There are two functional divisions to the body's immune system: the innate immune system and the acquired immune system. The innate immune system is your rapid-response system. It is the first to respond when it finds an invader. It is made up of the skin, the eye's cornea, and the mucous membrane that lines the respiratory, gastrointestinal, and genitourinary tracts. These all create physical barriers to help protect your body. They protect against harmful germs, parasites such as worms, or cells such as cancer. The innate immune system is inherited and is active from the moment you are born. When this system recognizes an invader, it goes into action right away. The cells of this immune system surround and cover the invader. The invader is killed inside the immune system cells called phagocytes.

The acquired immune system, with help from the innate system, makes special proteins called antibodies to protect your body from a specific invader. These antibodies are developed by cells called B lymphocytes after the body has been exposed to the invader. The antibodies stay in your body. It can take several days for antibodies to form; however, after the first exposure, the immune system will recognize the invader and defend against it. The acquired immune system changes during your life. Immunizations train your immune system to make antibodies to protect you from harmful diseases.