There are several important organs that make up the immune system. While the primary lymphoid organs allow lymphocytes to proliferate and mature, secondary lymphoid organs facilitate the maturation of these cells. These organs produce specialized immune cells and are responsible for defending the body from diseases. The organs include the thymus, spleen, tonsils, and mucosa-associated lymphoid tissues.
Lymphocytes are white blood cells that are involved in the immune response. These cells are activated by foreign antigens and clonally expand. Once mature, lymphocytes travel to other parts of the body and target specific foreign antigens. Lymphocytes are located in the lymphatic system, a network of blood vessels that transport fluid from the body. The largest lymphoid organ is the spleen, located in the upper left hypochondriac region of the abdominal cavity. It serves as a filter for blood and contains both T and B-cell populations.
White blood cells have many functions. They engulf microorganisms, present antigens, and secrete cytokines. To accomplish this, circulating leukocytes must adhere to the endothelial cells lining the blood vessels. They must also pass between the layers of endothelial cells to extravasate into the tissues. Similarly, neutrophils can penetrate the endothelial layer and migrate into underlying tissues. They also act as sensors.
The thymus is located behind the breastbone and above the heart. It is a gland-like organ and only reaches full maturity during childhood. It transforms into fatty tissue afterward. Thymus cells mature and coordinate the processes of the innate and adaptive immune systems. T cells also monitor the surface of all the cells in the body. They are also responsible for memory and self/nonself recognition.
