The invention relates to antibody polypeptides that monovalently bind CD40L.Antibody polypeptides that are monovalent for binding of CD40L can inhibit CD40Lactivity while avoiding potential undesirable effects that can occur with antibodiescapable of divalent or multivalent binding of CD40L. In one aspect, a monovalentanti-CD40L antibody polypeptide consists of or comprises a single immunoglobulinvariable domain that specifically binds and antagonizes the activity of CD40L,preferably without substantially agonizing CD40 and/or CD40L activity. Inanother aspect, the monovalent anti-CD40L antibody polypeptide is a human antibodypolypeptide. The invention further encompasses methods of antagonizing CD40/CD40Linteractions in an individual and methods of treating diseases or disordersinvolving CD40/CD40L interactions, the methods involving administering amonovalent anti-CD40L antibody polypeptide to the individual.
