An agent that inhibits the classical complement pathway, the alternative complement pathway and the lectin complement pathway for use in a method of treating or preventing a complement-mediated disease and/or disorder in a subject with a complement C5 polymorphism at the Arg885 residue of wildtype C5 which decreases the effectiveness of eculizumab, wherein said method comprises the steps of: 1) identifying a subject with a complement C5 polymorphism at the Arg885 residue of wildtype C5 which decreases the effectiveness of eculizumab; and 2) identifying an agent that inhibits the classical complement pathway, the alternative complement pathway and the lectin complement pathway in said subject; and 3) administering to said subject a therapeutically or prophylactically effective amount of said agent identified in step 2, wherein the agent is: (i) a protein comprising or consisting of amino acids 19 to 168 of the amino acid sequence in SEQ ID NO: 2 or is a functional equivalent of this protein; (ii) a protein comprising or consisting of amino acids 1 to 168 of the amino acid sequence in SEQ ID NO: 2 or is a functional equivalent of this protein; or (iii) a nucleic acid molecule encoding a protein as recited in (i) or (ii); wherein the complement-mediated disease and/or disorder is selected from acute pancreatitis, age related macular degeneration, atypical haemolytic uremic syndrome, Alzheimer's disease, Huntington's disease, Parkinson's disease, allergic encephalomyelitis, allotransplatation, asthma, adult respiratory distress syndrome, influenza, burn injuries, Crohn's disease, glomerulonephritis, haemolytic anaemia, haemodialysis, hereditary angioedema, ischaemia reperfusion injuries, multiple system organ failure, multiple sclerosis, myasthenia gravis, myocardial infarction, paroxysmal nocturnal haemoglobinuria, psoriasis, rheumatoid arthritis, septic shock, systemic lupus erythematosus, stroke, thrombotic thrombocytopaenic purpura, traumatic brain injury, vascular leak syndrome, and transplantation rejection and graft versus host disease, peripheral nerve disorders and respiratory disorders, and wherein the functional equivalent of (a) a protein comprising or consisting of amino acids 19 to 168 of the amino acid sequence in SEQ ID NO: 2; or (b) a protein comprising or consisting of amino acids 1 to 168 of the amino acid sequence of SEQ ID NO: 2; is (c) a homologue having greater than 80% sequence identity to the protein as defined in (a) or (b); or (d) a fragment of a protein, wherein the protein has the amino acid sequence of SEQ ID NO: 2, or a fragment of a homologue, wherein the homologue has greater than 80% identity to the sequence of amino acids 19 to 168 or amino acids 1 to 168 of the amino acid sequence SEQ ID NO: 2, wherein said functional equivalent retains the ability to bind complement C5 and to prevent the cleavage of complement C5 by C5 convertase into complement C5a and complement C5b-9. 1) identifying a subject with a complement C5 polymorphism at the Arg885 residue of wildtype C5 which decreases the effectiveness of eculizumab; and 2) identifying an agent that inhibits the classical complement pathway, the alternative complement pathway and the lectin complement pathway in said subject; and 3) administering to said subject a therapeutically or prophylactically effective amount of said agent identified in step 2, wherein the agent is: (i) a protein comprising or consisting of amino acids 19 to 168 of the amino acid sequence in SEQ ID NO: 2 or is a functional equivalent of this protein; (ii) a protein comprising or consisting of amino acids 1 to 168 of the amino acid sequence in SEQ ID NO: 2 or is a functional equivalent of this protein; or (iii) a nucleic acid molecule encoding a protein as recited in (i) or (ii); (i) a protein comprising or consisting of amino acids 19 to 168 of the amino acid sequence in SEQ ID NO: 2 or is a functional equivalent of this protein; (ii) a protein comprising or consisting of amino acids 1 to 168 of the amino acid sequence in SEQ ID NO: 2 or is a functional equivalent of this protein; or (iii) a nucleic acid molecule encoding a protein as recited in (i) or (ii); (a) a protein comprising or consisting of amino acids 19 to 168 of the amino acid sequence in SEQ ID NO: 2; or (b) a protein comprising or consisting of amino acids 1 to 168 of the amino acid sequence of SEQ ID NO: 2; is (c) a homologue having greater than 80% sequence identity to the protein as defined in (a) or (b); or (d) a fragment of a protein, wherein the protein has the amino acid sequence of SEQ ID NO: 2, or a fragment of a homologue, wherein the homologue has greater than 80% identity to the sequence of amino acids 19 to 168 or amino acids 1 to 168 of the amino acid sequence SEQ ID NO: 2,