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Immune system
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==Components of the Immune System== [[leukocyte|White blood cell]]s, also called leukocytes, are the main defenders of the body. They circulate throughout the bloodstream (the circulatory system), surveying the body for possible threats.<ref name="Shlomchik2001">{{Cite journal | last = Shlomchik | first = Mark J. | last2 = Walport | first2 = Mark | last3 = Travers | first3 = Paul | last4 = Charles A Janeway | first4 = Jr | date = 2001 | title=The components of the immune system | url = https://www.ncbi.nlm.nih.gov/books/NBK27092/|journal=Immunobiology: The Immune System in Health and Disease. 5th edition|language=en}}</ref> ===Types and Functions of White Blood Cells (Leukocytes) === ====[[Granulocyte]]s==== [[Granulocyte]]s are a type of white blood cell filled with double-membrane sacs called granules which contain a variety of substances. These substances aid in the immune response. The substances released include: * histamine, a molecule that responds to injury, allergies and inflammation by inducing smooth muscle contraction and increased blood flow; * cytokines, messenger proteins that induce other immune cells’ inflammatory functions; * enzymes, metabolic proteins that decrease the amount of time it takes to carry out chemical reactions, and that activate other white blood cells. The contents of granules can be released in two ways depending on the purpose of the substances that will be released. A granule can be ushered to the granulocyte’s membrane surface where the two will merge and the granule can dump its internal material into the area surrounding the cell. This method is helpful when the substances need to act directly on other tissues such as mast cells secreting histamine which act directly on the smooth muscle the mast cell in. Alternately, granules can also release their internal material directly into the granulocyte. This process is helpful in cases where the granulocyte has engulfed a pathogen and the released elements from the granules can break it down. There are several types of granulocytes that perform functions related to both innate and adaptive immunity. ===== [[Basophil granulocyte|Basophils]] ===== [[Basophil granulocyte|Basophils]], the least common granulocyte, help the body identify foreign substances. Once a pathogen is introduced to the body and consumed by a lymphocyte, such as a macrophage, the invader is broken down into smaller pieces. This eliminates it as a threat and makes disposal easier. Additionally, in order for the immune cell to signal to the body that an invader was present (and that most likely there are other microbes nearby), the white blood cell will save some of the proteins from the pathogen and place it on its surface like a flag. These surface proteins are called antigens, and the process is called antigen presentation. Basophils are both capable of placing these antigens on themselves and onto other cells. Now that the antigen can be identified, different white blood cells begin scanning cells’ surfaces in search of these markers. Helper T-cells (CD4 T-cells) are produced for this purpose. After sensing antigens, helper T-cells signal for the production of other while blood cell variants, which make inflammatory molecules that fight the infection. In addition, CD4 T-cells also help macrophages and killer T-cells perform their phagocytic and cytotoxic duties. However, before a helper T-cell does all of this, they need to be: 1) told to function as these particular cells, and 2) stimulated to function in that manner. Basophils are the cells responsible for the programming and stimulation of these immune cells. It was initially hypothesized that basophils did not contain an essential component necessary to the differentiation of naive T-cells into helper T-cells. New evidence supports the idea that basophils do in fact possess all the necessary proteins, such as major histocompatibility complex II (see section below) and the cytokine IL-4, that stimulate the conversion of naive T-cells into helper T-cells. Normal basophils amounts in the body range from 0-300/μL (0.000003L) of blood. Low levels are caused by afflictions such as hyperthyroidism or anaphylaxis. High levels are caused by [[Hashimoto's thyroiditis (hypothyroidism)|hypothyroidism]], and myeloproliferative disorders (blood disorders).{{citation needed | date = 2023}} ===== [[Eosinophil|Eosinophils]] ===== [[Eosinophil]]s are the next most common granulocyte. However, they store several enzymes and proteins with unidentified roles, so some the full extent of their purpose is unknown. Eosinophils inflict damage largely to parasites and to a smaller extent bacteria and viruses. These cells can also cause damage to the body’s own tissues during allergic reactions. Outside of the immune system, eosinophils help with organ development. Eosinophils are typically less than 500/μL of blood. Infection by parasitic worms, ulcerative colitis, allergies are examples of maladies that can cause high levels of eosinophils, known as eosinophilia. Low levels may indicate alcohol intoxication, Cushing’s disease or problems with the bone marrow. ===== [[Neutrophil|Neutrophils]] ===== Neutrophils are the most common immune cell and the first type of cell to arrive at the site of infection. These cells are flexible, bulbous, and multilobed. Normally, neutrophils travel through the blood and lymph systems but they have receptors that jut out of their surface like pins on a pincushion, that allow them to attach to cells stressed by damage or infection and perform their duty. With these receptors, neutrophils are also able to slip between cells if they’re needed in tissues outside of the blood and lymph systems. Neutrophils are activated by chemical signals that stressed cells release. Cytokines, messenger proteins that induce different immune cell functions, can function in this capacity: switching migratory neutrophils to potent eradicator cells. Once operative, neutrophils working alongside macrophages in the innate immune system, engulfing and destroying pathogens non-specifically. This mechanism works because there are molecules present on pathogens that are not present on any of the cells in the body. These are called pathogen-associated molecular patterns (PAMPs). Cells in the body that have pattern-recognition receptors (PRRs), which neutrophils do, can identify the PAMPs and eliminate the non-self cells. Once ingested into the lumen on the neutrophil, the contents of the granules are released, which break down the microbe. Alternatively, the granules from a neutrophil can be released to the surrounding environment and break down pathogens outside of the cell. Neutrophils are normally between [[Tel:1500-8000|1500-8000]] neutrophils/μL. High levels are caused by smoking, infection and non-infectious inflammation. Low levels appear with suppressed immune systems, autoimmune diseases and during drug treatments such as chemotherapy. ==== [[Monocyte]]s ==== [[Monocyte]]s are also white blood cells and are largely tasked with ingesting and eliminating microbial invaders, or identifying and presenting the proteins made by the infectious agent. They may also work in a restorative capacity to heal the affected area(s). Typically, monocytes become tissue macrophages or dendritic cells. ===== [[Macrophage]]s ===== [[Macrophage]]s are globular cells derived from monocytes that consume dysfunctional cells, cellular debris, and pathogens; they may be thought of as the garbage disposals of the immune system. They may be already present at sites where infection occurs, or they may migrate to the area of infection. As the name suggests, tissue [[macrophage]]s exist in the different tissues in the body (e.g. liver or skin); they also are not necessarily derived from [[monocyte]]s circulating in the [[blood]]and lymph system. ===== Dendritic cells===== [[Dendritic cell]]s are also monocyte-derived. They aid T-cell recognition of infectious material through the processing of antigens and production of proteins that major histocompatibility complexes (larger proteins on the surface of cells and tissues that allow the body to identify self from foreign) present to T-cells. === Production and Storage === ==== Bone marrow ==== White blood cells are produced in [[bone marrow]]. They begin as pluripotent hematopoietic stem cells (PSCs or HSCs) meaning they are capable of becoming any of the cells listed above, as well as red blood cells or platelets. T lymphocytes (T cells) are produced in the bone marrow while B lymphocytes (B cells) are both produced and develop to their full function in the bone marrow. ==== Thymus ==== The [[thymus]] is an immune organ that ceases having a functional capacity when puberty begins. During development, including during fetal development, the thymus stores [[T-cell]]s. The thymus is also a part of the [[endocrine system]] as it produces the hormone thymosin. This hormone initiates the maturation of T-cells. Once fully developed, the T-cells leave the thymus for the [[lymph node]]s, where they enter active circulation and begin their immune duties. ==== The lymph system ==== The lymph system, like the circulatory system, is comprised of vessels and nodes all throughout the body. Instead of blood, these vessels carry a slightly opaque, white fluid called lymph. This circulation system is responsible for removing toxins and infectious agents from tissues. Spread throughout the lymph tubes are small, grape-shaped compartments attached in clusters on the sides of vessels. These nodes, concentrated at the neck, upper chest, armpits and groin, constantly filter the lymph liquid of harmful substances. The lymph nodes contain B and T lymphocytes that specifically recognize dangerous material and make antibodies, which further help identify the pathogens, and eliminate them. ==== Spleen ==== The spleen filters the blood of dysfunctional or malformed red blood cells and platelets. It also clears away infectious agents from the blood. Macrophages neutralize all of these potential harms and recycle the products they can for further use by the body (e.g., hemoglobin from defective red blood cells can be put in new red blood cells). Lymphocytes are also stored in the spleen.
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