The invention relates to the inhibition of blood coagulation, especially during organ rejection, and in particular the inhibition of delayed vascular rejection. The invention provides anticoagulant proteins which are anchored to cell membranes. The anticoagulant function is preferably provided by heparin, antithrombin, hirudin, TFPI, tick anticoagulant peptide, or a snake venom factor. These anticoagulant proteins are preferably prevented from being constitutively expressed at the cell surface. In particular, expression at the cell surface is regulated according to cell activation, for instance by targeting the protein to a suitable secretory granule. Expression of these proteins renders cells, tissues and organs less vulnerable to rejection after transplantation (e.g. after xenotransplantation).