While some species develop a local immune response, sending special protective cells to the uterus itself, mare don’t, researchers learned recently. Rather, they appear to send those cells elsewhere as soon as semen enters the uterus. Where they go, nobody knows (yet). | Photo: iStock

Stallion semen is an essential part of breeding a mare. But it’s also a foreign body entering a mare’s uterus. The natural immune response to that foreign body could determine the success of the resulting pregnancy. But how exactly do mares react to this foreign substance?

While some species develop a local immune response in the uterus itself, calling special protective cells to the “battlefield front,” mare don’t, researchers in Austria learned recently. Rather, their reaction involves sending those same cells—T regulatory cells, or Tregs—elsewhere as soon as semen enters the uterus. Where they go, nobody knows (yet). But researchers know they’re pulled out of circulating blood to concentrate in some part of the body.

“It seems that exposure of the endometrium (the uterus lining) to semen attracts Tregs from the blood, and they are probably involved in some immune response,” said Christine Aurich, DVM, PhD, head of the Graf Lehndorff Institute, in Neustadt, Germany, and professor of artificial insemination and embryo transfer in the Vetmeduni veterinary school Department for Small Animals and Horses, in Vienna, Austria