Seminal fluid contains small molecules that act as biological signals. Once deposited in the vagina and the cervix of a woman, these persuade the woman’s immune system to adopt a profile that tolerates (that is, recognises and accepts) sperm proteins known as “transplantation antigens”.
The tolerant profile matters if fertilisation takes place. Immune cells recognise the same transplantation antigens on the developing baby, and so support the process through which the embryo implants into the wall of the uterus and forms a healthy placenta and fetus.
So over time, repeated contact with the same male partner acts to stimulate and strengthen a tolerant immune response to his transplantation antigens. The immune system of a woman responds to her partner’s seminal fluid to progressively build the chances of creating a healthy pregnancy over at least several months of regular sex.
Some forms of infertility and disorders of pregnancy are caused by immune rejection, when the process of tolerance is not adequately established.
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