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The immune system consists of a number of lymphoid organs, including the thymus, lymph nodes, spleen, and tonsils; aggregates of lymphoid tissue in nonlymphoid organs, such as Peyer patches in the gut; clusters of lymphoid cells dispersed throughout the connective and epithelial tissues of the body, as well as throughout the bone marrow and blood; and a variety of individual cells that travel from the various lymphoid organs to the rest of the body. The lymphocytes are derived from precursors in the bone marrow: the T cells develop in the thymus and are then exported to the periphery, whereas the B and NK cells develop in the bone marrow and then go out to the periphery. Of the nonlymphoid hematopoietic cells, the monocytes, macrophages, and dendritic cells are key elements of innate and acquired immunity, whereas granulocytes (e.g., neutrophils, eosinophils, basophils, and mast cells) and platelets play important accessory roles in the immune system.


In human anatomy, the thymus is an organ located in the upper anterior portion of the chest cavity. The thymus plays an important role in the development of the immune system in early life, and its cells form a part of the body's normal immune system. It is most active before puberty, after which it shrinks in size and activity in most individuals and is replaced with fat.

The thymus is also present in many other animals. When animal thymus tissue is sold in a butcher shop or at a meat counter, thymus is known as sweetbread.

The function of the thymus in the immune system was determined by Jacques Miller. In the two thymic lobes, lymphocytes precursors mature into T cells (where T stands for “thymus”)[1]. The key function of the thymus is the selection of the T cell repertoire that the immune system uses to combat infections. This involves selection of T cells that are functional (positive selection), and elimination of T cells that are autoreactive (negative selection). The thymus will, if examined when its growth is most active, be found to consist of two lateral lobes placed in close contact along the middle line, situated partly in the thorax, partly in the neck, and extending from the fourth costal cartilage upward, as high as the lower border of the thyroid gland. The two lobes generally differ in size; they are occasionally united, so as to form a single mass, and sometimes separated by an intermediate lobe. The thymus is of a pinkish-gray color, soft, and lobulated on its surfaces. Each lateral lobe is composed of numerous lobules held together by delicate areolar tissue; the entire gland being enclosed in an investing capsule of a similar but denser structure. Thymus continues to grow until the time of puberty and then begins to atrophy.

Lymph Nodes

Lymph nodes are small bean shaped structures lying along the course of lymphatics. They are aggregated in particular sites such as the neck, axillae, groins and para-aortic region. Knowledge of the sites of lymph nodes is important in physical examination of patients. Lymph nodes have two main functions: 1. phagocytic cells act as filters for particulate matter and micro-organisms 2. antigen is presented to the immune system . The lymphatic system is composed of lymph vessels, lymph nodes, and organs. The functions of this system include the absorbtion of excess fluid and its return to the blood stream, absorption of fat (in the villi of the small intestine) and the immune system function.

The infrastructure of the lymph node is an extensive reticular network where APCs and T cells meet and interact. For example, bone marrow-derived dendritic cells in the skin pick up antigen and travel through the lymphatics to the draining lymph node. The cells then migrate through the floor of the subcapsular sinus of the lymph node to the interfollicular regions, where they settle in the reticular network of the paracortex as interdigitating dendritic cells (IDCs). T cells from the blood migrate through specialized postcapillary venules, known as high endothelial venules (HEVs), and migrate along the same reticular network, where they come in contact with the numerous antigen-presenting IDCs.14The germinal centers contain B cells, which are derived from stem cells and differentiate in the bone marrow.

The lymphoid follicles contain follicular dendritic cells (FDCs), which are not derived from the bone marrow. FDCs, which are found only in lymphoid follicles, express complement receptors CR1, CR2, CR3, and Fc receptors. These receptors enable the FDCs to present antigen to activated B cells in the form of antigen-antibody-complement complexes. FDCs can retain these complexes for a long time.

Bone Marrow

All the cells of the immune system are derived from stem cells in the bone marrow. The bone marrow is the site of origin of red blood cells, white cells (including lymphocytes and macrophages) and platelets. The lymphocytes are derived from precursors in the bone marrow: the T cells develop in the thymus and are then exported to the periphery, whereas the B and NK cells develop in the bone marrow and then go out to the periphery




A spongy, soft organ about as big as a person's fist, is located in the upper left part of the abdomen, just under the rib cage. The splenic artery brings blood to the spleen from the heart. Blood leaves the spleen and moves to the liver.

The spleen is made up of the white pulp and the red pulp, each with different functions. The white pulp is part of the infection-fighting (immune) system. It produces white blood cells called lymphocytes, which in turn produce antibodies (specialized proteins that protect against invasion by a foreign substance). The red pulp filters the blood, removing unwanted material. The red pulp contains other white blood cells called phagocytes that ingest microorganisms, such as bacteria, fungi, and viruses. It also monitors red blood cells, destroying those that are abnormal or too old or damaged to function properly. In addition, the red pulp serves as a reservoir for different elements of the blood, especially white blood cells and platelets (cell-like particles involved in clotting). However, releasing these elements is a minor function of the red pulp.




Tonsils are the two masses on the back of the throat. Tonsils and adenoids are near the entrance to the breathing passages where they can catch incoming germs, which cause infections.

The tonsils function as the first outpost of the body's immune defense system at the portal of easiest entry for germs into the body - the mouth. We have six separate tonsils, you can only see two of them when you open your mouth. The two we can see are the palatine tonsils. Above these, just out of sight at the top of the mouth are the two tubal tonsils. Above these close to the nasal passage sits a single pharyngeal tonsil and at the bottom, at the back of the tongue sits a single lingual tonsil. These tonsils are pretty much the same in structure. They each sit under the epithelial cell wall and look shriveled and wrinkled. The wrinkles are called crypts and they subdivide the interior of a tonsil. Inside the tonsils have multiple lymphoid follicles again looking like those follicles seen in lymph nodes. Mostly consisting of B cells plus T cells and APCs the tonsils respond to antigenic challenge by proliferation of cells and production of antibody.

They become enlarged because immune cells (lymphocytes) take up residence there in response to infection or serious exposure to infectious or allergic stimuli.

Peyer's Patches

Along the length of the small intestine are up to 200 oval lumps of lymphoid tissue. Each is an accumulation of up to 60 lymphoid follicles composed of B cells with T lymphocytes in the interfollicular areas.. Lymphoid follicles similar to lymph nodes or the tonsils, located especially in the lower small intestine. They contain white blood cells, defend against invading bacteria and try to prevent inadequately digested food particles from crossing the gut wall and entering the blood. The epithelium covering Peyers patches is modified such that there is no secretion of mucus and the cells, called "M cells", readily allow passage of gut antigens straight through them. Peyers patches are involved in defense against pathogens that may be colonizing the gut but they are also involved in "oral tolerance". Food antigens are foreign and strictly speaking our immune system should be seeing food as an antigenic, potentially threatening challenge to the body's survival.



The appendix looks somewhat similar to the tonsils in that it has crypts penetrating from the gut opening deep into the tissue and around these are numerous lymphoid follicles.

So, you were probably told that the appendix is a vestigial organ that has no function in humans. Well, the body doesn't carry excess baggage around for nothing. The appendix has been adapted into a lymphoid organ too. The appendix looks somewhat similar to the tonsils in that it has crypts penetrating from the gut opening deep into the tissue and around these are numerous lymphoid follicles.


Adenoids are high in the throat behind the nose and the roof of the mouth (soft palate) and are not visible through the mouth without special instruments



That concludes our look at organs of the immune system. We have a range of organs with different functions. Some organs are vital to our survival such as the bone marrow. The thymus is crucial in the development of T cells and the spleen is extremely important in our defense but we can survive without them with the aid of modern medicine to keep us healthy. The peripheral organs, lymph nodes and mucosal associated lymphoid tissue, are expendable without too much damage to our defense.




John R. David, From ACP Medicine Online

Theresa Lorraine, The Immune System