Lymphoma is a type of blood cancer that affects lymphocytes, the white blood cells that form part of the immune system that helps fight infections. In lymphoma, either the B lymphocytes (B-cells) or T lymphocytes (T-cells) undergo a harmful change and grow out of control. These abnormal lymphocytes crowd out the healthy cells, affecting the normal functioning of the immune system. Types of lymphoma The main types of lymphoma are Hodgkin's lymphoma (common in young adults who are 15 – 30 years old and adults over 50) and non-Hodgkin's lymphoma (more common in older people). Hodgkin's lymphoma Several types of Hodgkin's lymphoma exist, which may include rarer forms that are difficult to identify. The subtypes of lymphoma include: Nodular sclerosis – More common in young adults than in other age groups Lymphocyte-rich – More common in men Lymphocyte-depleted – More rare and aggressive Mixed cellularity – More prevalent in adult men In developing a treatment plan, accurate diagnosis and staging is important in shaping a treatment plan. The general stages of lymphoma are: Stage I – Lymphoma occurs in 1 lymph node, a group of lymph nodes, or 1 organ Stage II – Lymphoma occurs in 2 or more lymph node regions, or when it has invaded an organ and the nearby lymph nodes. The cancer sites are still limited to the same side of the diaphragm. Stage III – The cancer occurs on both sides of the diaphragm Stage IV – The cancer occurs in several sites in 1 or more organs and tissues. This is the most advanced stage of Hodgkin's lymphoma, and affects not only the lymph nodes but also other parts of the body, such as the liver, lungs or bones. Non-Hodgkin's lymphoma This is a heterogeneous group of malignancies of the lymphoid system. The most common non-Hodgkin's lymphoma is the diffuse large B-cell lymphoma. The staging of the non-Hodgkin's lymphoma is similar to Hodgkin's lymphoma.