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Just as blood vessels carry blood, lymph vessels carry lymph. Lymph vessels are very thin, and usually you can't see them.

Lymph is a fluid that surrounds tissues and organs. It is taken up by the lymph vessels and flows back into the bloodstream. This fluid:

• Carries waste products from around your body
• Carries cells that are part of the immune system and help fight infections
• Contains protein.

The lymph vessels take the fluid to your lymph nodes. These small collections of cells are part of your immune system. They are found all over your body, such as in your groin, the backs of your knees and under your chin. They are small, round or oval lumps that you can't usually feel unless you are skinny or have an infection that has made them swollen.

Lymph fluid from the breast travels to your armpit (which doctors sometimes call the axilla). If there are cancer cells in your breast, they may be picked up by the lymph fluid and carried to the lymph nodes in your armpit. The cancer cells may continue growing in your armpit or travel on through your lymph vessels to other parts of your body. These cancer cells may die off or start growing.

About 5 percent of the lymph fluid from your breast goes to another group of lymph nodes called the internal mammary chain. These lymph nodes lie under your breastbone in the center of your chest.

You can't tell if breast cancer has spread to the lymph nodes in your armpit unless at least one of them is removed by surgery and checked for cancer cells under a microscope.

Breast cancer cells can also be picked up by blood vessels in the breast and can travel around the body in your bloodstream. They may then grow somewhere else in the body. This is why women with breast cancer are sometimes given drugs that treat the whole body. This type of treatment (known as systemic treatment) reduces the chance that the cancer will appear somewhere else in your body.

This information is for educational use only, and is not a substitute for prompt professional medical advice. Readers should always consult a physician or other professional for advice and treatment. ©BMJ Publishing Group Limited 2008. All rights reserved.