Lymphoid (or lymphatic) tissues, which mainly consist of dense accumulations of lymphocytes, are widely distributed in the body. Lymphoid tissues are typically located at sites that provide a possible route of entry of pathogens and/or sites that are liable to infections. Epithelia delimit all other tissues from the "outside world" and it is not surprising that lymphoid tissues are often found near them. Such lymphoid tissues are grouped together as epithelium-associated lymphoid tissues. Depending on their precise location these lymphoid tissues may be referred to as e.g. mucosa-associated lymphoid tissue (MALT) or bronchus-associated lymphoid tissue (BALT). The tonsils or Peyer's patches are examples of mucosa-associated lymphoid tissues. Lymphoid tissues represent the sites of proliferation and differentiation of lymphocytes.
Lymphoid organs may be defined as anatomical "entities" which consists chiefly of lymphoid tissues. The thymus is a primary lymphoid organ in that it supplies other lymphoid organs and tissues with T-lymphocytes. Inserted into the blood and lymph vascular system, the spleen and lymph nodes (secondary lymphoid organs) monitor the internal environment of the body.
