The result of a reaction of the immune system against sperm cells (spermatozoa) and, by limiting the ability of sperm to show motility, a contributory (or occasionally the only) cause of infertility. Can be present in serum, in cervical mucus or in semen. Can be agglutinating (which make sperm stick in clumps), immobilising (which cripple sperm particularly effectively and kill them), or coating, which interfere with sperm attachment to the zona pellucida of the egg (the secondary oocyte), preventing fertilisation. The screening test for sperm antibodies involves immunobeads. See also: serum sperm antibodies