In two seconds I found this ( I found a lot of sites on both sides of this question):

The relevance of melatonin in partially or wholly restoring optimal function, in a series of disorders related to immune dysfunction, is addressed in this report. This includes the potential relief of both autoimmune diseases and many other ailments involving abnormal immune responses, including the overall diminished effectiveness of body defenses occurring with aging.
https://pubmed.ncbi.nlm.nih.gov/32651969