Our immune system, certainly one of the great marvels of nature, equips us with 100 billion (100,000,000,000) immunological receptors. Each of these tiny receptors has the uncanny natural capacity to distinguish the self from the nonself. Consequently, they are able to immunize or protect our bodies against the invasion of foreign substances that could be harmful to us.
Marvelous as nature is, it is never extremist. From a purely immunological point of view (from the standpoint of an all out defensive strategy), a woman’s body would reject the oncoming sperm, recognizing it as a foreign substance. But this is precisely the point at which nature, we might say, becomes wise. If our immune system regards sperm as a potential enemy, then fertilization would never take place, and the human race would have come to an early demise with the passing of Adam and Eve.
But something extraordinary occurs that makes fertilization and the continuation of the human race possible. Traveling alongside the sperm in the male’s seminal fluid is a mild immunosuppressant. Immunologists refer to it as consisting of “immunoregulatory macromolecules.” This immunosuppressant is a chemical signal to the woman’s body that allows it to recognize the sperm not as a nonself, but as part of her own self. It makes possible, despite the immune system’s usual preoccupation with building an airtight defense system, a self-to-self union or, from an immunological perspective, a “two-in-one-flesh” intimacy.
Source: Donald DeMarco. “Can Immunology Corroborate the Two-in-One-Flesh Image in Genesis?” Linacre Quarterly Volume 77, Number 2 (May, 2010): 181-185.