Sperm cells are not normally exposed to blood, but if they are, the immune system may produce against sperm cells' and render someone sterile.

Sperm cells are not normally exposed to blood, but if they are, the immune system may produce __auto-antibodies__ against sperm cells' __auto-antigens_ and render someone sterile.

Explanation:

Auto-antibodies are antibodies that can destroy cells from the same organism. They are mistakenly directed against organs or tissues of the same organisms, causing auto-immune diseases or affections. The autoantibodies action occurs when the organism fails in recognizing its own cells and faces them as strange elements.

Auto-antigen is the molecule produces by the body that fails in being recognized due to a break up in the tolerance mechanisms, triggering an immune response.

In the exposed example, sperm cells do not usually mix with blood. So when they do, blood cells recognize them as strange bodies and act against them. Sterility might be an effect of this auto-immune response.