Considering the multiple and important immunoregulatory and muscle functions of the myokine IL-15 [22,[24], [25], [26], [27]], which decisively influences the muscle-immune crosstalk, it is not surprising that a deterioration of skeletal muscle homeostasis, e.g. due to aging, which interestingly is associated with a reduction of IL-15 serum levels [40,41] but also with vitamin D inadequacy [59], also affects the function of the immune system and can lead to sarcopenia and immunosenescence, states of reduced quantity and function of IL-15 [22,59,60]. This evidence concerns the gene IL15 and sarcopenia.