Lymphoma is a blood (hematologic) cancer. It starts in the lymph system, which is a major part of your immune system. The lymph system helps to protect your body from infection and disease. It consists of lymph, lymphoid tissue, lymph nodes and lymph vessels (see Figure 1).

Lymph is fluid that carries cells and travels through lymph vessels. Lymphoid tissue is mostly made up of white blood cells (lymphocytes). It is in many parts of your body such as the lymph nodes, bone marrow, thymus, digestive tract, and adenoids and tonsils. Lymph vessels connect hundreds of lymph nodes, which are structures in your neck, underarm, chest, abdomen and groin that help filter substances through lymph, the clear fluid that travels through the lymphatic system and carries cells that help fight infections.

    How Lymphoma Occurs

    Lymphoma develops when lymphocytes change into cancer cells. They multiply and collect in parts of the lymph system. Lymphoma cells can cause the lymph nodes, spleen or other organs to get bigger.

    Two main types of lymphocytes can transform into lymphoma. They are B-lymphocytes (B-cells) and T-lymphocytes (T-cells). B-cells and T-cells work in different ways to defend your body against infection.

    Lymphomas are frequently divided into Hodgkin lymphoma and non-Hodgkin lymphoma. Both can arise in any lymphoid tissue.

    • Hodgkin lymphoma most often begins in lymph nodes found in the upper half of the body, such as the neck, chest and underarms. Hodgkin lymphoma often spreads from one group of lymph nodes to others in a predictable way. It can often be cured.
    • Non-Hodgkin lymphoma (NHL) is more common than Hodgkin lymphoma. NHL occurs when T-cells, B-cells or natural killer (NK) cells grow uncontrollably. It spreads in a less orderly way than Hodgkin lymphoma. More than 60 subtypes of NHL exist. They look different under a microscope and have distinct molecular features. They affect the body in a variety of ways and may require different types of treatment. The subtypes also grow and spread at diverse rates. Slow-growing types are indolent lymphomas. Fast-growing types are aggressive lymphomas. The subtype of NHL you have affects your outcome.