rhesus

Rhesus may refer to multiple subjects in history and science.

In Greek mythology, Rhesus was a Thracian king whose forces fought for Troy during the Trojan War. In some versions of the narrative, he is slain in the Greek night raid led by Diomedes and Odysseus, and his horses are recovered by the Greeks. The episode appears in later epic traditions and has been cited in discussions of Thracian and Trojan war lore.

In biology, the name is associated with the rhesus macaque (Macaca mulatta), a primate native to South and Southeast Asia that has long been used in medical and behavioral research. The term also appears in the Rh blood group system. The Rh (Rhesus) factor refers to an antigen on red blood cells, most notably the D antigen. The presence or absence of this antigen determines Rh-positive or Rh-negative blood type. The Rh system was first identified in studies of rhesus monkey blood; in humans the D antigen is the most clinically significant. Rh incompatibility between an Rh-negative mother and an Rh-positive fetus can lead to hemolytic disease of the newborn, a risk managed today with screening and preventive treatment such as anti-D immunoglobulin. Rh typing is standard in blood transfusion and pregnancy care.

Rhesus macaques are also used as model organisms in neuroscience, infectious disease research, and vaccine development due to physiological similarities to humans.