C3d is derived from human C3 after a series of proteolytic cleavages. Trypsin-like enzymes (such as plasmin, thrombin, elastase, etc.) cleave iC3b separating C3c from C3dg (38,900 Da) and C3dg is further digested releasing C3g (5,100 Da) to leave C3d. iC3b (inactivated C3b) is derived from C3b. C3b itself is produced by all three pathways of complement (Law, S.K.A. and Reid, K.B.M. (1995)) when native C3 is cleaved releasing C3a. C3d is prepared at CompTech from purified human C3. The C3 is converted to C3b by treatment with the natural human C3 convertase and this C3b is converted to iC3b by treatment with complement factors H and I. Finally, this iC3b is cleaved with trypsin to yield C3d. The C3d made by CompTech is made from soluble C3b. If the C3b precursor was attached to a surface, such as occurs during complement activation, the iC3b, C3dg and C3d fragments would remain covalently attached to that surface. C3c is released into the fluid phase. The C3d sold by CompTech is not capable of attaching to a surface. This can only occur during complement activation on a surface. Surface-bound C3d is linked to the target through an ester or an amide covalent bond. Ester bonds are unstable resulting in the gradual release of C3d from the particle.