Abstract: A group of substances derived from human leukocytes (white blood cells) are described that amplify, suppress, or otherwise modulate the response of the immune system to reintroduction of antigens. The described materials are generically effective as to antigens rather than specific as to particular antigens, but the described modulators of immune response are effective only in respect of antigens with which the recipient has previously been challenged. The described materials are extracted from leukocyte dialysates by processes involving high pressure liquid chromatography with specified resin/solvent systems. Uses and pharmaceutical compositions for use of the materials are also described.