A term used to describe a number of neoplasms, at different stages of maturity, originating in lymphoid cells. The lymphatic system is a network of organs (spleen, thymus), lymph nodes, ducts and lymphatic vessels that produce and transport lymph from the tissues to the blood stream. It is a fundamental part of the body's immune system. There are two main types of lymphocyte in the blood called B lymphocytes (B cell) and T lymphocytes (T cell).

- B lymphocytes: account for 5 - 15% of all lymphocytes and give rise to plasma cells that produce antibodies which help the body protect itself against bacteria and viruses.

- T lymphocytes: are responsible for coordinating the immune cell response and have the role of cooperating to develop all the forms of the immune response, as well as the production of antibodies by the B lymphocytes.

B and T circulating lymphocytes derive, like all the other blood cells, from the hematopoietic stem cells in the bone marrow. The stem cell gives rise, amongst others, to the lymphoblasts (less mature lymphoid cells) which, after successive stages of maturation, become lymphocytes. The neoplastic transformation of these cells can occur during any of these stages of the lymphocyte's maturation and this is what leads to the various kinds of lymphoproliferative syndrome (LPS). It also explains the great variety and biological and clinical heterogeneity of these tumours.

For the purposes of clarification the following large groups of lymphoid neoplasms are usually distinguished:

— Immature lymphoid cell neoplasms

— Acute lymphoblastic leukaemia (various subtypes, see ALL)

— Mature lymphoid cell neoplasms

— Non-Hodgkin lymphoma (NHL) (more than 20 types, see table)

— Other LPS with leukaemic expression (hairy cell leukaemia, splenic marginal zone lymphoma, prolymphocytic leukemia, granular lymphocytic leukaemia, amongst others)

— Monoclonal gammopathies (multiple myeloma, Waldenström macroglobulinemia, primary amyloidosis, heavy chain disease, amongst others)

— Chronic lymphocytic leukaemia (LLC)

— Hodgkin lymphoma (LH) (various subtypes, see table)

Furthermore, many of these diseases can be sub-classified according to their origin in B or T cells.

Consequently, the classification proposed for lymphoid neoplasms by the WHO (2008) is rather complex: