CD4 and cancer: Immunogenic HSP–peptide complexes are displayed to a larger extent on cancer cells after heat treatment at least in some cancers (Udono and Srivastava, 1993; Multhoff et al, 1995), an interesting feature since these complexes can activate natural killer (NK) cells (Botzler et al, 1996), offering a second defence line independent of MHC-restricted immunogenicity provided by CD4+/CD8+ T cells.