Immunity is a system of organs, white blood cells and chemicals that protect you from germs, such as bacteria, viruses and fungi. It also helps your body heal from infections and injuries.

Natural immunity includes barriers, like the skin and mucous membranes, that keep harmful substances from entering the body. It also includes certain cells and proteins that recognize and destroy invading pathogens.

A special group of white blood cells, called leukocytes, patrol the blood and tissues of your body to detect foreign substances. If they find them, they send signals that activate and mobilize the immune response. Two types of leukocytes are important to the immune response: phagocytes and lymphocytes.

B lymphocytes (or B cells) and T lymphocytes are the two main types of lymphocytes in the immune system. Both are made in the bone marrow and then sent to the thymus gland, where they mature. When they encounter a germ or other pathogen, they immediately become activated and start producing antibodies that lock onto the pathogen or foreign substance and then help destroy it. They can “remember” each antigen they encounter, allowing them to respond quickly and more effectively the next time that same pathogen appears. This is why it’s possible for someone to have a single infection with a disease, such as chickenpox or hepatitis A, and gain lifelong natural immunity from that particular disease.

Another type of immunity is acquired immunity, which happens when you get sick with a disease and your immune system builds a defense against that particular antigen. This is why vaccines work, and is the reason why you need to receive a booster shot for some diseases.