What Organs Make Up Your Immune System?
You probably know that your heart pumps blood. You may even know that the kidneys filter the blood and create urine. But do you know what your spleen does? Did you know you have an organ called a thymus? These last two organs, along with lymph nodes, bone marrow, and other tissues are all important parts of the immune system.
While bone marrow isn't considered an organ, it is vital to the immune system and shouldn't go unmentioned. Bone marrow is a tissue that lies inside most of your bones. It creates hematopoietic stem cells through the process of hematopoiesis. These stem cells differentiate into all sorts of blood and lymphatic cells - such as red blood cells, white blood cells, platelets, immature thymocytes, B cells, etc. - in response to hormones released by cells which make up the bone marrow. Some cells of the immune system mature in the bone marrow, and some travel to other areas of the body to complete their maturation.
The thymus is a small, butterfly-shaped organ that lies between your breastplate and your heart. You probably haven't heard of it because, after adolescence, it doesn't do much. It is fully developed at birth and grows until puberty, after which it becomes fatty and shrinks to about 15% of its maximum size. During its most active time, the thymus is responsible for directing the maturation of immature thymocytes into T cells. T cells are the managers of the immune system, instructing other cells how to react to foreign substances. During this maturation in the thymus, T cells learn to differentiate between "self" (the body's own cells) and "nonself" (foreign objects, organisms, or diseased cells). If a T cell thinks a self cell is foreign, it is destroyed, as it could cause the effects of an autoimmune disease if allowed to leave the thymus.
The spleen performs several functions; it filters the blood, destroying old or damaged blood vessels. It also contains many specialized cells (e.g. T cells) of the immune system that look for foreign particles as blood circulates. T cells also receive information from migratory dendritic cells and macrophages, which ingest foreign microorganisms and present their pieces to T cells. If a T cell recognizes a presented particle as foreign, it instructs a B cell (which also recognizes the particle) to create antibodies against it.
Lymph nodes are a bit like the spleen, but instead of filtering blood, they filter lymph. Lymph is composed of fluids which drain from tissues. It is collected at various locations throughout the body and circulates through a series of lymph nodes, eventually returning to the blood for circulation. Unwelcome microorganisms enter the lymph nodes either by circulating with lymph or by ingestion by a cell that then travels to the lymph node. Lymph nodes contain the same specialized cells found in the spleen, which sample the incoming particles and respond accordingly. These important organs make up the framework of the immune system, providing meeting locations for specialized cells to communicate.