Re: Why doesn't a woman's body immunologically reject male sperm.

Your question is very good.  I've done a bit of literature searching for 
the answer, and it was difficult to find a precise answer.  What is known 
is the following:

Seminal fluid appears to contain a number of factors which suppress the 
immune response of the female from attacking the sperm.  Some of these 
factors that have been suggested are:

1) Certain cytokines, possibly TGF beta.  This is a normal chemical of the 
immune system which can suppress T cell and macrophage responses.

2) Prostaglandins, which may also suppress the immune response.

3) Proteins specific to semen which block T cell responses.

4) Proteins which block complement-mediated destruction of sperm.

The sperm themselves appear to be special in that they lack expression of 
MHC molecules, which are necessary for T cell mediated rejection.  The 
surface of the sperm also has molecules which deactivate complement.

Hope this helps.