The present invention relates to isolated protein sequences that correspond to cell binding peptides, fragments, neo-structures and/or neo-epitopes of a normally occurring serum protein present in human tissue, wherein the peptide, fragment, neo-structure and/or neo-epitope has an immunoregulatory activity and is the result of either an enhanced proteolytic activity and/or conformational changes in a tissue, or a malignant tumour. In the present patent application, a common structure of several of these peptides, fragments, neo-structures and/or neo-epitopes, having immunoregulatory activity by binding to receptors on immune cells, has been identified. The present invention further also relates to monoclonal and/or polyclonal antibodies directed to a cell binding fragment of a normally occurring serum protein present in human tissue, as described above.