The good guys
The players in the immune system include:

• Lymph — a clear fluid that travels through the lymphatic system and carries cells that help fight infections and other diseases.

• Lymph nodes — rounded masses of lymphatic tissue that is surrounded by a capsule of connective tissue. Lymph nodes filter lymph, and they store white blood cells.

• Lymph vessels — thin tubes that carry lymph and white blood cells through the lymphatic system. They branch, like blood vessels, into all the tissues of the body.

• The thymus — an organ in the chest behind the breastbone. T lymphocytes grow and multiply in the thymus.

• The spleen — an organ on the left side of the abdomen, near the stomach. It produces some white blood cells, filters the blood, stores blood cells, and destroys old blood cells.

• White blood cells — cells are made by bone marrow and help the body fight infection and other diseases. There are lots of types of white blood cells.

The Enemy

• Antigen—a foreign substance that causes a response in the immune system. Antigens can be bacterium, viruses, etc. There's a different antigen for every cold that you've ever had and every type of flower that's ever made you sneeze.

How it works

White blood cells patrol the body. When they come across an antigen, they produce an antibody. The antibody binds to the antigen. Each antigen is shaped differently. The immune system has to produce the antibody that fits it exactly. Some antibodies destroy antigens when they bind with them. Others make it easier for white blood cells to destroy the antigen.