Why are sperm cells not attacked by the female immune system?
Studies show semen does mount a considerable immune response in the female mucosa, however:


  Seminal plasma redirects the differentiation of human dendritic cells (DCs) toward a regulatory profile. By promoting a tolerogenic profile in DCs, seminal plasma might favor fertility, but might also compromise the capacity of the receptive partner to mount an effective immune response against sexually transmitted pathogens.


Semen promotes the differentiation of tolerogenic dendritic cells.