Our body's first line of defense include skin, muscles, tears from the eye, urinnation, acid in the stomach and white blood cells.. These are actually the chemical and physical barriers that fist try to defend against any infection initially. All the micro organisms that try to attack the body has to pass through these initial defense of the human body to attack the humans. Whenever this primary defense gets broken down the secondary line of defense gets activated and they try to fight the germs that have attacked.