Abstract: A modified immunoglobulin (Ig) molecule incorporates, preferably in one or more non-CDR loops, one or more foreign antigenic peptides such as a ras peptide. The antigen binding site of the immunoglobulin preferably recognises dendritic antigen presenting cells (APCs). The modified Ig can thus be taken up by dendritic APCs and the foreign antigenic peptide presented on MHC II to naive T-helper cells which stimulate cytotoxic T-cells via the production inter alia of IL-2. Modified Igs of the invention can be used to stimulate the immune system which has apparently become tolerant of a mutant protein, e.g., in the case of certain types of cancer, or it could be used for vaccination against viral infections. The modified Ig can be expressed from recombinant host cells from which it is secreted, notwithstanding the presence of the foreign pepide in a loop of the molecule.