Provided are: a method for enhancing immune cell function by activating ex vivo various immune cells； and immune cells with enhanced function. Further provided is a method for assessing cell multifunctionality related to immunity. The multifunctionality of immune cells can be enhanced using a biguanide-based antidiabetic medication selected from any one of metformin, phenformin and buformin to increase CD8+ T cells, which have a high capacity for producing IL-2, TNFa, and IFN?. Cell multifunctionality related to immunity can be assessed by comparing the multifunctionality of immune cells treated with a biguanide-based antidiabetic medication selected from any one of metformin, phenformin and buformin, and control immune cells not treated with the biguanide-based antidiabetic medication. If the immune cells treated with a biguanide-based antidiabetic medication selected from any one of metformin, phenformin and buformin have significantly improved multifunctionality compared to the control cells, said immune cells can be assessed as having improved sensitivity to said drug.
