lymphocytes inside them. If large numbers of foreign substances are filtered through a lymph node or series of lymph nodes, swelling may occur and the nodes may become tender to the touch. Most swollen lymph nodes are a reaction to infection and are not cancerous. Lymph nodes can also become enlarged in states of inflammation, such as in autoimmune diseases like rheumatoid arthritis. How Does the Immune System Work? The immune system is the body’s defense against things that might cause it harm. The immune system is made up of a network of cells, tissues, and organs that work together to detect and destroy invaders, such as bacteria, viruses, and parasites, that can make people sick. The immune system provides two different types of immunity: ■ Innate (meaning “inborn” or “natural”) immunity — This type of immunity is provided by natural barriers in the body, substances in the blood, and specific cells that attack and kill foreign cells. Examples of natural barriers include skin, mucous membranes, stomach acid, and the cough reflex. These barriers keep germs and other harmful substances from entering the body. Inflammation (redness and swelling) is also a type of innate immunity. Blood cells that are part of the innate immune system include neutrophils, macrophages, eosinophils, and basophils. 13 Understanding Hodgkin Lymphoma ■ Adaptive (meaning adapting to external forces or threats) immunity — This type of immunity is provided by the thymus gland, spleen, tonsils, bone marrow, circulatory system, and lymphatic system. B cells and T cells, the two main types of lymphocytes, carry out the adaptive immune response by recognizing and either inactivating or killing invading organisms. The adaptive immune system can then “remember” the identity of the invader, so that the next time the body is infected by the same invader, the immune response will develop more quickly and strongly. What Is Lymphoma? A lymphoma is a type of cancer that originates from lymphocytes in the lymph nodes and other tissues in the lymphatic system. There are two major categories of lymphomas: Hodgkin lymphoma (HL) and non-Hodgkin lymphoma (NHL). Both of these categories are further subdivided into numerous types that differ in the way they develop and spread, whether they come from B cells or T cells or NK cells, as well as in the way they are treated. Unlike other cancers, therapy and prognosis are not based solely on the stage at which the disease is diagnosed but rather are determined by the lymphoma type, in addition to a variety of other factors like age and other medical issues. What Is Hodgkin Lymphoma? Hodgkin lymphoma (previously called Hodgkin’s disease or Hodgkin disease) is a type of cancer that starts in the lymphocytes. HL is named after Dr. Thomas Hodgkin, a British physician who first described the disease in 1832. In the United States, approximately 8,260 people a year are diagnosed with HL. Although both children and adults can develop HL, the disease is most common among young adults aged 20 to 34. Learning the Basics 14 Part 1 How Does HL Develop? HL develops when abnormal (cancerous) B cells called ReedSternberg (RS) cells start to multiply and grow in an unregulated manner. Most patients with HL have either RS cells or RS-cell variants in their lymphatic tissue. When examined under a microscope, RS cells are usually surrounded by large numbers of inflammatory cells such as T cells, histiocytes, eosinophils, and neutrophils. For this reason, HL was not initially recognized as a cancer; for years doctors thought of HL as a type of infection. A REED-STERNBERG CELL The presence of cells that look like RS cells does not necessarily mean that a person has HL. In fact, RS-like cells can be found in many other conditions, some of which are benign (not cancerous). However, one characteristic that is unique to most types of HL is that the RS cells have an antigen (marker) on their surfaces called CD30. To make an HL diagnosis, a hematopathologist (a doctor who specializes in the Reed-Sternberg cell Normal lymphocyte 15 Understanding Hodgkin Lymphoma diagnosis of blood diseases) examines a sample of lymphatic tissue under a microscope and