THE ORGANS OF THE IMMUNE SYSTEM

Bone marrow (moelle osseuse/midollo osseo/medulla ósea) [E1], [E2], [E3], [E5], [F], [ES]
All the cells of the immune system are initially derived from the bone marrow. They form through a process called hematopoiesis (hématopoïèse/ematopoiesi) [E1], [E2]. During hematopoiesis, bone-marrow-derived stem cells differentiate either into mature cells or into precursors of cells that migrate out of the bone marrow to continue their maturation elsewhere. The bone marrow produces B cell, natural killer cell and granulocyte precursors and thymocytes (precursors of T cells).

Lymphatic circulatory system (système lymphatique, sistema linfatico, sistema linfàtico) [E], [F], [ES]
The intracellular spaces of almost all tissues contain small endothelial-lined tubes, which are blind ending, but otherwise identical in structure to blood capillaries. These are lymphatic capillaries and are permeable to fluids and dissolved molecules in the interstitial fluid. In some areas the lymphatic capillaries allow the entry of larger molecules (e.g. large proteins, triglycerides etc.) and of immune system cells. The lymphatic capillary network acts as a drainage system removing surplus fluid (lymph/lymphe/linfa/linfa) from tissue spaces. The lymphatic capillaries merge to form larger vessels. On its way to larger vessels, lymph passes through one or more lymph nodes, where antigens are processed and activated lymphocytes are added to lymph. The larger lymphatic vessels terminate in two main lymphatic vessels: the toracic duct and the right main lymphatic duct, which empty lymph into the venous system.

Thymus (thymus/timo/timo) [E1], [E2]
Immunologically naive lymphocytes (thymocytes) from bone marrow differentiate into mature T cells in the thymus. During this process, referred to as “thymic education”, the immune system distinguishes self from foreign antigens and develops self-tolerance. T cells that are beneficial to the immune system are spared, while those T cells that might evoke a detrimental autoimmune response are eliminated. The mature T cells are then released into the blood stream. The thymus also secretes hormones which control T cell production, differentiation, maturation. It is located in the superior and anterior mediastinum and undergoes involution after puberty.

Spleen (rate/milza/bazo) [F]
The spleen lies in the left upper abdomen. The two main functions of the human spleen are: to mount a primary immune response to antigens in the blood and to act as a filter to remove particulate matter and aged or abnormal red cells and platelets from the circulation. It comprises B cells, T cells, macrophages, dendritic cells, natural killer cells and red blood cells. Migratory macrophages and dendritic cells bring antigens to the spleen via the bloodstream. An immune response is initiated when macrophages or dendritic cells present the antigen to the appropriate B or T cells.

Lymph nodes (ganglion lymphatiques/linfonodi/nódulos linfàticos) [E]
The lymph nodes function as an immunologic filter for the bodily fluid known as lymph. Lymph nodes are found throughout the body. Composed mostly of T cells, B cells, dendritic cells and macrophages, the nodes drain fluid from most of our tissues. Antigens are filtered out of the lymph in the lymph node before the lymph returns to the circulation. In a similar fashion as the spleen, the macrophages and dendritic cells, that capture antigens, present these foreign materials to T and B cells, consequently initiating an immune response. Lymph nodes are the main site for expansion of lymphoid cells in an immune response.

Mucosa-associated lymphoid tissue (MALT)
Mucosa-associated lymphoid tissue (MALT) consists of non-encapsulated lymphoid tissue, which is located in the walls of the gastrointestinal (gut-associated lymphoid tissue, GALT), respiratory and urogenital tracts. MALT takes the form of diffuse infiltrates or more discrete nodules. MALT includes: the palatine, lingual and parapharyngeal tonsils (adenoids/adenoidi/adenoides), mucosal nodules in the esophagus, Peyer’s patches in the small intestine and the appendix (appendice/appendice/apéndice).

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