There are more than 25 different types of tumors that are considered lymphomas. These tumors can be classified into two categories – Hodgkin’s lymphoma and non-Hodgkin’s lymphoma (NHL). Hodgkin’s lymphoma category, also known as Hodgkin’s disease, is named after the physician who identified it in 1832. All other types of lymphomas are considered non-Hodgkin’s lymphoma.

Cases of non-Hodgkin’s lymphoma are far more common. In 2003, an estimated 7,600 new cases of Hodgkin’s lymphoma were diagnosed in the United States, compared with 53,400 new cases of non-Hodgkin’s lymphoma.

Hodgkin’s lymphoma can usually be distinguished from NHL when tissue is examined under a microscope. The cells found in Hodgkin’s lymphoma, known as Reed-Sternberg cells, are named after the physicians who first described them in detail. Many researchers believe that Reed-Sternberg cells are a form of malignant B-cells.

Many classification systems have been developed for lymphomas, and experts disagree on exactly how they should be organized. The most recent classification system was an update of the Revised European American Lymphoma (REAL) Classification by the World Health Organization (WHO). While previous classification mainly relied on the appearance of the lymphoma cells, the updated WHO/REAL classification system relies on the genetic and chemical characteristics of the cells as well.

Hodgkin’s lymphoma is classified as:

• Nodular lymphocyte-predominant Hodgkin’s lymphoma
  
  
• Classical Hodgkin’s lymphoma:
  
  
• • Nodular sclerosis Hodgkin’s lymphoma
  • Mixed-cellularity Hodgkin’s lymphoma
  • Lymphocyte-depleted Hodgkin’s lymphoma
  • Lymphocyte-rich Hodgkin’s lymphoma
    

Types of non-Hodgkin’s lymphoma are first divided by cell type into B-cell or T-cell groups. They are then further classified by whether the tumors were derived from precursor (immature) or peripheral (mature) cells.

Non-Hodgkin’s lymphoma classification is lengthy and complicated. The most common forms of non-Hodgkin’s lymphomas include:

• B-cell chronic lymphocytic leukemia (CLL)/small cell lymphocytic lymphoma (SCLL)
  
  
• Follicular lymphoma
  
  
• Extranodal marginal zone B-cell lymphomas of mucosa-associated lymphoid tissue (MALT) type
  
  
• Hairy cell leukemia
  
  
• Diffuse large cell B–cell lymphoma (DLBCL)
  
  
• Burkitt’s lymphoma
  
  
• Precursor T-cell acute lymphoblastic leukemia/lymphoblastic lymphoma

Occasionally, lymphoma can develop from bone marrow lymphocytes. Lymphocytic leukemia is another form of cancer that develops from bone marrow lymphocytes. As a result, cases of leukemia and lymphoma can be very similar. For example in some forms of lymphoma, the malignant (cancerous) cells may be the same and the only difference is where the cancer occurs. B-cell chronic lymphocytic leukemia (CLL)/small cell lymphocytic lymphoma (SCLL) is one such disease. When the cancer is mostly in the blood, the disease is considered CLL. It is considered SCLL when the cancer is mainly in the lymph nodes. In addition, CLL can transform into a high-grade or aggressive non-Hodgkin’s lymphoma (Richter syndrome).

Precursor T-cell acute lymphoblastic leukemia/lymphoblastic lymphoma is another example. The disease can be classified as lymphoma or leukemia, depending on the amount of bone marrow cells which are lymphoma cells. When more than 25 percent of the bone marrow is involved, the disease is classified and treated as leukemia, whereas lymphoma involves less bone marrow. There is also another group of leukemias unrelated to lymphomas called myeloid and monocytic leukemias.