Yes, so then antibodies can be present in the blood, and can even combine with sperm (outside the bloodstream) without stimulating a system-wide anti-immune response.
For this reason, vasectomies and reversals normally don't have adverse effects over their paients.
And also for this reason, most doctors can't accept the fact that someone could have an "allergy" to his sperm. It is just very rare (they think impossible).
So as you imply with your question, there must be a "gap" or point through which the semen passes to come into direct contact with the blood.
It's been mentioned a few times that some sort of imaging would be "interesting" to try to find this "gap". And in some cases I think it could probably be feasible.
However, the gap may not necesarily be so visible. In the journey from it's origin in the testicle to the tip of the penis, the sperm travels a very long distance. Much of which is through very delicate and potentially "permiable" tissue.
If at some point a type I reaction can be initiated, which is virtually superficial (internally), tissue permeability can be reduced even more by inflamation caused y IgE activity.
Perhaps where there is inflamation, there must be a method of "seeing it". But can that assure us that there is actual "transgression" through a "gap".
I'm a prime candidate for auto-immune cause. have had a vasectomy AND a reversal some 20 yrs. later. Such a long time after, in a auto-immune damaged system, a potential gap is much more likely. My POIS stared about 10 yrs after the reversal. In retrospect, I would have been surprised not to have ended up with POIS.....
I'm probably goin gto have to have work done down in that area soon, so maybe I'll see if I can take advantage of the situation to do some cool scans.
But, awaiting budget!!