What in the body makes blood clot?

Multiple : Platelets, proteins called factors, and red blood cells all make up a clot.

Cells and proteins: Clotting is initiated by cells called platelets that are made in the bone marrow and circulated everywhere. Platelets are attracted to an area of injury and aggregate to each other and adhere to the damaged tissue. They release a signal in the serum which attracts a cascade of proteins that criss-cross and weave fibrin which completes the clot.