Figure 2

Possible pathways for complement activation; the key factor for hyperacute rejection (HAR). The classical pathway is activated by the presence of a specific antigen. In xenografts in humans this is the alpha-gal epitope against which humans have natural antibodies. The alternative pathway is normally activated by some non-specific cell surface structures, but activation of this pathway against xenografts may be prevented if the xenograft contains either membrane cofactor (MCP or CD 46) or decay accellerating factor (H-DAF or CD 55), which both prevent conversion of C3 to C5 or CD 59, which prevents the C6 C7 C8 C9 cascade.