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The immune system is the body's natural defense system that helps fight infections. The immune system is made up of antibodies, white blood cells, and other chemicals and proteins that attack and destroy substances such as bacteria and viruses that they recognize as foreign and different from the body's normal healthy tissues. The immune system also includes:

• The tonsils and thymus, which make antibodies.
• The lymph nodes and vessels (the lymphatic system). This network of lymph nodes and vessels throughout the body carries lymph fluid, nutrients, and waste material between the body tissues and the bloodstream. The lymphatic system is an important part of the immune system. The lymph nodes filter lymph fluid as it flows through them, trapping bacteria, viruses, and other foreign substances, which are then destroyed by special white blood cells called lymphocytes.
• Bone marrow. This is soft tissue found mainly inside the long bones of the arms and legs, the vertebrae, and the pelvic bones of the body. It is made up of red marrow, which produces red and white blood cells and platelets, and yellow marrow, which contains fat and connective tissue and produces some white blood cells.
• The spleen, which filters the blood by removing old or damaged blood cells and platelets and helps the immune system by destroying bacteria and other foreign substances.
• White blood cells. These blood cells are made in the bone marrow and protect the body against infection. If an infection develops, white blood cells attack and destroy the bacteria, virus, or other organism causing it.