Immune system

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The immune system is a system of biological structures and processes within an organism that protects against disease. To function properly, an immune system must detect a wide variety of agents, from viruses to parasitic worms, and distinguish them from the organism's own healthy tissue. DELL XPS M1210 Fan

Pathogens can rapidly evolve and adapt, and thereby avoid detection and neutralization by the immune system, however, multiple defense mechanisms have also evolved to recognize and neutralize pathogens. Even simple unicellular organisms such as bacteria possess a rudimentary immune system, in the form of enzymes that protect against bacteriophageinfections. DELL Alienware m15x Series CPU FAN basic immune mechanisms evolved in ancient eukaryotes and remain in their modern descendants, such as plants and insects. These mechanisms include phagocytosis, antimicrobial peptides called defensins, and the complement system. Jawed vertebrates, including humans, have even more sophisticated defense mechanisms,^[1] DELL Alienware PR2H1 Series CPU FAN including the ability to adapt over time to recognize specific pathogens more efficiently. Adaptive (or acquired) immunity creates immunological memory after an initial response to a specific pathogen, leading to an enhanced response to subsequent encounters with that same pathogen. This process of acquired immunity is the basis of vaccination. DELL Inspiron 1100 Series CPU FAN Disorders of the immune system can result in autoimmune diseases, inflammatory diseases and cancer.^[2][3]Immunodeficiency occurs when the immune system is less active than normal, resulting in recurring and life-threatening infections. In humans, immunodeficiency can either be the result of a genetic disease such as severe combined immunodeficiency, DELL Inspiron 1110 Series CPU FAN acquired conditions such as HIV/AIDS, or the use of immunosuppressive medication. In contrast,autoimmunity results from a hyperactive immune system attacking normal tissues as if they were foreign organisms. Common autoimmune diseases include Hashimoto's thyroiditis, rheumatoid arthritis, diabetes mellitus type 1, and systemic lupus erythematosus. DELL Inspiron 1150 Series CPU FAN Immunology covers the study of all aspects of the immune system.

Immunology is a science that examines the structure and function of the immune system. It originates from medicine and early studies on the causes of immunity to disease. The earliest known reference to immunity was during the plague of Athens in 430 BC. DELL Inspiron 11Z Series CPU FAN Thucydides noted that people who had recovered from a previous bout of the disease could nurse the sick without contracting the illness a second time.^[4] In the 18th century, Pierre-Louis Moreau de Maupertuis made experiments with scorpion venom and observed that certain dogs and mice were immune to this venom.^[5] DELL Inspiron 1300 Series CPU FAN This and other observations of acquired immunity were later exploited byLouis Pasteur in his development of vaccination and his proposed germ theory of disease.^[6] Pasteur's theory was in direct opposition to contemporary theories of disease, such as the miasma theory. It was not until Robert Koch's 1891 proofs, for which he was awarded a Nobel Prize in 1905, DELL Inspiron 1318 CPU FAN that microorganisms were confirmed as the cause of infectious disease.^[7] Viruses were confirmed as human pathogens in 1901, with the discovery of the yellow fever virus by Walter Reed.^[8]

Immunology made a great advance towards the end of the 19th century, through rapid developments, in the study of humoral immunityand cellular immunity.^[9] DELL Inspiron 13R Series CPU FAN Particularly important was the work of Paul Ehrlich, who proposed the side-chain theory to explain the specificity of the antigen-antibody reaction; his contributions to the understanding of humoral immunity were recognized by the award of a Nobel Prize in 1908, which was jointly awarded to the founder of cellular immunology, Elie Metchnikoff. DELL Inspiron 1405 CPU FAN

The immune system protects organisms from infection with layered defenses of increasing specificity. In simple terms, physical barriers prevent pathogens such as bacteria and viruses from entering the organism. If a pathogen breaches these barriers, the innate immune system provides an immediate, but non-specific response. DELL Inspiron 1410 CPU FAN Innate immune systems are found in all plants and animals.^[11] If pathogens successfully evade the innate response, vertebrates possess a second layer of protection, the adaptive immune system, which is activated by the innate response. Here, the immune system adapts its response during an infection to improve its recognition of the pathogen. DELL Inspiron 1420 Series CPU FAN This improved response is then retained after the pathogen has been eliminated, in the form of an immunological memory, and allows the adaptive immune system to mount faster and stronger attacks each time this pathogen is encountered.

Both innate and adaptive immunity depend on the ability of the immune system to distinguish between self and non-self molecules. DELL Inspiron 1421 CPU FAN In immunology, self molecules are those components of an organism's body that can be distinguished from foreign substances by the immune system.^[13] Conversely, non-self molecules are those recognized as foreign molecules. One class of non-self molecules are called antigens (short for antibody generators) and are defined as substances that bind to specific immune receptors and elicit an immune response. DELL inspiron 1425 CPU FAN Several barriers protect organisms from infection, including mechanical, chemical, and biological barriers. The waxy cuticle of many leaves, the exoskeleton ofinsects, the shells and membranes of externally deposited eggs, and skin are examples of mechanical barriers that are the first line of defense against infection.^[14] DELL inspiron 1427 CPU FAN However, as organisms cannot be completely sealed against their environments, other systems act to protect body openings such as the lungs,intestines, and the genitourinary tract. In the lungs, coughing and sneezing mechanically eject pathogens and other irritants from the respiratory tract. DELL Inspiron 14R Series CPU FAN The flushing action of tears and urine also mechanically expels pathogens, while mucus secreted by the respiratory and gastrointestinal tract serves to trap and entangle microorganisms.^[15]

Chemical barriers also protect against infection. The skin and respiratory tract secrete antimicrobial peptides such as the β-defensins.^[16] DELL Inspiron 1501 Series CPU FAN Enzymes such aslysozyme and phospholipase A2 in saliva, tears, and breast milk are also antibacterials.^[17][18] Vaginal secretions serve as a chemical barrier followingmenarche, when they become slightly acidic, while semen contains defensins and zinc to kill pathogens.^[19][20] In the stomach, gastric acid and proteases serve as powerful chemical defenses against ingested pathogens. DELL Inspiron 1520 Series CPU FAN Within the genitourinary and gastrointestinal tracts, commensal flora serve as biological barriers by competing with pathogenic bacteria for food and space and, in some cases, by changing the conditions in their environment, such as pH or available iron.^[21] This reduces the probability that pathogens will reach sufficient numbers to cause illness. DELL Inspiron 1521 Series CPU FAN However, since most antibiotics non-specifically target bacteria and do not affect fungi, oral antibiotics can lead to an "overgrowth" of fungi and cause conditions such as a vaginal candidiasis (a yeast infection).^[22] There is good evidence that re-introduction of probiotic flora, DELL Inspiron 1525 Series CPU FAN such as pure cultures of the lactobacilli normally found in unpasteurized yogurt, helps restore a healthy balance of microbial populations in intestinal infections in children and encouraging preliminary data in studies on bacterial gastroenteritis, inflammatory bowel diseases, urinary tract infection and post-surgical infections. DELL Inspiron 1526 Series CPU FAN Microorganisms or toxins that successfully enter an organism encounter the cells and mechanisms of the innate immune system. The innate response is usually triggered when microbes are identified by pattern recognition receptors, which recognize components that are conserved among broad groups of microorganisms,^[26]or when damaged, injured or stressed cells send out alarm signals, DELL Inspiron 1545 Series CPU FAN many of which (but not all) are recognized by the same receptors as those that recognize pathogens.^[27] Innate immune defenses are non-specific, meaning these systems respond to pathogens in a generic way.^[14] This system does not confer long-lastingimmunity against a pathogen. The innate immune system is the dominant system of host defense in most organisms. DELL Inspiron 1546 Series CPU FAN

Inflammation is one of the first responses of the immune system to infection.^[28] The symptoms of inflammation are redness, swelling, heat, and pain, which are caused by increased blood flow into tissue. Inflammation is produced by eicosanoids and cytokines, which are released by injured or infected cells. DELL Inspiron 1720 CPU FAN Eicosanoids include prostaglandins that produce fever and the dilation of blood vessels associated with inflammation, and leukotrienes that attract certain white blood cells (leukocytes).^[29][30] Common cytokines include interleukins that are responsible for communication between white blood cells; chemokines that promotechemotaxis; DELL Inspiron 1721 CPU FAN and interferons that have anti-viral effects, such as shutting down protein synthesis in the host cell.^[31] Growth factors and cytotoxic factors may also be released. These cytokines and other chemicals recruit immune cells to the site of infection and promote healing of any damaged tissue following the removal of pathogens. DELL Inspiron 2500 Series CPU FAN The complement system is a biochemical cascade that attacks the surfaces of foreign cells. It contains over 20 different proteins and is named for its ability to "complement" the killing of pathogens by antibodies. Complement is the major humoral component of the innate immune response.^[33][34] DELL Inspiron 2600 CPU FAN Many species have complement systems, including non-mammals like plants, fish, and some invertebrates.^[35]

In humans, this response is activated by complement binding to antibodies that have attached to these microbes or the binding of complement proteins tocarbohydrates on the surfaces of microbes. DELL Inspiron 2650 CPU FAN This recognition signal triggers a rapid killing response.^[36] The speed of the response is a result of signal amplification that occurs following sequential proteolytic activation of complement molecules, which are also proteases. After complement proteins initially bind to the microbe, they activate their protease activity, which in turn activates other complement proteases, and so on. DELL Inspiron 4000 CPU FAN This produces a catalytic cascade that amplifies the initial signal by controlled positive feedback.^[37] The cascade results in the production of peptides that attract immune cells, increasevascular permeability, and opsonize (coat) the surface of a pathogen, marking it for destruction. DELL Inspiron 4100 CPU FAN This deposition of complement can also kill cells directly by disrupting their plasma membrane.