Claesson R, Kanasi E, Johansson A, Kalfas S. A new cleavage site for elastase within the complement component 3. APMIS 2010; 118: 765–68. The lysosomal enzyme elastase was earlier shown to cleave the complement molecule C3. During some preliminary experiments on the interactions of certain pathogenic bacteria with the innate defence mechanisms, we observed C3 cleavage, in the presence of elastase, to fragments not previously described. To elucidate this proteolytic reaction, the present study was conducted. Degradation of C3 in mixtures with elastase or cathepsin G was detected by an immunoblot procedure using anti-C3c and anti-C3d antibodies after separating the proteins by SDS-PAGE. Certain C3 fragments were analysed for amino acid sequence. The results revealed the existence of a cleavage site for elastase at the position alanine1350/lysine1351 of the C3 molecule, which has not been previously described. The fragment resulted from this cleavage has a size of about 39 kDa and it contains a part or the whole of C3d. This cleavage was distinct from the one previously described at position 987/988, which gives a 34 kDa C3d-containing fragment.