Abstract: An immunogen comprising a residue of a histamine-releasing peptide comprising a cationic N-terminal head and a hydrophobic C-terminal tail, together with a residue capable of eliciting antibodies against said peptide whilst inhibiting histamine release by said peptide is useful in anti-allergy treatment. Preferably the histamine-releasing peptide is of formula:Lys-Thr-Lys-Gly-Ser-Gly-Phe-Phe-Val-Phe,optionally amidated at the C terminal.Antibodies to the histamine-releasing peptide are useful for passive immunisation.

Abstract: Peptides of the above-mentioned sequence wherein Xaa is any amino acid residue, but most preferably Val or Ile, and their simple derivatives, including those chain-extended at the N-terminus or C-terminus, are useful in anti-allergy treatment.

Abstract: The assay of rheumatoid arthritis by reference to IgA-.alpha..sub.1 antitrypsin complex present in analytes is facilitated by certain novel antibody reagents. These are ligands comprising an antibody domain specific for an antigenic determinant of a complex of human IgA and .alpha..sub.1 -antitrypsin, this antibody domain being substantially non reactive with free human IgA and free human .alpha..sub.1 -antitrypsin. Monoclonal antibodies to the naturally occurring IgA-.alpha..sub.1 AT complex and monoclonal or polyclonal antibodies to a synthetic peptide are preferred.