How Estrogen and Progesterone levels affect women. These hormones play a key role in pregnancy and are also present in men.

Biologist Raven the Maven explains antibodies through rap music.

An important part of the brain's immune system, cells called microglia constantly extend and retract "branches" from their cell body to survey their environment.

Testosterone is the main sex hormone found in men. It controls male physical features.. The testes (testicles) make testosterone.