Why are surface antigens on sperm cells not recognized as “self,” and why do they require a blood-testis barrier to prevent antibody formation?

Sperm cells are recognised as foreign cells by the immune system and requires blood testis barrier to prevent antibody formation.

Explanation:

Shortly after birth, the immune system of an individual starts to recognize the surface antigens of its body cells as 'self' and avoids forming an antibody against it. Spermatozoa (sperm cells) are formed at puberty, between the age of 10 and 14. Hence, the body immune system will recognise the antigen on the sperm cells as 'non-self' and stimulate antibodies against it. This is prevented by the blood testis barrier.

The blood testis barrier which are formed from the Sertoli cells at the seminiferous tubules helps in the isolation of the sperm cells and protects it from immune surveillance. This barrier also prevents sperm cells from coming in contact with blood.