To be immune (say: ih-MYOON) means to be protected. So it makes sense that the body system that helps fight off sickness is called the immune system. The immune system is made up of a network of cells, tissues, and organs that work together to protect the body.

White blood cells, also called leukocytes (say: LOO-kuh-sytes), are part of this defense system. There are two basic types of these germ-fighting cells:

• phagocytes (say: FAH-guh-sytes), which chew up invading germs
  
• lymphocytes (say: LIM-fuh-sytes), which allow the body to remember and recognize previous invaders

Leukocytes are found in lots of places, including your spleen, an organ in your belly that filters blood and helps fight infections. Leukocytes also can be found in bone marrow, which is a thick, spongy jelly inside your bones.

Your lymphatic (say: lim-FAH-tik) system is home to these germ-fighting cells, too. You've encountered your lymphatic system if you've ever had swollen "glands" on the sides of your neck, like when you have a sore throat. Although we call them "glands," they are actually lymph nodes, and they contain clusters of immune system cells. Normally, lymph nodes are small and round and you don't notice them. But when they're swollen, it means your immune system is at work.

Lymph nodes work like filters to remove that could make you sick. Lymph nodes, and the tiny channels that connect them to each other, contain lymph, a clear fluid with leukocytes (white blood cells) in it. Beside your neck, where else do you have lymph nodes? Behind your knees, in your armpits, and in your groin — just to name a few.

So you have this great system in place. Is it enough to keep you from getting sick? Well, everyone gets sick sometimes. But your immune system helps you get well again. And if you've had your shots (also called vaccines), your body is extra-prepared to fight off serious illnesses that your immune system alone might not handle very well. If you get the shot that covers measles, for instance, it can protect you from getting measles, if you're ever exposed to it.

Immune System Problems

Sometimes a person has a problem with his or her immune system. Allergies are one kind of problem — the immune system overreacts and treats something harmless, like peanuts, as something really dangerous to the body.

With certain medical conditions, such as lupus or juvenile rheumatoid arthritis, instead of fighting germs, the immune system fights the good cells and this can cause problems. Other immune system problems may develop due to an illness like HIV/AIDS or cancer.

You can't prevent most immune system disorders. But if they happen, they can be treated with medicine and in other ways to help the person feel good and be healthy again. If you have an immune system problem, your doctor can help teach you ways to take care of yourself so you stay strong and are able to fight off illness. Immunologists (say: ih-myoon-ALL-oh-jists) are doctors who specialize in immune system problems.

Healthy kids can help their immune systems by washing their hands regularly to prevent infections, eating nutritious foods, getting plenty of exercise, getting enough sleep, and getting regular medical checkups. And if you feel great today, thank your immune system!

Reviewed by: Yamini Durani, MD
Date reviewed: October 2012