Lymphoma is cancer of the lymphoid tissue. Lymphoid tissue comprises the lymphatic system, which is a critically important component of the immune defense system. Although other forms of cancer may spread to parts of the lymphatic system, lymphoma is the only cancer that originates in the system.

The lymphatic system consists of organs, such as the spleen, lymph nodes and lymphatic vessels that manufacture leukocytes. It is also responsible for moving lymph fluid to the bloodstream. This fluid is made of plasma and various types of white blood cells, which fight some types of infection and other threats, such as cancer. 

Lymphoid tissue is formed by various types of immune cells that work together to resist infection and other threats, such as cancer.  Lymphocytes are the main type of cell found in the lymphoid tissue. In patients with lymphoma, the lymphocytes become abnormal and grow in an uncontrolled fashion. As the cells continue to grow and divide, the lymph glands or other organs in which the lymphocytes grow begin to enlarge. The cells form tumors that begin to develop in the body. Organ function may become affected as the lymphocyte masses grow larger, making it more difficult for normal cells to function.

The two main types of lymphocytes are:

• B lymphocytes (B-cells). B-cells defend the body from bacteria and other invading threats by changing into plasma cells, which make antibodies. These antibodies then mark antigens or other invaders for destruction.
  
  
• T lymphocytes (T-cells). T-cells destroy germs, cancer cells and infected cells directly.

Both forms can develop into lymphoma (either the Hodgkin’s or non-Hodgkin’s type). However, B-cell lymphomas are more common.  

In this condition, tumors generally develop in the lymph nodes, or in some situations, in patches of lymphatic tissue in organs, such as the stomach or intestines. Lymph nodes are small, bean-shaped organs located in groups in various areas of the body, including the neck, armpit, chest and groin. Their functions include producing immune cells (e.g., lymphocytes and plasma cells), and filtering bacteria, cancer cells and other foreign material from lymph. These materials have surface proteins called antigens. When the lymph nodes recognize antigens and other foreign material in the lymph fluid, the nodes respond by enlarging and producing additional white blood cells to help combat these threats.

Because lymphoid tissue is present in many parts of the body, lymphomas can develop almost anywhere. In addition, lymphomas are capable of spreading to other areas of the body including the liver, bone marrow and spleen.

Most lymphomas develop as a result of noninherited mutations in the genes of growing leukocytes. Some forms of cancer are caused by DNA mutations that “turn on” oncogenes (genes that speed cell division) or “turn off” tumor suppressor genes (genes that slow down cell division or cause cells to die at the right time). In most people with lymphoma, these mutations are acquired after birth. The mutations may occur from exposure to radiation or cancer-causing chemicals, but many times the mutations occur for no apparent reason. Researchers have noted that they appear to happen more often as people age, making age the greatest risk factor for some forms of lymphoma.

Another defect that can cause lymphoma is a translocation. A translocation is the transfer of DNA from one chromosome to another. This abnormality can "turn on" oncogenes or "turn off" tumor suppressor genes.

According to the National Cancer Institute (NCI), lymphomas account for about 5 percent of all cases of cancer in the United States. They can occur in children and adults.

Although there is no single cause of lymphoma, researchers have identified some risk factors that can increase the chances of developing lymphoma. Regardless of type, lymphoma tends to occur more often in:

• White men
  
  
• People with certain immune deficiencies, such as infection with human immunodeficiency virus (HIV)
  
  
• Individuals who have been treated with immunosuppressive drugs, as after a kidney or other organ transplant.
  

Certain medical conditions have also been linked to lymphoma. The Epstein Barr virus (which causes infectious mononucleosis) has been identified as a risk factor for Hodgkin's lymphoma. In addition, researchers have noted high rates of hepatitis C among lymphoma patients. It is believed that infection with hepatitis C virus affects the immune system, resulting in lymphoma. A bacterium known as Helicobacter pylori also has been associated with the development of lymphoma in the stomach wall.

In general, people who have received either radiation treatments or chemotherapy for other types of cancer as a child, appear to have a higher risk of developing lymphoma later in life. There is also an apparent increase in incidences of the condition in areas where farming is prevalent. Researchers believe certain herbicide ingredients may be responsible for increasing lymphoma in this population.

Scientists also have discovered an association between some genes and the development of lymphoma. In research studies, HMG-1, an oncogene (cancer-causing gene), was linked to aggressive lymphoma and leukemia in children and adults.

Further studies have identified a gene associated with the human herpes virus 8 (HHV8) that may cause lymphoma by activating a pathway involved in the production of lymphocytes. These abnormal lymphocytes are cells that repeatedly divide and eventually develop into lymphoma. More research is necessary to determine the exact relationship between genes and the development of lymphoma

The annual rate of occurrence of lymphoma has nearly doubled over the last 35 years. The increase may be partly due to AIDS-related non-Hodgkin’s lymphoma, because the incidence of Hodgkin’s lymphoma has stabilized.  In 2007, the American Cancer Society (ACS) predicts nearly 72,000 new cases of lymphoma and nearly 20,000 deaths from the disease. These statistics include both Hodgkin’s and non–Hodgkin’s lymphoma.