"Fraternal birth order" (FBO) effect is a theory that has strong proof of its prenatal origin. No proof so far links this to an exact prenatal process. Tests give the idea that this may be from the immune system. This is a immune reaction from the mother against a substance important to the development of the male fetus. This reaction is even more probable after every successful male birth. It is thought that this immune effect changes future males prenatal development. This process is the maternal immunization hypothesis (MIH). This starts when cells from a male fetus enter the circulation of the mother. The immune system of the mother cannot support the Y proteins because she is female. This causes the development of antibodies. An antibody travels through the placenta into the section with the fetus. From here, an antibody crosses the blood–brain barrier of the brain of the fetus. This alters sex differences in brain structures relative to sexual orientation. This cause the son to experience more attraction to a man than a woman.