Tissue, lymphoid: A part of the body's immune system that helps protect it from bacteria and other foreign entities.
Lymphoid tissue is rich in lymphocytes (and accessory cells such as macrophages and reticular cells). The lymphoid tissue includes the lymph nodes, spleen, tonsils, adenoids and the thymus (an organ in the chest that is particularly large during infancy).
In medicine, a tissue is a broad term that is applied to any group of cells that perform specific functions. A tissue in medicine need not form a layer. Thus, the bone marrow is also a tissue.