Lymphoma is a type of cancer that originates in lymphocytes of the immune system. They often originate in lymph nodes, presenting as an enlargement of the node (a tumor). Lymphomas are closely related to lymphoid leukemias, which also originate in lymphocytes but typically involve only circulating blood and the bone marrow (where blood cells are generated in a process termed haematopoesis) and do not usually form static tumours. There are many types of lymphomas, and in turn, lymphomas are a part of the broad group of diseases called hematological neoplasms.
Thomas Hodgkin published in 1832 the first description of lymphoma, specifically of the form named after him, Hodgkin's lymphoma. Since then many other forms of lymphoma have been described, grouped under several proposed classifications. The 1982 Working formulation classification became very popular. It introduced the category non-Hodgkin lymphoma (NHL), itself divided into 16 different diseases. However, since these different lymphomas have little in common with each other, the NHL label is of limited usefulness for doctors or patients and is slowly being abandoned. The latest classification by the WHO (2001) lists 43 different forms of lymphoma divided in four broad groups.
Some forms of lymphoma are indolent (e.g. small lymphocytic lymphoma), compatible with a long life even without treatment, whereas other forms are aggressive (e.g. Burkitt's lymphoma), causing rapid deterioration and death. The prognosis therefore depends on the correct classification of the disease, established by a pathologist after examination of a biopsy.
Although older classifications referred to histiocytic lymphomas, these are recognized in newer classifications as of B, T or NK cell lineage. True histiocytic malignancies are rare and are classified as sarcomas.