oKnown as immunological surveilence cells.Lymphoid tissuesThe lymphoid tissues are present as NODULESwhich are made up of loose connective tissue densely packed with LYMPHOCYTES. The nodules are around 1mm in size and have a central region called the GERMINAL CENTREwhich contains dividing LYMPHOCYTES.Lymph nodules are commonly found in the connective tissue that lines the respiratory, digestive and urinary tracts.The group of nodules associated with the digestive system is called the MUCOSA ASSOCIATED LYMPHOID TISSUE (MALT).Groups of these nodules that are found in the intestine are known as PEYERS PATCHES. The walls of the appendix (small pouch at junction of small and large intestine) also contains lymphoid nodulesThe TONSILS arealso lymphoid tissues. Most people have 5 tonsils –left and right palatine, single pharyngeal tonsil are also known as the ADENOIDS and two lingual tonsils.Lymphocytes that lie in a nodule can’t always destroy bacteria or viruses that they encounter. If the bacteria or viruses become established we get an infection of which most of you would be familiar with TONSILITIS. The treatments associated with this include antibiotics and sometimes removal of the nodules. Lymphatic organs – Lymph nodesThere are three different types of lymphoid organs. The LYMPH NODES, the THYMUS and the SPLEENThe LYMPH NODESare small organs that range from 1-25mm in diameter. They are shaped similar to a kidney bean with a small indentation called a HILLUSwhere blood vessels and nerves attach. The Lymph Nodes are connected to two types of lymphatic vessels, the AFFERENT LYMPHATICSand the EFFERENT LYMPHATICS. AFFERENT LYMPHATICSoBring lymph to the lymph node from the peripheral tissues. They enter the node on the opposite side of the HILLUS.The EFFERENET LYMPHATICSoTake lymph away from the lymph node to the veins. They leave the node at the HILLUS.The role of the lymph nodes are to act as FILTERSto remove most of the pathogens and debri from the lymph before it re-enters the circulation. We have mentioned previously how all cells have their own ANTIGENSand it is at the lymph nodes where ANTIGENS of any pathogens in the body are PRESENTED to the lymphocytes. This ANTIGEN PRESENTATIONis one of the first steps in activating the immune response.The lymph nodes also provide an early warning system thus when there is damage or pathological invasion, there is an increased activity of the lymphocytes in the lymph nodes. We often refer to ‘swollen lymph glands’ when we have a viral infection or cold.Lymphatic organs – ThymusThe THYMUSis found in the MEDIASTINUMwhich is a collective term for the central tissue mass in the thoracic cavity and includes the heart, lungs and vessels associated.