If pig tissue were transplanted as is, molecules of complement would call immune cells to attack the foreign protein.
With the Protective Proteins
Protective proteins appear on all human cells that are in contact with the blood stream. Pigs that have been genetically engineered to produce these proteins don't trigger complement attack. Here's how the protective proteins work:
a) Pig tissue with the protective proteins MCP and DAF on the surface.
b) The complement proteins approach the tissue.
c) DAF pushes complement aside, and MCP breaks it up, before they can signal the macrophages to attack. c) The transplant survives the complement attack, and the immune system is cannot attack the graft. (At least, that's the way the system is supposed to work -- this experiment has not been done -- yet.)
