significantly less melatonin during lifetime than others; this may have an impact in terms of aging 768. The loss of amplitude of melatonin rhythm in the advanced age is both an indication as well as a cause of age-related disturbances in the circadian pacemaker leading to chronobiological disorders 69. This is accompanied by a general deterioration of cognitive, psychological and social functioning as well as by sleep disturbances 707172.

The age-related impairment of the immune system first appears around 60 years of age coinciding with the decrease of plasma melatonin concentration. Indeed, melatonin has a defined immunomodulatory role both in animals and humans 1373. The diurnal and seasonal changes in the immune system have been shown to correlate with melatonin synthesis and secretion 74. Melatonin is synthesized by human lymphocytes and this finding adds further support to the hypothesis that melatonin plays a role in the regulation of human the immune system 75.

Melatonin exerts its many physiological actions by acting on membrane and nuclear receptors although many of its actions are receptor-independent (e.g., scavenging of free radicals, interaction with cytosol proteins like calmodulin). The two melatonin receptors cloned (MT1 and MT2) are membrane receptors that have seven membrane domains and belong to the superfamily of G-protein coupled receptors 76. Melatonin receptor activation induces a variety of responses that are mediated both by pertussis-sensitive and insensitive G proteins 77. In the cytosol melatonin interacts with calmodulin 78. Nuclear binding receptors have been identified in human lymphocytes and monocytes 79.

In recent years much attention has been devoted to the possible interaction between melatonin and the immune system 137380. Melatonin has significant immunomodulatory roles in immunocompromised states. In 1986, Maestroni et al. first showed that inhibition of melatonin synthesis causes inhibition of cellular and humoral responses in mice 81. Mice kept under constant light, or receiving injections of betaadrenergic blockers (propranolol) to inhibit melatonin synthesis, exhibited an inability to mount a primary antibody response to sheep red blood cells (SRBC), a decreased cellularity in thymus and spleen and a depressed autologous mixed lymphocyte reaction; all these were reversed by melatonin administration at the late afternoon 81. Late afternoon injection of melatonin increases both the primary and secondary antibody responses to SRBC 82. Indeed, the immunoenhancing effect of melatonin was evident only when melatonin was administered in the afternoon or in the presence of T-dependent antigenic stimulation. Since melatonin was ineffective in vitro, Maestroni and co-workers concluded that it exerts its immunostimulating effect through other neuroendocrine mechanisms in antigen-activated cells 83. Hamsters exposed to short photoperiods had increased spleen weight and number of splenic lymphocytes and macrophages 84. A key finding-albeit in young adult humans – with respect to the interplay of melatonin and the immune system, was the observation that the nocturnal rise of blood melatonin in humans correlated with the increase of thymic production of peptides like thymosin-1 alpha and thymulin 85.

A number of studies support the immunoregulatory action of melatonin on the body's innate immunity 80. Melatonin stimulates the production of progenitor cells for granulocytes and macrophages (GM-CFU) and has a general stimulatory action on hemopoiesis 8687. Melatonin receptors are detectable in monocyte/macrophage lineage 79 and melatonin binding to these receptors stimulates the production of GMCFU cells 8889. A recent pivotal study, although carried out in young adult mice, has revealed a profound, time-dependent influence of melatonin on certain cells fundamentally important to the immune system. Exogenous melatonin augments NK cells and monocytes in both the bone marrow and the spleen with a latency of 7 to 14 days 90. As both these cells are components of the non-specific immune system, the findings suggest that melatonin could be an effective way for arresting neoplastic growth and for destroying virus infected cells. The action of melatonin on monocyte production can be partly due to its direct action on melatonin receptors or may be due to an increase of monocyte sensitivity to stimulants like IL-3, IL-4, IL-6 or GM-colony stimulating factor (GM-CSF) 888990. As stromal cells contain receptors for kappa opioid cytokine peptides, melatonin-induced release of opioid peptides from these stromal cells in bone marrow could be involved in the regulation of hemopoietic cell proliferation 91. In addition to monocytes, the bone marrow precursor cells for the granulocyte lineage increase in absolute numbers after melatonin administration. The study of Currier et al. 90 revealed that melatonin increases the actual production of the GM-cell lineage and not the inter-organ trafficking of myeloid precursors. An increased activation of monocytes/macrophages by melatonin has been reported in yet another study in rodents 92. As both macrophage cells and neutrophils form important components of the innate immune system, the stimulatory action of melatonin reflects a significant immunoenhancing property. Melatonin treatment restores the decreased total leukocyte count in peripheral blood and bone marrow of pinealectomized squirrels 93. Macrophages have been shown to form large amounts of nitric oxide (NO) upon activation by ROS that mediate their microbiocidal properties. This excessive production of NO can be harmful to the body as it can result in the development of degenerative diseases 94. In a recent study melatonin was found to decrease NO concentration in macrophages by suppressing inducible NO synthase expression 95. When melatonin's effects on phagocytic activity of macrophages were tested at different concentrations, the greatest phagocytic stimulation was obtained when melatonin was added resembling the unstressed situation 96.

NK cells play an important role in immunosurveillance against neoplasia and virus infected cells 9798. IFN-gamma enhances NK cell activity 99. An observation of potentially high prophylactic significance, was the demonstration that exogenous melatonin given acutely at 1800 h to young healthy males increased their responsiveness to IFN while the chronic administration of melatonin augmented the spontaneous NK cell activity and also the circulating number of NK cells 100. The increased NK cell number brought about by melatonin administration was attributed partly to the increased production of cytokines by melatonin-stimulated T helper cells. IL-2, IL-6, IL-12 and IFN-gamma have all been suggested as the possible cytokines that mediate melatonin-induced increase of NK cell number 90. T helper cells contain melatonin receptors that presumably mediate melatonin action in releasing cytokines 101102103.

Melatonin has been proposed to regulate the immune system by affecting cytokine. production in immunocompetent cells 104. Melatonin enhances the production of IL2, IFN-gamma and IL-6 by cultured human mononuclear cells 101. Melatonin, by. activating monocytes 105, increases the production of IL-1, IL-6, TNF-alpha and ROS. Melatonin also increases IL-12 production by monocytes 105. Repeated stimulation of T helper (Th) cells in the presence of IL-12 causes Th cells to differentiate into Th1 cells, which produce IL-2 and IFN-gamma and are particularly effective in enhancing immune responses that involve macrophages and other phagocytes. Melatonin augments IFN-gamma production by Th1 cells 104. The enhancement of NK cell activity by melatonin is attributed to the increased production of IL-2 and IL-12 104106107.

Human lymphocytes themselves play an important role in stimulating IL-2. production in an autocrine or paracrine fashion 75. After melatonin treatment, up-regulation of gene expression for TGF-β, M-CSF, TNF-α, and stem cell factor (CSF) in peritoneal exudate cells, and the level of gene expression of IL-1β, M-CSF, TNF-α, IFN-γ, and SCF in splenocytes were reported 108. Melatonin's immunoenhancing effect depends upon its ability to enhance the production of cytokines as well as its anti-apoptotic and antioxidant action. As a functional impairment of macrophages and granulocytes (as shown by the diminished intracellular phagocytic activity, degranulation and decrease in chemotactic activity) has been reported in the elderly 2844 and a parallel decrease in melatonin production occurs 60616263646566 it may not be unreasonable to speculate that immunosenescence can be partly attributed to a decreased production of melatonin. To restore the defective phagocytic function the use of adjuvants with immunizations and nutritional supplements has been proposed 109.

Micronutrients like zinc, selenium and vitamin E play a vital role in phagocytic function 110. Since melatonin can stimulate the immune response and correct immunodeficiencies by causing up-regulation of cytokine production it can be used therapeutically for correcting the immunodeficiency state associated with aging.

Besides its stimulatory action on the production of several cytokines that regulate immune function, melatonin's immunoenhancing properties have been attributed to a direct action on the immunocompetent cells (e.g. granulocyte-macrophage cells, NK cells and lymphocytes). Earlier studies demonstrated that the thymus is a primary target of melatonin's action. The thymus is an organ of youth in mammals, yet any influences on the thymus in youth will have profound effects on the immune system of elderly mammals. A milestone, earlier demonstration revealed that pinealectomized, young mice underwent accelerated involution of the thymus 111. The presence of melatonin binding sites in membrane preparations of non-mammalian (duck) thymus has also been reported 112. Mice kept under constant light, or administered with beta-adrenergic blockers exhibited decreased cellularity of thymus and spleen that was reversed by late afternoon administration of exogenous melatonin 8182113.

The severe loss of thymocytes with age is the main cause of structural thymic atrophy and thymic weight loss. Melatonin administration increased the total number of thymocytes in old mice 114. In that study, thymic cell number in 2 months-old mice was 12.6 × 10⁷, while it dropped to 7.3 × 10⁷ cells in 24 months-old animals; in melatonin treated old mice the total number of thymocytes was 9.1 × 10⁷ cells 114. This protective effect of melatonin on thymocytes was attributed to its antiapoptotic action. Melatonin inhibited glucocorticoid- or hydroxyl radical-induced thymic apoptosis 115116. The reversal of age-associated thymic involution by melatonin added further support to the concept that melatonin can be a potential therapeutic agent for correcting immunodeficiency state associated with aging and possibly other immunocompromised states like severe stress 117. Finally, Yu et al. 118 have demonstrated that orally administered melatonin can substantially promote the survival (anti-apoptosis) of precursor B lymphocytes (responsible for humoral immunity) in the B lymphocyte generating site, i.e., the bone marrow. This indicates that melatonin treatment can boost the survival of mature B cells which are the functional elements in humoral immunity.

Melatonin enhances both cell-mediated and humoral immunity. The administration of melatonin to normal or immunocompromised mice elevated in vitro and in vivo antibody responses 73. The immunoenhancing effect of melatonin involves opiod peptides; melatonin stimulates Th cells to secrete opiod peptides that have upregulatory effects on a variety of immune cells 73. According to Nelson and Drazen 119, melatonin is a part of a complex physiological system that coordinates reproductive, immunological and other physiological processes to cope up with energetic stressors during winter. Studies in birds also indicate that melatonin stimulates both cellular and humoral responses and that the response involves opiate intermediates 120121.

The immunostimulatory role of melatonin is exerted mainly on Th cells and on T lymphocyte precursors. There is a possibility that melatonin could act as an autocoid in bone marrow as shown by the demonstration of melatonin synthesis in bone marrow cells of mice and humans 122. The existence of specific melatonin binding sites inlymphoid cells provides evidence for a direct effect of melatonin in the regulation of the immune system. By using the melatonin agonist 2 [¹²⁵I]-melatonin high affinity binding sites and a signal tranduction pathway for melatonin have been characterized in human lymphocytes 123124. Melatonin also counteracted the inhibitory effect of prostaglandin E2 on IL-2 production in human lymphocytes via its MT1 membrane receptor 125. Melatonin augments CD4+ lymphocytes and decreases CD8+ lymphocytes in rat submaxillary lymph nodes 126. Collectively, these studies indicate that melatonin possesses important immunoenhancing properties and suggest that melatonin may favor a Th-1 response. During the natural history of human immunodeficiency virus type I (HIV-1) infection, an impairment of IL-12 production precedes a switch from a Th-1 to a Th-2 stage of cellular immunity. A recent study indicated a correlation of serum levels of melatonin and IL-12 in a cohort of 77 HIV-1 infected individuals, the decreased levels of serum melatonin found in HIV-1-infected individuals being possibly instrumental in the impairment of Th-1 immune response 127.

Besides the release of proinflammatory Th-1 cytokines, such as IFN-gamma and IL2 administration of melatonin to antigen-primed mice increased the production of IL10, indicating that melatonin can also activate anti-inflammatory Th-2-like immune responses in certain circumstances 128. Therefore, it is not yet clear whether melatonin acts only on Th-1 cells or also affects Th-2 cells. This is an important subject as the Th-1/Th-2 balance is significant for the immune response 73. Relevant to this, melatonin treatment suppressed the subsequent in vitro stimulation by the mitogenic agents LPS (that stimulates B cells) and Con A (that stimulates T cells) in submaxillary lymph nodes 126. In addition, an inhibitory influence of melatonin on parameters of the immune function has also been demonstrated, i.e., in human NK cell activity, DNA synthesis, IFN-gamma and TNF-alpha synthesis, as well as the proliferation of T lymphocytes and lymphoblastoid cell lines were depressed by melatonin 73. Melatonin can correct immunodeficiencies secondary to acute stress, viral diseases and drug treatment. In immunodepressed conditions, the immunoenhancing action of melatonin seems to be restricted to T lymphocyes 129. In conditions of immunodeficiency, as in other pathologies and the normal, melatonin appears to favour a Th1 lymphocyte response 108. Finally, a recent study (130) has estabished a significant role for melatonin, i.e., as an adjuvant with vaccination in sheep afflicted with ovine footrot, indicating that this agent clearly has significant benefits in health maintenance and disease treatment.

Studies by Drazen and Nelson 102 indicated that melatonin receptor subtype MT2 but not MT1, is involved in melatonin-induced enhancement of cell-mediated and humoral function in mice. cAMP signal transduction plays an important role in regulating lymphocyte function and this pathway appeared to be abnormal in aged mice131. Melatonin antagonized partially forskolin-induced increase of cAMP levels of lymphocytes; indeed, G1 protein coupled adenylate cyclase-cAMP signal pathway may be one of the important mechanisms for the anti-inflammatory immunoregulation by melatonin 132. Melatonin enhanced significantly met-enkephalin in 2 and 11 months old mice, and the effect was blocked by nifedipine, a Ca2+ antagonist 132. This suggests that melatonin promotes the production of met-enkephalin through L-type Ca2+ channel. Melatonin-induced immunoregulation may depend upon immuno-opiod interaction 133.

It has been suggested that Th-1 responses are readily transformed into Th-2 dominance through depletion of intracellular GSH 134. GSH in its reduced form is the single most important protective and regulatory antioxidant in cells. The work of Peterson and his coworkers 135 showed that depletion of glutathione from antigen presenting cells in vivo resulted in lowered Th-1 activity and higher Th-2 activity. Murata et al. 136137 showed that oxidized macrophages exhibited higher levels of oxidized glutathione as they polarized to type Th-2 cells. Thus the immune activity can have Th-1 or Th-2 characteristics depending upon the relative antioxidant status of the cells.

Since melatonin stimulates the production of glutathione 138 its immunoenhancing role may be partly due to its influence on the maintenance of intracellular glutathione level. Indeed, melatonin acts as a hypnotic-chronobiotic 139140 with cytoprotective properties 141142 as well as an immunoenhancing agent. Indeed, melatonin not only acts as a hypnotic-chronobiotic with cytoprotective properties but also as an immunoenhancing agent. Melatonin provides a functional link between the neuroendocrine and immuno-hematopoietic systems 143.

Recent studies reveal that not only melatonin but also its oxidation product N1 acetyl-N2-formyl-5-methoxykynuramine (AFMK) is very effective in acting onneutrophils 144145. Both melatonin and AFMK have been shown to inhibit IL-8 release from neutrophils and AFMK has been found to be more active than melatonin in this aspect. The production of TNF-alpha by neutrophils is also inhibited by melatonin and AFMK. Since TNF-alpha and IL-8 contribute to the severity of inflammatory conditions 146, the finding of melatonin inhibiting the release of IL-8 and TNF-alpha assumes significance for it may help to reduce acute and chronic inflammation. Neutrophils are more responsive than monocytes to AFMK suggesting that melatonin biosynthesis and metabolism participate in the chemical communication among leukocytes. Melatonin may be effective in optimizing intrinsic immune responses rather than acting simply as an antioxidant 147. Dietary supplementation of melatonin has been shown to change mRNA levels of many genes and to arrest the attenuated immune responses associated with senescence 147.

A number of recent studies point out that seasonal changes exert influence on immune function and melatonin may play an important role in this aspect. Seasonal changes of immune function in animals are mediated by the duration of melatonin secretion, which acts as a photoperiodic signal 119. Such seasonal changes in immune function have been observed in humans also. Increased production of proinflammatory cytokines IFN-gamma and alpha occurred during winter 148. Highest production of IL-6 was reported in healthy volunteers during autumn/winter season 149. In humans the seasonal changes in immune functions can be mediated by the changes in duration of melatonin secretion. Seasonal changes in cytokines like IL-6, IFN-alpha, IFN or the balance of Th-1 and Th-2 response can account for seasonal changes in mood and behavior, such as Seasonal Affective Disorder.

The age-associated decline in immune function, known as immunosenescence, is characterized by a decrease in the functional activity of NK cells, granulocytes and macrophages. There is significant reduction in IL-1 and diminished generation of ROS from monocytes. In addition, there is an increase of IL-6 production. Besides causing changes in innate immunity, aging is associated with changes in cellular and humoral immunity. Decreases of CD3 and CD4 and increases of CD8 cells occur in elderly individuals. The decrease in IL-2 production that occurs during aging causes a reduced antibody formation. Melatonin seems to play a significant immunomodulatory role. Melatonin enhances both innate and cellular immunity. It stimulates the production of progenitor cells of granulocytes and macrophages and of NK cells. Production of IL-2, IL-6 and IL-12 is stimulated by melatonin. Increased T-helper production, particularly of CD4+ cells, occurs after melatonin supplementation. Melatonin decreases CD8+ cells. Melatonin may act through the immune-opiod network. The regulation of immune function by melatonin appears to involve cAMP signal transduction, L-type Ca2+ channels and glutathione. The seasonal changes in immune function observed in animals and humans are likely to be mediated by the changes in the duration of melatonin secretion.