The lymphatic system includes lymph nodes, lymph vessels, and lymph. The lymph contains many lymphocytes, which are a type of leukocyte, white blood cell, or immune cell. The lymphatic system contributes greatly to ridding an organism of foreign invasion. The invader can be bacteria, viruses, parasites, fungi, or toxins. If an invasion gets out of control, it can lead to an infection, which the lymphatic system then plays a roll in subduing.
The lymph nodes will swell when combating an infection, often being one of the confirming signs of illness due to a foreign organism.
Lymph drains through the lymph system in the direction of venous blood flow (towards the heart). It is pumped by the gross muscular movement of the body as it squeezes the intercellular spaces where the lymph moves.
People who have had cancerous tissue removed will often have lymph nodes removed as well to check for spreading of the cancer. After such removal the person must be careful to avoid injury to the body along that route of lymph as the immune response to infection is impaired (e.g. a mastectomy on the left side with removal of lymph nodes means that arm needs to be treated with caution).