The major lymphoid organs are the lymph nodes, thymus, and spleen. There are approximately 600 lymph nodes, ranging in diameter up to 1 inch (about 25 millimeters), scattered along the lymphatic vessels. The greatest concentrations of lymph nodes are found at the neck, armpit, thorax, abdomen, and groin.

The thymus serves both as an endocrine organ that secretes hormones and as a lymphoid organ. It consists of a large number of lymphocytes, many of which become specialized T cells. The spleen is the largest lymphoid organ. It is about 5 inches (12 centimeters) long and weighs 5.6 ounces (160 grams).