Abstract: The present invention provides a ligand isolated from Moraxella catarrhalis outer membrane protein which binds to CEACAM receptors, said ligand comprising a receptor binding domain comprising an amino acid sequence selected from the group disclosed, or a fragment, homologue, functional equivalent, derivative, degenerate or hydroxylation, sulphonation or glycosylation product or other secondary processing product thereof. The invention also provides medicaments and vaccines comprising said ligand, and their use in the treatment or prophylaxis of infection. Also provided is a screening method for the identification of novel therapeutic compounds.