A novel class of embryo derived peptides are described (Preimplantation factor) that were generated synthetically and were tested on peripheral blood immune cells and shown to block activated but not basal immunity, inhibiting cell proliferation and creating a TH2 type cytokine bias. In addition PIF peptides enhance endometrial receptivity by increasing adhesion molecules expression. PIF biological activity appears to be exerted by specific binding to inducible receptors present on the several white cell lineages. PIF peptides, which are immune modulators, therefore may have diagnostic and non toxic therapeutic applications in improving fertility, reducing pregnancy loss as well may be useful when administered for the treatment of autoimmune diseases and for prevention xenotransplants rejection.