A method to increase the immunoreactivity of cells of the immune system by reducing or inhibiting the Cbl-b function of the cells by using a short DNA or RNA sequence complementary to a part of the Cbl-b mRNA sequence. The method can be used to transiently increase the immunoreactivity of CD8+ or CD4+ T cells to a desired antigen. The inhibitory DNA or RNA sequences can be used to treat immunoinsufficiency in diseases such as AIDS, multiple myeloma, chronic lymphatic leukaemia, drug induced immunosuppression or cancer.