The invention relates to inhibitors that bind to C5 and C5a, but which do not prevent the activation of C5 and do not prevent formation of or inhibit the activity of C5b. One example of such an inhibitor molecule is the monoclonal antibody designated MAb137-26, which binds to a shared epitope of human C5 and C5a. These inhibitors may be used to inhibit the activity of C5a in treating diseases and conditions mediated by excessive or uncontrolled production of C5a. The inhibitor molecules are also useful for diagnostic detection of the presence/absence or amount of C5 or C5a.