Cells, tissues, and organs composing the immune system , including the bone marrow , thymus , spleen , and lymph node s.
The most highly organized components are the thymus and lymph nodes, and the least organized are the cells that wander in the loose lymphocyte production centre) in response to antigen s. The most common lymphoid tissue cell is the lymphocyte. Others are macrophages, which engulf foreign materials and probably alter them to initiate the immune response, and reticular cells, which produce and maintain thin networks of fibres as a framework for most lymphoid organs. See also immunity ; lymphatic system .