Melatonin and the Immune System
White blood cells have melatonin receptors and are an essential part of the immune system. Many studies have shown that melatonin has anti-inflammatory properties, while some arguing that melatonin is an immunostimulant. However both sides are correct because melatonin is actually an immune buffer. It acts as a stimulant under immunosuppressive conditions, or as an anti-inflammatory compound in the presence of exacerbated immune responses, such as acute inflammation.
Melatonin affects the immune response acting as both an activator and an inhibitor of the inflammatory process. The hormone acts as an "immunological buffer" activating impaired immunity in immunosuppression, chronic stress, or old age as well as suppressing overreaction of the immune system.
Melatonin mediates between neurohormonal and immune systems by means of the immune-pineal axis acting as a negative feedback mechanism. The axis connects development of the immune reaction with pineal activity and melatonin secretion induced by inflammatory mediators.
If you are looking for a pharmaceutical grade melatonin we carry Annie's Melatonin CR which is a controlled release capsule that has 5mg of melatonin.
Melatonin is not only synthesized by the pineal gland but also in many other organs and tissues of the body, particularly by lymphoid organs such as the bone marrow, thymus and lymphocytes. Melatonin participates in various functions of the body, among which its immunomodulatory role has assumed considerable significance in recent years. Melatonin has been shown to be involved in the regulation of both cellular and humoral immunity. Melatonin not only stimulates the production of natural killer cells, monocytes and leukocytes, but also alters the balance of T helper (Th)-1 and Th-2 cells mainly towards Th-1 responses and increases the production of relevant cytokines such as interleukin (IL)-2, IL-6, IL-12 and interferon-gamma. The regulatory function of melatonin on immune mechanisms is seasonally dependent. This fact may in part account for the cyclic pattern of symptom expression shown by certain infectious diseases, which become more pronounced at particular times of the year. Moreover, melatonin-induced seasonal changes in immune function have also been implicated in the pathogenesis of seasonal affective disorder and rheumatoid arthritis. The clinical significance of the seasonally changing immunomodulatory role of melatonin is discussed in this review.
References: Immunomodulation by melatonin: its significance for seasonally occurring diseases. https://www.ncbi.nlm.nih.gov/pubmed/18679047