Lymphoma is a type of cancer involving cells of the immune system, called lymphocytes. Just as cancer represents many different diseases, lymphoma represents many different cancers of lymphocytes—about 35 different subtypes, in fact.
Lymphoma is a group of cancers that affect the cells that play a role in the immune system, and primarily represents cells involved in the lymphatic system of the body.
- The lymphatic system is part of the immune system. It consists of a network of vessels that carry a fluid called lymph, similar to the way that the network of blood vessels carry blood throughout the body. Lymph contains white blood cells called lymphocytes. Lymphocytes attack a variety of infectious agents as well as many cells in the precancerous stages of development.
- Lymph nodes are small collections of lymph tissue that occur throughout the body. The lymphatic system involves lymphatic channels that connect thousands of lymph nodes scattered throughout the body. Lymph flows through the lymph nodes, as well as through other lymphatic tissues including the spleen, the tonsils, the bone marrow, and the thymus gland.
- These lymph nodes filter the lymph, which may carry bacteria, viruses, or other microbes. The lymph nodes, or glands as they may be called, filter the lymph, which may on various occasions carry different microbial organisms. At infection sites, large numbers of these microbial organisms collect in the regional nodes and produce the swelling and tenderness typical of a localized infection. These enlarged and occasionally confluent collections of lymph nodes (so-called lymphadenopathy) are often referred to as "swollen glands."
- B lymphocytes produce antibodies (proteins that circulate through the blood and lymph and attach to infectious organisms and abnormal cells). The combination attachment cell or antibody microbial organism essentially alerts other cells of the immune system recognize and destroy these intruders, also known as pathogens.
- T cells, when activated, can kill pathogens directly. T cells also play a part in the mechanisms of immune system control, to prevent the system from inappropriate overactivity or underactivity.
- After fighting off an invader, some of the B and T lymphocytes "remember" the invader and are prepared to fight it off if it returns.
Cancer occurs when normal cells undergo a transformation whereby they grow and multiply uncontrollably. Lymphoma is a malignant transformation of either lymphocytes B or T cells or their subtypes.
- As the abnormal cells multiply, they may collect in 1 or more lymph nodes or in other lymph tissues such as the spleen.
- As the cells continue to multiply, they form a mass often referred to as a tumor.
- Tumors often overwhelm surrounding tissues by invading their space, thereby depriving them of the necessary oxygen and nutrients needed to survive and function normally.
- Because of their uncontrolled growth, lymphomas can encroach on and/or invade neighboring tissues or distant organs.
- In lymphoma, abnormal lymphocytes travel from one lymph node to the next, and sometimes to remote organs, via the lymphatic system.
- While lymphomas are often confined to lymph nodes and other lymphatic tissue, they can spread to other types of tissue almost anywhere in the body. Lymphoma development outside of lymphatic tissue is called extranodal disease.
Lymphomas fall into 1 of 2 major categories. Hodgkin lymphoma (HL, previously called Hodgkin's disease) and all other lymphomas (non-Hodgkin lymphomas or NHLs).
- These 2 types occur in the same places, may be associated with the same symptoms, and often have similar gross physical characteristics. However, they are readily distinguishable via microscopic examination.
- Hodgkin disease develops from a specific abnormal B lymphocyte lineage. NHL may derive from either abnormal B or T cells and are distinguished by unique genetic markers.
- There are 5 subtypes of Hodgkin disease and about 30 subtypes of non-Hodgkin lymphoma.
- Because there are so many different subtypes of lymphoma, the classification of lymphomas is complicated and includes both the microscopic appearance and well-defined genetic and molecular rearrangements.
- Many of the NHL subtypes look similar, but they are functionally quite different and respond to different therapies with different probabilities of cure. HL subtypes are microscopically distinct, and typing is based upon the microscopic differences as well as extent of disease.
Lymphoma is the most common type of blood cancer in the United States. It is the sixth most common cancer in adults and the third most common in children. Non-Hodgkin lymphoma is far more common than Hodgkin disease.
- In the United States, about 54,000 new cases of NHL and 7000 new cases of HL were diagnosed in 2004, and the overall incidence is increasing.
- About 24,000 people die of NHL and 1400 of HL each year, with the survival rate of all but the most advanced cases of HL greater than that of other lymphomas.
- Lymphoma can occur at any age, including childhood. Hodgkin disease is most common in 2 age groups: young adults aged 16-34 years and in older people aged 55 years and older. Non-Hodgkin lymphoma is more likely to occur in older people.