Abstract: The invention provides methods for stimulating the immune system using antibodies capable of activating immune cells, such as T cells, the antibodies having reversibly inhibited antigen binding sites. Selective activation of the antigen binding sites at a site where an immune response is required (e.g. by irradiation) leads to a corresponding activation of the immune response at that site. Surprisingly good results are achieved even though the antibodies do not possess any targeting moiety which causes them to selectively accumulate at the site where immune cell activation is required.