The immune system is your body's defense system. It helps protect you from illness and infection. The immune system's job is to attack things that don’t belong in your body, including:
Your immune system fights infections and diseases. This is called an immune response. Your immune system needs to:
To do its job, your immune system needs to be able to tell what does and doesn't belong in your body. That way it knows which things to fight off and which to leave alone.
An antigen is something that doesn't belong in your body. So it triggers a response from your immune system. Antigens may be chemicals that are on the outside or inside of germs or cancer cells. Antigens may also exist on their own—for example, as food molecules or pollen. Your immune system already knows how to recognize some antigens. It has to learn to recognize other antigens.
The important parts of your immune system include:
White blood cells (leukocytes) travel through your blood to find and fight germs and other problems. Once they fight an antigen and destroy it, they can usually remember it. If your white blood cells can remember that particular antigen, they'll fight it more quickly the next time it appears in your body.
Antibodies are chemicals that certain white blood cells make. Antibodies float through the bloodstream to find and attack antigens. You have many different antibodies. Each antibody can attack only one specific antigen. Your white blood cells learn to make new antibodies every time they need to defend you from a new antigen. However, your body remembers how to make those antibodies for a long time.
The lymphatic system is a network of vessels. These vessels drain excess fluid from your tissues along with dead germs and dead cells from your body. The fluid is called lymph. Lymph passes through pea-sized collection points called lymph nodes. Lymph nodes filter out the dead germs and cells. If you have an infection, nearby lymph nodes can swell up. For example, a throat infection can make lymph nodes in your neck swell up. People call these "swollen glands," but lymph nodes aren't really glands.
Organs that are part of your immune system include your bone marrow, thymus gland, spleen, tonsils, and appendix. Your bone marrow and thymus gland make white blood cells. Your spleen, tonsils, and appendix trap germs and other antigens and serve as a place for immune system cells to get stronger.
In its fight to attack germs or other targets, your immune system releases chemicals. These chemicals cause inflammation, which is pain, redness, warmth, and swelling. For example, inflammation from a throat infection makes your throat red, swollen and sore.
Sometimes your immune system doesn't work as it should. When this happens, it may cause problems such as:
An immunodeficiency disorder, such as HIV—your immune system is deficient so it can't fight off germs properly and you get sick more easily
An allergic reaction, such as hay fever—your immune system overreacts to something harmless, such as food, plants, or medicine, which causes sneezing, a rash, hives, or trouble breathing