Tuesday, October 8, 2013

Several layers of passive protection are provided by the mother

Several layers of passive protection are provided by the mother. During pregnancy, a particular type of antibody, called IgG, is transported from mother to baby directly across the placenta, so human babies have high levels of antibodies even at birth, with the same range of antigen specificities as their mother.[64] DELL Latitude D420 CPU FAN

 Breast milk or colostrum also contains antibodies that are transferred to the gut of the infant and protect against bacterial infections until the newborn can synthesize its own antibodies.[65] This is passive immunitybecause the fetus does not actually make any memory cells or antibodies—it only borrows them. DELL Latitude D430 CPU FAN This passive immunity is usually short-term, lasting from a few days up to several months. In medicine, protective passive immunity can also be transferred artificially from one individual to another via antibody-richserum.

Long-term active memory is acquired following infection by activation of B and T cells. DELL Latitude D500 Series CPU FAN Active immunity can also be generated artificially, through vaccination. The principle behind vaccination (also calledimmunization) is to introduce an antigen from a pathogen in order to stimulate the immune system and develop specific immunity against that particular pathogen without causing disease associated with that organism.[14] DELL Latitude D505 Series CPU FAN This deliberate induction of an immune response is successful because it exploits the natural specificity of the immune system, as well as its inducibility. With infectious disease remaining one of the leading causes of death in the human population, vaccination represents the most effective manipulation of the immune system mankind has developed.[35][67] DELL Latitude D510 Series CPU FAN

Most viral vaccines are based on live attenuated viruses, while many bacterial vaccines are based onacellular components of micro-organisms, including harmless toxin components.[14] Since many antigens derived from acellular vaccines do not strongly induce the adaptive response, DELL Latitude D531 CPU FAN most bacterial vaccines are provided with additional adjuvants that activate the antigen-presenting cells of the innate immune systemand maximize immunogenicity.

The immune system is a remarkably effective structure that incorporates specificity, inducibility and adaptation. DELL Latitude D600 Series CPU FAN Failures of host defense do occur, however, and fall into three broad categories: immunodeficiencies, autoimmunity, and hypersensitivities.

Immunodeficiencies occur when one or more of the components of the immune system are inactive. The ability of the immune system to respond to pathogens is diminished in both the young and the elderly, DELL Latitude D610 Series CPU FAN with immune responses beginning to decline at around 50 years of age due to immunosenescence.[69][70] In developed countries, obesity, alcoholism, and drug use are common causes of poor immune function.[70] However, malnutrition is the most common cause of immunodeficiency indeveloping countries.[70] DELL Latitude D620 CPU FAN

Diets lacking sufficient protein are associated with impaired cell-mediated immunity, complement activity, phagocyte function, IgAantibody concentrations, and cytokine production. Additionally, the loss of the thymus at an early age through genetic mutation or surgical removal results in severe immunodeficiency and a high susceptibility to infection.[71] DELL Latitude D630 CPU FAN

Immunodeficiencies can also be inherited or 'acquired'.[14] Chronic granulomatous disease, where phagocytes have a reduced ability to destroy pathogens, is an example of an inherited, or congenital, immunodeficiency. AIDS and some types of cancer cause acquired immunodeficiency. DELL Latitude D800 Series CPU FAN Overactive immune responses comprise the other end of immune dysfunction, particularly the autoimmune disorders. Here, the immune system fails to properly distinguish between self and non-self, and attacks part of the body. Under normal circumstances, many T cells and antibodies react with "self" peptides.[74] DELL Latitude D810 Series CPU FAN One of the functions of specialized cells (located in the thymus and bone marrow) is to present young lymphocytes with self antigens produced throughout the body and to eliminate those cells that recognize self-antigens, preventing autoimmunity.

Hypersensitivity is an immune response that damages the body's own tissues. DELL Latitude D820 CPU FAN They are divided into four classes (Type I – IV) based on the mechanisms involved and the time course of the hypersensitive reaction. Type I hypersensitivity is an immediate or anaphylactic reaction, often associated with allergy. Symptoms can range from mild discomfort to death. DELL Latitude D830 CPU FAN Type I hypersensitivity is mediated by IgE, which triggers degranulation of mast cells and basophils when cross-linked by antigen.[75] Type II hypersensitivity occurs when antibodies bind to antigens on the patient's own cells, marking them for destruction. This is also called antibody-dependent (or cytotoxic) hypersensitivity, and is mediated by IgG and IgM antibodies.[75] DELL Latitude E4200 Series CPU FAN Immune complexes (aggregations of antigens, complement proteins, and IgG and IgM antibodies) deposited in various tissues trigger Type III hypersensitivity reactions.[75] Type IV hypersensitivity (also known as cell-mediated or delayed type hypersensitivity) usually takes between two and three days to develop. DELL Latitude E4300 Series CPU FAN Type IV reactions are involved in many autoimmune and infectious diseases, but may also involve contact dermatitis (poison ivy). These reactions are mediated by T cells, monocytes, and macrophages.

It is likely that a multicomponent, adaptive immune system arose with the first vertebrates, as invertebrates do not generate lymphocytes or an antibody-based humoral response.[1] DELL LATITUDE E4310 Series CPU FAN Many species, however, utilize mechanisms that appear to be precursors of these aspects of vertebrate immunity. Immune systems appear even in the structurally most simple forms of life, with bacteria using a unique defense mechanism, called the restriction modification system to protect themselves from viral pathogens, called bacteriophages.[76] DELL Latitude E5400 CPU FAN Prokaryotes also possess acquired immunity, through a system that uses CRISPR sequences to retain fragments of the genomes of phage that they have come into contact with in the past, which allows them to block virus replication through a form of RNA interference.[77][78] DELL Latitude E5500 CPU FAN Pattern recognition receptors are proteins used by nearly all organisms to identify molecules associated with pathogens. Antimicrobial peptides called defensins are an evolutionarily conserved component of the innate immune response found in all animals and plants, and represent the main form of invertebrate systemicimmunity.[1] DELL Latitude E6400 Series CPU FAN The complement system and phagocytic cells are also used by most forms of invertebrate life. Ribonucleases and the RNA interference pathway are conserved across all eukaryotes, and are thought to play a role in the immune response to viruses.[79]

Unlike animals, plants lack phagocytic cells, but many plant immune responses involve systemic chemical signals that are sent through a plant.[80] DELL Latitude E6410 CPU FAN Individual plant cells respond to molecules associated with pathogens known as Pathogen-associated molecular patterns or PAMPs.[81] When a part of a plant becomes infected, the plant produces a localized hypersensitive response, whereby cells at the site of infection undergo rapid apoptosis to prevent the spread of the disease to other parts of the plant. DELL Latitude E6410 Series CPU FAN Systemic acquired resistance (SAR) is a type of defensive response used by plants that renders the entire plant resistant to a particular infectious agent.[80] RNA silencing mechanisms are particularly important in this systemic response as they can block virus replication. DELL Latitude E6420 CPU FAN Another important role of the immune system is to identify and eliminate tumors. The transformed cells of tumors expressantigens that are not found on normal cells. To the immune system, these antigens appear foreign, and their presence causes immune cells to attack the transformed tumor cells. DELL Latitude E6500 CPU FAN The antigens expressed by tumors have several sources;[84]some are derived from oncogenic viruses like human papillomavirus, which causes cervical cancer,[85] while others are the organism's own proteins that occur at low levels in normal cells but reach high levels in tumor cells. DELL Latitude E6500 Series CPU FAN One example is anenzyme called tyrosinase that, when expressed at high levels, transforms certain skin cells (e.g. melanocytes) into tumors called melanomas.[86][87] A third possible source of tumor antigens are proteins normally important for regulatingcell growth and survival, that commonly mutate into cancer inducing molecules called oncogenes.[84][88][89] DELL Latitude E6510 CPU FAN The main response of the immune system to tumors is to destroy the abnormal cells using killer T cells, sometimes with the assistance of helper T cells.[87][90] Tumor antigens are presented on MHC class I molecules in a similar way to viral antigens. This allows killer T cells to recognize the tumor cell as abnormal.[91] DELL Latitude E6510 Series CPU FAN

 NK cells also kill tumorous cells in a similar way, especially if the tumor cells have fewer MHC class I molecules on their surface than normal; this is a common phenomenon with tumors.[92] Sometimes antibodies are generated against tumor cells allowing for their destruction by the complement system.[88] DELL Precision M4400 Series CPU FAN Clearly, some tumors evade the immune system and go on to become cancers.[93] Tumor cells often have a reduced number of MHC class I molecules on their surface, thus avoiding detection by killer T cells.[91] Some tumor cells also release products that inhibit the immune response; DELL Precision M4500 CPU FAN for example by secreting the cytokine TGF-β, which suppresses the activity ofmacrophages and lymphocytes.[94] In addition, immunological tolerance may develop against tumor antigens, so the immune system no longer attacks the tumor cells.[93] DELL Precision M6400 Series CPU FAN Paradoxically, macrophages can promote tumor growth [95] when tumor cells send out cytokines that attract macrophages, which then generate cytokines and growth factors that nurture tumor development. In addition, a combination of hypoxia in the tumor and a cytokine produced by DELL Precision M65 CPU FAN macrophages induces tumor cells to decrease production of a protein that blocks metastasis and thereby assists spread of cancer cells.

Hormones can act as immunomodulators, altering the sensitivity of the immune system. For example, DELL Precision M70 Series CPU FAN female sex hormones are known immunostimulators of both adaptive[96] and innate immune responses.[97] Some autoimmune diseases such as lupus erythematosus strike women preferentially, and their onset often coincides with puberty. By contrast, male sex hormones such as testosterone seem to be immunosuppressive.[98] DELL Precision M90 CPU FAN Other hormones appear to regulate the immune system as well, most notably prolactin, growth hormone and vitamin D.[99][100]

When a T-cell encounters a foreign pathogen, it extends a vitamin D receptor. DELL Pricision M6300 CPU FAN This is essentially a signaling device that allows the T-cell to bind to the active form of vitamin D, the steroid hormone calcitriol. T-cells have a symbiotic relationship with vitamin D. Not only does the T-cell extend a vitamin D receptor, in essence asking to bind to the steroid hormone version of vitamin D, calcitriol, but the T-cell expresses the gene CYP27B1, DELL Studio 1435 CPU FAN which is the gene responsible for converting the pre-hormone version of vitamin D, calcidiol into the steroid hormone version, calcitriol. Only after binding to calcitriol can T-cells perform their intended function. Other immune system cells that are known to express CYP27B1 and thus activate vitamin D calcidiol, are dendritic cells,keratinocytes and macrophages.[101][102] DELL Studio 1450 CPU FAN It is conjectured that a progressive decline in hormone levels with age is partially responsible for weakened immune responses in aging individuals.[103]Conversely, some hormones are regulated by the immune system, notably thyroid hormone activity.[104] DELL Studio 1457 CPU FAN

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