There are three main classes of blood cells: ■ Red blood cells (or erythrocytes) — Red blood cells carry oxygen from the lungs to all the tissues in the body. Red blood cells also remove the carbon dioxide waste produced by cells and bring it to the lungs to be exhaled. A low number of red blood cells is called anemia. A person with anemia may feel tired, weak, and short of breath. ■ White blood cells (or leukocytes) — White blood cells work as part of the immune system to help the body fight infections. The main types of white blood cells are: – Lymphocytes — These are discussed on the following page. – Granulocytes — There are three types of granulocytes: neutrophils, basophils, and eosinophils. Neutrophils help fight bacterial infections. A low number of neutrophils in the blood is called neutropenia. People with neutropenia are more likely to get infections (mostly bacterial infections) than people with normal numbers of neutrophils. Basophils are cells that take part in inflammatory reactions. Eosinophils also help fight infections— particularly those caused by parasites—and they become plentiful during allergic reactions. – Monocytes — These also play an important role in immunity and are usually the first cells to recover after an episode of neutropenia. Learning the Basics 10 Part 1 ■ Platelets (or thrombocytes) — Platelets are cell fragments produced by cells in the bone marrow. They clump together in a blood clot to stop bleeding from broken blood vessels. A low number of platelets is called thrombocytopenia. People with thrombocytopenia are more likely to bruise and bleed with minor trauma. They are also more likely to have severe and recurring nosebleeds and bleeding gums. Because blood cells have a limited lifespan, the body needs to constantly replenish its supply of these cells. Red blood cells live for about 120 days; most white blood cells have a much shorter life, ranging from a few hours to a few weeks. New blood cells are made by hematopoietic (blood-forming) stem cells, which are immature (nonspecialized) cells that can develop into any kind of blood cell. Hematopoietic stem cells are found in the bone marrow (the spongy, fatty material inside large bones such as the pelvis, vertebrae, and ribs). What Are Lymphocytes? Lymphocytes are one type of white blood cell. There are three main types of lymphocytes: ■ B lymphocytes (B cells) — B cells make antibodies to fight infections. They are called “B” cells because they were first discovered in the “Bursa of Fabricius” in birds (similar to the appendix in humans). Later, similar cells were found in humans. ■ T lymphocytes (T cells) — There are many types of T cells. Some help B cells make antibodies, some attack and kill infected cells, and others help control or regulate the way other parts of the immune system fight infections. They are called “T” cells because they develop in the thymus gland, a small organ in the front of the chest. ■ Natural killer (NK) cells — NK cells attack and kill cancer cells and virus-infected cells. They also make chemicals called cytokines that help the body get rid of viruses and tumor cells. 11 Understanding Hodgkin Lymphoma What Is the Lymphatic System? As shown in the image below, the lymphatic system is a circulatory system made up of a spidery network of thin tubes called lymph vessels or lymphatic vessels. Like blood vessels, lymphatic vessels branch out into all tissues of the body. Lymphatic vessels carry lymph, a liquid that contains lymphocytes to help fight infection. THE LYMPHATIC SYSTEM Tonsil Lymph vessels Thymus Diaphragm Spleen Lymph nodes Lymph node Vein Artery Learning the Basics 12 Part 1 Within this huge network of vessels are groups of small, bean-shaped organs called lymph nodes, which are also commonly known as “glands.” Lymph nodes filter the lymph fluid, removing bacteria, viruses, and other foreign substances from the body. Hundreds of lymph nodes are found at locations throughout the body, including the neck, underarms, chest, abdomen, and groin. Lymphocytes can mostly be found in lymph nodes, where they monitor the body’s immune system for signs of infection. The lymph nodes can change in size, becoming bigger or smaller depending on the number of