ANATOMY

Detailed explanation-1: -The thyroid gland and pancreas are not considered lymphoid tissue or lymphoid organs.

Detailed explanation-2: -The primary lymphoid organs are the red bone marrow, in which blood and immune cells are produced, and the thymus, where T-lymphocytes mature. The lymph nodes and spleen are the major secondary lymphoid organs; they filter out pathogens and maintain the population of mature lymphocytes.

Detailed explanation-3: -Primary lymphoid organs: These organs include the bone marrow and the thymus. They create special immune system cells called lymphocytes. Secondary lymphoid organs: These organs include the lymph nodes, the spleen, the tonsils and certain tissue in various mucous membrane layers in the body (for instance in the bowel).

Detailed explanation-4: -The internal lymphoid organs include the thymus, spleen, bone marrow and some of the lymph nodes.