Tuesday, October 8, 2013

Leukocytes (white blood cells)

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 When B cells and T cells are activated and begin to replicate, some of their offspring become long-lived memory cells. Throughout the lifetime of an animal, these memory cells remember each specific pathogen encountered and can mount a strong response if the pathogen is detected again. DELL Latitude D400 Series CPU FAN This is "adaptive" because it occurs during the lifetime of an individual as an adaptation to infection with that pathogen and prepares the immune system for future challenges. Immunological memory can be in the form of either passive short-term memory or active long-term memory.
Newborn infants have no prior exposure to microbes and are particularly vulnerable to infection. DELL Latitude D410 Series CPU FAN

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