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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.

Leukocytes (white blood cells) act like independent, single-celled organisms and are the second arm of the innate immune system.[14] The innate leukocytes include the phagocytes (macrophages, neutrophils, and dendritic cells), DELL Inspiron 5100 Series CPU FAN
mast cells,eosinophils, basophils, and natural killer cells. These cells identify and eliminate pathogens, either by attacking larger pathogens through contact or by engulfing and then killing microorganisms.[35] Innate cells are also important mediators in the activation of the adaptive immune system.[12]DELL Inspiron 5150 CPU FAN
Phagocytosis is an important feature of cellular innate immunity performed by cells called 'phagocytes' that engulf, or eat, pathogens or particles. Phagocytes generally patrol the body searching for pathogens, but can be called to specific locations by cytokines.[14] Once a pathogen has been engulfed by a phagocyte, DELL Inspiron 5160 CPU FAN
it becomes trapped in an intracellular vesicle called aphagosome, which subsequently fuses with another vesicle called a lysosome to form a phagolysosome. The pathogen is killed by the activity of digestive enzymes or following a respiratory burst that releases free radicals into the phagolysosome.[38][39]DELL Inspiron 6000 Series CPU FAN
Phagocytosis evolved as a means of acquiring nutrients, but this role was extended in phagocytes to include engulfment of pathogens as a defense mechanism.[40] Phagocytosis probably represents the oldest form of host defense, as phagocytes have been identified in both vertebrate and invertebrate animals. DELL Inspiron 600m Series CPU FAN
Neutrophils and macrophages are phagocytes that travel throughout the body in pursuit of invading pathogens.[42] Neutrophils are normally found in the bloodstream and are the most abundant type of phagocyte, normally representing 50% to 60% of the total circulating leukocytes.[43] During the acute phase of inflammation, DELL Inspiron 630m CPU FAN
particularly as a result of bacterial infection, neutrophils migrate toward the site of inflammation in a process called chemotaxis, and are usually the first cells to arrive at the scene of infection. Macrophages are versatile cells that reside within tissues and produce a wide array of chemicals including enzymes, DELL Inspiron 6400 Series CPU FAN
complement proteins, and regulatory factors such as interleukin 1.[44] Macrophages also act as scavengers, ridding the body of worn-out cells and other debris, and as antigen-presenting cells that activate the adaptive immune system.[12]DELL Inspiron 640m CPU FAN
Dendritic cells (DC) are phagocytes in tissues that are in contact with the external environment; therefore, they are located mainly in the skin, nose, lungs,stomach, and intestines.[45] They are named for their resemblance to neuronal dendrites, as both have many spine-like projections, but dendritic cells are in no way connected to the nervous system. DELL Inspiron 700M Series CPU FAN
Dendritic cells serve as a link between the bodily tissues and the innate and adaptive immune systems, as they present antigen to T cells, one of the key cell types of the adaptive immune system.[45]

Mast cells reside in connective tissues and mucous membranes, and regulate the inflammatory response.[46]DELL Inspiron 710M Series CPU FAN
They are most often associated with allergy andanaphylaxis.[43] Basophils and eosinophils are related to neutrophils. They secrete chemical mediators that are involved in defending against parasites and play a role in allergic reactions, such as asthma.[47] Natural killer (NK cells) cells are leukocytes that attack and destroy tumor cells, or cells that have been infected by viruses. DELL Inspiron 8000 Series CPU FAN
Natural killer cells, or NK cells, are a component of the innate immune system which does not directly attack invading microbes. Rather, NK cells destroy compromised host cells, such as tumor cells or virus-infected cells, recognizing such cells by a condition known as "missing self." DELL Inspiron 8100 Series CPU FAN
This term describes cells with low levels of a cell-surface marker called MHC I (major histocompatibility complex) - a situation that can arise in viral infections of host cells.[35] They were named "natural killer" because of the initial notion that they do not require activation in order to kill cells that are "missing self." DELL Inspiron 8500 Series CPU FAN
For many years it was unclear how NK cells recognize tumor cells and infected cells. It is now known that the MHC makeup on the surface of those cells is altered and the NK cells become activated through recognition of "missing self". Normal body cells are not recognized and attacked by NK cells because they express intact self MHC antigens. DELL Inspiron 8600 Series CPU FAN
Those MHC antigens are recognized by killer cell immunoglobulin receptors (KIR) which essentially put the brakes on NK cells.

The adaptive immune system evolved in early vertebrates and allows for a stronger immune response as well as immunological memory, DELL Inspiron 9200 Series CPU FAN
where each pathogen is "remembered" by a signature antigen.[50] The adaptive immune response is antigen-specific and requires the recognition of specific "non-self" antigens during a process called antigen presentation. Antigen specificity allows for the generation of responses that are tailored to specific pathogens or pathogen-infected cells. DELL Inspiron 9300 Series CPU FAN
The ability to mount these tailored responses is maintained in the body by "memory cells". Should a pathogen infect the body more than once, these specific memory cells are used to quickly eliminate it.

The cells of the adaptive immune system are special types of leukocytes, called lymphocytes. DELL Inspiron 9400 Series CPU FAN
B cells and T cells are the major types of lymphocytes and are derived from hematopoietic stem cells in the bone marrow.[35] B cells are involved in the humoral immune response, whereas T cells are involved in cell-mediated immune response.

Both B cells and T cells carry receptor molecules that recognize specific targets. DELL Inspiron B120 Series CPU FAN
T cells recognize a "non-self" target, such as a pathogen, only after antigens (small fragments of the pathogen) have been processed and presented in combination with a "self" receptor called a major histocompatibility complex(MHC) molecule. There are two major subtypes of T cells: the killer T cell and the helper T cell. DELL Inspiron B130 Series CPU FAN
Killer T cells only recognize antigens coupled to Class I MHCmolecules, while helper T cells only recognize antigens coupled to Class II MHC molecules. These two mechanisms of antigen presentation reflect the different roles of the two types of T cell. A third, minor subtype are the γδ T cells that recognize intact antigens that are not bound to MHC receptors.[51]DELL Inspiron E1405 CPU FAN
In contrast, the B cell antigen-specific receptor is an antibody molecule on the B cell surface, and recognizes whole pathogens without any need for antigen processing. Each lineage of B cell expresses a different antibody, so the complete set of B cell antigen receptors represent all the antibodies that the body can manufacture. DELL Inspiron E1501 Series CPU FAN
Killer T cells are a sub-group of T cells that kill cells that are infected with viruses (and other pathogens), or are otherwise damaged or dysfunctional.[52] As with B cells, each type of T cell recognizes a different antigen. DELL Inspiron E1505 Series CPU FAN
Killer T cells are activated when their T cell receptor (TCR) binds to this specific antigen in a complex with the MHC Class I receptor of another cell. Recognition of this MHC:antigen complex is aided by a co-receptor on the T cell, called CD8. The T cell then travels throughout the body in search of cells where the MHC I receptors bear this antigen. DELL Inspiron E1705 Series CPU FAN
When an activated T cell contacts such cells, it releasescytotoxins, such as perforin, which form pores in the target cell's plasma membrane, allowing ions, water and toxins to enter. The entry of another toxin calledgranulysin (a protease) induces the target cell to undergo apoptosis.[53] T cell killing of host cells is particularly important in preventing the replication of viruses. DELL Inspiron m140 CPU FANOther     T cell activation is tightly controlled and generally requires a very strong MHC/antigen activation signal, or additional activation signals provided by "helper" T cells (see below).

Helper T cells regulate both the innate and adaptive immune responses and help determine which immune responses the body makes to a particular pathogen.[54][55] DELL Inspiron N3010 Series CPU FAN
These cells have no cytotoxic activity and do not kill infected cells or clear pathogens directly. They instead control the immune response by directing other cells to perform these tasks.

Helper T cells express T cell receptors (TCR) that recognize antigen bound to Class II MHC molecules. DELL Inspiron N4010 Series CPU FAN
The MHC:antigen complex is also recognized by the helper cell's CD4 co-receptor, which recruits molecules inside the T cell (e.g., Lck) that are responsible for the T cell's activation. Helper T cells have a weaker association with the MHC:antigen complex than observed for killer T cells, DELL Latitude 100L CPU FAN
meaning many receptors (around 200–300) on the helper T cell must be bound by an MHC:antigen in order to activate the helper cell, while killer T cells can be activated by engagement of a single MHC:antigen molecule. Helper T cell activation also requires longer duration of engagement with an antigen-presenting cell.[56]DELL Latitude 120L Series CPU FAN

 The activation of a resting helper T cell causes it to release cytokines that influence the activity of many cell types. Cytokine signals produced by helper T cells enhance the microbicidal function of macrophages and the activity of killer T cells.[14] In addition, helper T cell activation causes an upregulation of molecules expressed on the T cell's surface, such as CD40 ligand (also called CD154), DELL Latitude 131L Series CPU FAN
which provide extra stimulatory signals typically required to activate antibody-producing B cells.

γδ T cells possess an alternative T cell receptor (TCR) as opposed to CD4+ and CD8+ (αβ) T cells and share the characteristics of helper T cells, cytotoxic T cells and NK cells. DELL Latitude C500 CPU FAN
The conditions that produce responses from γδ T cells are not fully understood. Like other 'unconventional' T cell subsets bearing invariant TCRs, such asCD1d-restricted Natural Killer T cells, γδ T cells straddle the border between innate and adaptive immunity.[58]On one hand, γδ T cells are a component of adaptive immunity as they rearrange TCR genes to produce receptor diversity and can also develop a memory phenotype. DELL Latitude C540 CPU FAN
On the other hand, the various subsets are also part of the innate immune system, as restricted TCR or NK receptors may be used as pattern recognition receptors. For example, large numbers of human Vγ9/Vδ2 T cells respond within hours to common molecules produced by microbes, and highly restricted Vδ1+ T cells in epithelia respond to stressed epithelial cells. DELL Latitude C600 CPU FAN
A B cell identifies pathogens when antibodies on its surface bind to a specific foreign antigen.[60] This antigen/antibody complex is taken up by the B cell and processed by proteolysis into peptides. The B cell then displays these antigenic peptides on its surface MHC class II molecules. DELL Latitude C610 CPU FAN
This combination of MHC and antigen attracts a matching helper T cell, which releases lymphokines and activates the B cell.[61] As the activated B cell then begins to divide, its offspring (plasma cells) secrete millions of copies of the antibody that recognizes this antigen. These antibodies circulate in blood plasma and lymph, DELL Latitude C640 CPU FAN
bind to pathogens expressing the antigen and mark them for destruction by complement activation or for uptake and destruction by phagocytes. Antibodies can also neutralize challenges directly, by binding to bacterial toxins or by interfering with the receptors that viruses and bacteria use to infect cells. DELL Latitude C800 Series CPU FAN
Although the classical molecules of the adaptive immune system (e.g., antibodies and T cell receptors) exist only in jawed vertebrates, a distinct lymphocyte-derived molecule has been discovered in primitive jawless vertebrates, such as the lampreyand hagfish. DELL Latitude C810 Series CPU FAN
These animals possess a large array of molecules called variable lymphocyte receptors (VLRs) that, like the antigen receptors of jawed vertebrates, are produced from only a small number (one or two) of genes. These molecules are believed to bind pathogenic antigens in a similar way to antibodies, and with the same degree of specificity. DELL Latitude C840 Series CPU FAN

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