The lymph system and lymphoid tissue. To know what lymphoma is, it helps to understand the body’s lymph system. The lymph system (also known as the lymphatic system) is composed mainly of lymphoid tissue, lymph vessels, and a clear fluid called lymph.Lymphoid tissue includes the lymph nodes and related organs that are part of the body’s immune and blood-forming systems, such as the spleen and bone marrow.ymphocytes. Lymphoid tissue is made up of several types of immune system cells that help the body fight infections. Most of the cells in lymphoid tissue are lymphocytes, a type of white blood cell. The main types of lymphocytes are B lymphocytes (B cells) and T lymphocytes (T cells).