Factor VIII protein. Computer molecular model of Factor VIII (antihaemophilic globulin), a protein which helps to clot blood after an injury. Absence of Factor VIII can lead to haemophilia, an inherited condition which causes abnormal bleeding. The Factor VIII molecule is extremely large and complex, being symmetrically built from three similar units known as domains (marked red, blue and green). Factor VIII is present in an inactive form in blood fluid (plasma). It is activated by complex reactions initiated by chemicals released by cells at injury sites. It helps to form fibrin protein fibres which trap blood cells to form a clot.