The present invention relates to methods and immunonutritional compositions for preventing the impairment of the immune function during anti-cancer therapy, thereby attaining a better efficacy of the treatment. More particularly, the present invention relates to methods and immunonutritional compositions that can transiently augment or enhance the immunocompetence of an immune cell and the immunogenecity of a tumor cell of a subject undergoing anti-cancer therapy-induced apoptosis and tumor-cell-enhanced immunogenicity such that the innate and adaptive immune functions and normal physiology of the immune cell are preserved, which, in turn, lead to (i) a better tolerance and increased efficacy to anti-cancer therapy; (ii) transient augmentation or enhancement of immunocompetence of the immune cell and immunogenecity of the tumor cell; and (iii) optimization of the effects of and increase of immunocompetence of the immune cell weakened by anti-cancer therapy.