Human sera , old world monkeys and primates contain unique anti- Gal antibody directed against an epitope Gala1 - 3Galb1 - 4GlcNAc (a- gal ) . Antibodies with such specificity are the most common antibodies in the sera of a group of species. The anti- Gal are involved in a number of processes related to the defense of the organism , but they may also be involved in the pathogenesis of certain diseases. Anti -Gal could potentially be a first line of defense against certain parasites , viruses and bacteria ; activate the complement cascade on the surface of some tumor cells , regulate the disposal of old and abnormal red blood cells. Elevated levels of anti -Gal demonstrated in several autoimmune diseases , including Graves' disease , where these antibodies activate the thyroid follicular cells . Anti- Gal is also the first barrier for successful transplantation of organs of mammals such as swine , human, as most of the antibodies responsible for hyperacute rejection of such transplants is anti -Gal .