The spleen is an important part of the lymphatic system. It is a deep red organ situated in the abdomen caudal to the stomach (see diagram 10.3). It is composed of two different types of tissue. The first type makes and stores lymphocytes, the cells of the immune system. The second type of tissue destroys worn out red blood cells, breaking down the haemoglobin into iron, which is recycled, and waste products that are excreted. The spleen also stores red blood cells. When severe blood loss occurs, it contracts and releases them into the circulation.
The thymus is a large pink organ lying just under the sternum (breastbone) just cranial to the heart (see diagram 10.1). It has an important function processing lymphocytes so they are capable of recognising and attacking foreign invaders like bacteria.
Other lymph organs are the bone marrow of the long bones where lymphocytes are produced and lymph nodules, which are like tiny lymph nodes. Large clusters of these are found in the wall of the small intestine (called Peyer’s Patches) and in the tonsils.