Some scientists think that parts of semen, such as sperm and seminal plasma, can make another person's immune system weaker. Experiments show that when substances in a man's semen enter another person's body, that person's body makes antibodies. Antibodies are large Y-shaped proteins used by the body's immune system to stop foreign objects from harming the body. However, the antibodies made in response to substances in semen attack one of the body's own cells, called T lymphocytes. This weakens the body's immune system.