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Encyclopedia of Immunobiology: The Human Immune System

By: , Posted on: April 6, 2016

encylopedia of immunology

We are thrilled to announce that the Encyclopedia of Immunobiology will be publishing May 2016! This 5 volume set will provide the largest integrated source of immunological knowledge currently available. A one-stop go-to reference, this encyclopedia will be ideal for students and professionals alike, providing relevant topics to those working on experimental and clinical immunology, microbiology, biochemistry, genetics, veterinary science, physiology, and hematology. Check out the below excerpt from Human Immune System to learn about the basic components of the human body immune system and look out for the Encyclopedia of Immunobiology publishing in both print and electronic next month!

Our world is full of microorganisms such as viruses, bacteria, fungi, and parasites, many of which may cause diseases. However, most of the time people are healthy; and even if they become ill, they can recover health after a while. So how do they defend themselves against infection? That is owing to the immune system in the human body, which defends us by rejecting foreign viruses, bacteria, and endogenous tumors. When the immune system is defective or in disorder, it can fail to protect the human body, causing chronic infectious disease such as acquired immunodeficiency syndrome (AIDS) and tumors, or even attack it, leading to autoimmune diseases and allergies.

The Components of the Immune System

The immune responses are mediated by leukocytes (white blood cells). All the cells of the immune system initially originate in the bone marrow, develop and mature in the bone marrow or thymus, then migrate to peripheral tissues or the bloodstream to defend against systemic infections. Most of them reside within peripheral lymphoid organs such as the spleen and lymph nodes (LNs) to be responsible for the induction of the systemic immune response; others enter and circulate in the bloodstream and the lymphatic system. A large subpopulation of leukocytes is located at the epithelial or the mucosal sites to respond to the mucus-derived pathogens, which is called mucus-associated lymphoid tissue (MALT), responsible for the induction of the mucosal or local immune response.

Immune Organs

The cells involved in the immune response are organized into tissues and organs to perform their immune functions effectively. The major lymphoid organs are classified as primary (central) lymphoid organs where lymphocytes are generated, and secondary (peripheral) lymphoid organs, where mature naïve lymphocytes are maintained and adaptive immune responses are initiated.

Central immune organs: Bone marrow and thymus

The bone marrow and the thymus are considered primary lymphoid organs that are responsible for the formation and maturation of various immune cells. The bone marrow is the primary site for the formation of all the blood cells and is the site where B lymphocytes mature. T lymphocytes originate in the bone marrow, the precursor T cells then migrate to the thymus for further maturation. The developing T lymphocytes in the thymus are also called thymocytes.

Peripheral immune organs: Lymphoid node, spleen, and MALT

The LNs and the spleen are the main peripheral lymphoid organs where most of the mature lymphocytes reside. The approximately 1200 (Qatarnhet et al., 2006) kidney-shaped LNs in the body transport the extracellular fluids between tissues and the blood through lymphatic vessels. In the LN, B cells are localized in follicles, which make up the outer (superficial) cortex of the LN, with T cells distributed in the paracortical areas. Lymphocytes migrating from the blood enter the T-cell zone first where antigen-presenting dendritic cells (DCs) and macrophages are also localized. The outer cortex contains primary and secondary follicles with germinal centers in it (Figure 1(a)).

peripheral lymphoid tissues
Figure 1. Organization of the peripheral lymphoid tissues. (a) Depiction of the structure of a lymph node. In the lymph node, B cells are localized in primary lymphoid follicles, which is the primary site for naïve B residence and make up the outer (superficial) cortex of the lymph node. When a B-cell clone is activated by an antigen, and helped by Th cells distributed in the paracortical areas (T-cell zone), it proliferates and migrates into the secondary follicles forming a germinal center in it. FDC, follicular dendritic cell. (b) The MALT of the human mucosal immune system. The mucosal immune system, MALT consists of lamina propria (LP), diffuse lymphocyte aggregates, and organized lymphoid tissue in the submucosa of the respiratory, gastrointestinal, and genitourinary tracts. Exogenous antigens from the mucosal surfaces are taken up by M cells within the follicle-associated epithelium (FAE). Fig 1a modified from Fig 1.18, Murphy K, Travers P and Walport M (2008) Janeway’s Immunobiology 7th ed. Garland Sci, New York.

The spleen consists of red pulp interspersed with lymphoid white pulp around central arterioles, which can respond to blood-borne antigens. Surrounding the arteriole is the periarteriolar lymphoid sheath (PALS), also known as the T-cell zone. Nearby or surrounding the PALS are follicles consisting mainly of B cells (follicular B (FOB) cell). The follicles are surrounded by a marginal zone (MZ) with MZ B cells (MZB) in it. Blood carrying lymphocytes and pathogens flows into a central arteriole ensuring the immune recognition of pathogens by splenic lymphocytes.

The vast majority of exogenous microbes make contact with mucosal surfaces rather than the skin. To maintain homeostasis in the extensive and vulnerable mucosae, they are protected by specialized anti-inflammatory immune defenses such as the formation and export of secretory IgA (sIgA) antibodies and induction of tolerance against innocuous foods, as well as commensal bacteria. The mucosal immune system is responsible for the induction of sIgA against antigens from mucosal surfaces, whereas induction of systemic immunity depends on the activation of lymphocytes within the LN and spleen.

The mucosal immune system, MALT, consists of lamina propria (LP), diffuse lymphocyte aggregates, and organized lymphoid tissue in the submucosa of the respiratory, gastrointestinal, and genitourinary tracts. Peyer’s patches in the distal small intestine are typical MALT structures and a main source of sIgA-expressing mucosal B cells. All MALT structures resemble LNs – with variable T-cell zones intervening between the B-cell follicles – and contain a variety of antigen-presenting cells (APCs). MALT actively samples exogenous antigens from the mucosal surfaces through a characteristic follicle-associated epithelium containing ‘microfold’ or ‘membrane’ (M) cells ( Figure 1(b)).

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