The primary lymphoid organs are the thymus and red bone marrow, which produce lymphocytes, while the secondary lymphoid organs are the lymph nodes, spleen, tonsils.
The lymphoid organs are categorized as primary or secondary lymphoid organs. Primary lymphoid organs—red bone marrow and the thymus—are locations where lymphocytes form and mature. A lymphocyte is a type of white blood cell, or leukocyte, that functions in immune response. Two types of lymphocyte that form and mature in lymphoid organs are B cells and T cells. Red bone marrow is the tissue within flat bones, such as the pelvis and sternum, that forms blood cells, including lymphocytes. Red bone marrow is also found in the spongy bone tissue of long bones, such as the femur. The thymus is an organ located just below the sternum and between the lungs. It is involved in immune response and is the location where T cells mature.
A stem cell destined to become a B cell (B lymphocyte) originates and matures in the red bone marrow. In contrast, a stem cell destined to become T cell (T lymphocyte) originates in red bone marrow and then migrates to the thymus, where the cell matures. B cells and T cells recognize pathogens, such as viruses and bacteria. B cells produce antibodies that bind to pathogens and neutralize them. T cells produce enzymes that kill infected cells, thereby destroying the pathogens.
Location of the Thymus
Although lymphocytes circulate throughout the body, they accumulate in lymph nodes. A lymph node is a small, bean-shaped organ of the lymphatic system that filters lymph throughout the body and traps circulating pathogens. The high concentration of lymphocytes in the lymph nodes usually allows these pathogens to be quickly destroyed. This is one of the body's main defenses against infection. Lymph, a colorless fluid, enters the nodes through afferent vessels, which have specialized valves that prevent lymph from flowing back along the vessels. Once delivered to the node, the lymph is filtered. The filtered lymph exits the node by means of efferent vessels and circulates through the body. Lymph nodes are found throughout the body, but the greatest densities of nodes are in the neck, groin, and armpits. Swollen lymph nodes are an indication of disease, because high concentrations of pathogens may cause the nodes to swell and become inflamed. Lymph nodes also are susceptible to becoming secondary cancer sites. Cancerous cells can enter the lymphatic system and make their way to the lymph nodes, where they accumulate. For this reason, the lymphatic system is an important part of the diagnosis and treatment of cancer.
Location of Red Bone Marrow in Femur
The secondary lymphoid organs are the spleen, the tonsils, and the lacteals. The spleen is the body's largest lymphoid organ, and it acts as a filter for the blood. It is found in the abdominal cavity beneath the diaphragm. The spleen fulfills many functions in the body, including recycling old red blood cells, filtering lymph, and storing platelets and white blood cells. When acting as part of the immune system, the spleen recognizes antibody-coated pathogens, traps them, and effectively filters them out of the lymphatic and circulatory systems. The spleen also contains B cells and T cells that can destroy these pathogens, and it filters out damaged blood cells. Further, the spleen produces monocytes. A monocyte is a white blood cell that digests other body cells. When another organ is diseased or damaged, the spleen releases large quantities of monocytes. Monocytes migrate to the damaged area, where they speed healing and reduce inflammation.
Anatomy of a Lymph Node
Each tonsil is a lymphoid organ located in the back of the throat that traps inhaled or swallowed pathogens. The pharyngeal tonsils are located just beyond the soft palate. The palatine tonsils are the easiest to see, located to the sides of the throat just behind the teeth. The lingual tonsils are found just behind the tongue. The primary role of the tonsils is to capture pathogens that arrive in the throat via food or inhaled air and prevent them from spreading to the rest of the body. However, this can lead to the tonsils becoming infected themselves, which is a condition called tonsillitis. The mouth is a fertile ground for bacteria and viruses. Because it is the tonsils' function to fight these pathogens, they regularly come in contact with a variety of viruses and bacteria. Common colds, caused by viruses, and strep throat, caused by bacteria, invade and infect the delicate tissues of the tonsils. For this reason, many people have their tonsils removed—a procedure called a tonsillectomy.