immune response towards sperm cells
Body does not act against sperms because the testes are isolated immunologically by the blood testis barrier.

In mumps due to this barrier becoming ineffective due to inflammation, orchiditis (inflammation of testis) may occur and this may render the person infertile in some cases.

Such antigens which are not exposed to body's immune system are called cryptic antigens.

One more example of cryptic antigen is the antigens of the cornea.

For more information see here.

For more you can search for blood testis barrier.

For the second question, immune reaction will occur if the injected cell contains cryptic antigens. If there are no cryptic antigens then there will be no reaction.

For cryptic antigens see here.