A lymphocyte is one of the subtypes of a white blood cell in a vertebrate's immune system. Lymphocytes include natural killer cells, T cells, B cells, they are the main type of cell found in lymph, which prompted the name "lymphocyte". The three major types of lymphocyte are B cells and natural killer cells. Lymphocytes can be identified by their large nucleus. T cells and B cells are the major cellular components of the adaptive immune response. T cells are involved in cell-mediated immunity, whereas B cells are responsible for humoral immunity; the function of T cells and B cells is to recognize specific "non-self" antigens, during a process known as antigen presentation. Once they have identified an invader, the cells generate specific responses that are tailored to maximally eliminate specific pathogens or pathogen-infected cells. B cells respond to pathogens by producing large quantities of antibodies which neutralize foreign objects like bacteria and viruses. In response to pathogens some T cells, called T helper cells, produce cytokines that direct the immune response, while other T cells, called cytotoxic T cells, produce toxic granules that contain powerful enzymes which induce the death of pathogen-infected cells.
Following activation, B cells and T cells leave a lasting legacy of the antigens they have encountered, in the form of memory cells. Throughout the lifetime of an animal, these memory cells will "remember" each specific pathogen encountered, are able to mount a strong and rapid response if the same pathogen is detected again. NK cells are a part of the innate immune system and play a major role in defending the host from tumors and virally infected cells. NK cells distinguish infected cells and tumors from normal and uninfected cells by recognizing changes of a surface molecule called MHC class I. NK cells are activated in response to a family of cytokines called interferons. Activated NK cells release cytotoxic granules which destroy the altered cells, they are named "natural killer cells" because they do not require prior activation in order to kill cells which are missing MHC class I. Mammalian stem cells differentiate into several kinds of blood cell within the bone marrow; this process is called haematopoiesis.
All lymphocytes originate, during this process, from a common lymphoid progenitor before differentiating into their distinct lymphocyte types. The differentiation of lymphocytes follows various pathways in a hierarchical fashion as well as in a more plastic fashion; the formation of lymphocytes is known as lymphopoiesis. B cells mature into B lymphocytes in the bursa equivalent, which in humans is the GALT, thought to be located in the Peyer's patches of the intestine, while T cells migrate to and mature in a distinct organ, called the thymus. Following maturation, the lymphocytes enter the circulation and peripheral lymphoid organs where they survey for invading pathogens and/or tumor cells; the lymphocytes involved in adaptive immunity differentiate further after exposure to an antigen. Effector lymphocytes function to eliminate the antigen, either by releasing antibodies, cytotoxic granules or by signaling to other cells of the immune system. Memory T cells remain in the peripheral tissues and circulation for an extended time ready to respond to the same antigen upon future exposure.
Microscopically, in a Wright's stained peripheral blood smear, a normal lymphocyte has a large, dark-staining nucleus with little to no eosinophilic cytoplasm. In normal situations, the coarse, dense nucleus of a lymphocyte is the size of a red blood cell; some lymphocytes show a clear perinuclear zone around the nucleus or could exhibit a small clear zone to one side of the nucleus. Polyribosomes are a prominent feature in the lymphocytes and can be viewed with an electron microscope; the ribosomes are involved in protein synthesis, allowing the generation of large quantities of cytokines and immunoglobulins by these cells. It is impossible to distinguish between B cells in a peripheral blood smear. Flow cytometry testing is used for specific lymphocyte population counts; this can be used to determine the percentage of lymphocytes that contain a particular combination of specific cell surface proteins, such as immunoglobulins or cluster of differentiation markers or that produce particular proteins.
In order to study the function of a lymphocyte by virtue of the proteins it generates, other scientific techniques like the ELISPOT or secretion assay techniques can be used. In the circulatory system, they move from lymph node to lymph node; this contrasts with macrophages. A lymphocyte count is part of a peripheral complete blood cell count and is expressed as the percentage of lymphocytes to the total number of white blood cells counted. A general increase in the number of lymphocytes is known as lymphocytosis, whereas a decrease is known as lymphocytopenia. An increase in lymphocyte concentration is a sign of a viral infection. A high lymphocyte count wi
In molecular biology, CD4 is a glycoprotein found on the surface of immune cells such as T helper cells, monocytes and dendritic cells. It was discovered in the late 1970s and was known as leu-3 and T4 before being named CD4 in 1984. In humans, the CD4 protein is encoded by the CD4 gene. CD4 + T helper cells are white blood cells, they are referred to as CD4 cells, T-helper cells or T4 cells. They are called helper cells because one of their main roles is to send signals to other types of immune cells, including CD8 killer cells, which destroy the infectious particle. If CD4 cells become depleted, for example in untreated HIV infection, or following immune suppression prior to a transplant, the body is left vulnerable to a wide range of infections that it would otherwise have been able to fight. Like many cell surface receptors/markers, CD4 is a member of the immunoglobulin superfamily, it has four immunoglobulin domains that are exposed on the extracellular surface of the cell: D1 and D3 resemble immunoglobulin variable domains.
D2 and D4 resemble immunoglobulin constant domains. The immunoglobulin variable domain of D1 adopts an immunoglobulin-like β-sandwich fold with seven β-strands in 2 β-sheets, in a Greek key topology. CD4 interacts with the β2-domain of MHC class II molecules through its D1 domain. T cells displaying CD4 molecules on their surface, are specific for antigens presented by MHC II and not by MHC class I. MHC class I contains Beta-2 microglobulin; the short cytoplasmic/intracellular tail of CD4 contains a special sequence of amino acids that allow it to recruit and interact with the tyrosine kinase Lck. CD4 is a co-receptor of the T cell receptor and assists the latter in communicating with antigen-presenting cells; the TCR complex and CD4 each bind to distinct regions of the antigen-presenting MHCII molecule - α1/β1 and β2, respectively. In CD4 the interaction involves its extracellular D1 domain; the resulting close proximity between the TCR complex and CD4 allows the tyrosine kinase Lck bound to the cytoplasmic tail of CD4 to tyrosine-phosphorylate the Immunoreceptor tyrosine activation motifs on the cytoplasmic domains of CD3 to amplify the signal generated by the TCR.
Lck is essential for the activation of many molecular components of the signaling cascade of an activated T cell. Depending on the signal, different types of T helper cells result. Phosphorylated ITAM motifs on CD3 recruit and activate SH2 domain-containing protein tyrosine kinases such as Zap70 to further mediate downstream signalling through tyrosine phosphorylation, leading to transcription factor activation including NF-κB and consequent T cell activation. CD4 has been shown to interact with SPG21, Lck and Protein unc-119 homolog. HIV-1 uses CD4 to gain entry into host T-cells and achieves this through its viral envelope protein known as gp120; the binding to CD4 creates a shift in the conformation of gp120 allowing HIV-1 to bind to a co-receptor expressed on the host cell. These co-receptors are chemokine receptors CCR5 or CXCR4. Following a structural change in another viral protein, HIV inserts a fusion peptide into the host cell that allows the outer membrane of the virus to fuse with the cell membrane.
HIV infection leads to a progressive reduction in the number of T cells expressing CD4. Medical professionals refer to the CD4 count to decide when to begin treatment during HIV infection, although recent medical guidelines have changed to recommend treatment at all CD4 counts as soon as HIV is diagnosed. A CD4 count measures the number of T cells expressing CD4. While CD4 counts are not a direct HIV test—e.g. They do not check the presence of viral DNA, or specific antibodies against HIV—they are used to assess the immune system of a patient. National Institutes of Health guidelines recommend treatment of any HIV-positive individuals, regardless of CD4 count Normal blood values are expressed as the number of cells per microliter of blood, with normal values for CD4 cells being 500–1200 cells/mm3. Patients undergo treatments when the CD4 counts reach a level of 350 cells per microliter in Europe but around 500/μL in the US. Medical professionals refer to CD4 tests to determine efficacy of treatment.
Viral load testing provides more information about the efficacy for therapy than CD4 counts. For the first 2 years of HIV therapy, CD4 counts may be done every 3–6 months. If a patient's viral load becomes undetectable after 2 years CD4 counts might not be needed if they are above 500/mm3. If the count remains at 300–500/mm3 the tests can be done annually, it is not necessary to schedule CD4 counts with viral load tests and the two should be done independently when each is indicated. CD4 continues to be expressed in most neoplasms derived from T helper cells, it is therefore possible to use CD4 immunohistochemistry on tissue biopsy samples to identify most forms of peripheral T cell lymphoma and related malignant conditions. The antigen has been associated with a number of autoimmune diseases such as vitiligo and type I diabetes mellitus. T-cells play a large part in autoinflammatory diseases; when testing a drug's efficacy or studying diseases, it is helpful to quantify the amount of T-cells. on fresh-frozen tissue with CD4+, CD8+, CD3+ T-cell markers.
CD4+ T cells and antitumor immunity CD1+Ant
Buttock cells are cells having a notched appearance that are found in certain malignancies, such as non-Hodgkin's lymphoma, mycosis fungoides, Sézary syndrome. Clue cell Koilocyte Large cell
A biopharmaceutical known as a biologic medical product, or biologic, is any pharmaceutical drug product manufactured in, extracted from, or semisynthesized from biological sources. Different from synthesized pharmaceuticals, they include vaccines, blood components, somatic cells, gene therapies, recombinant therapeutic protein, living cells used in cell therapy. Biologics can be composed of sugars, proteins, or nucleic acids or complex combinations of these substances, or may be living cells or tissues, they are isolated from living sources—human, plant, fungal, or microbial. Terminology surrounding biopharmaceuticals varies between groups and entities, with different terms referring to different subsets of therapeutics within the general biopharmaceutical category; some regulatory agencies use the terms biological medicinal products or therapeutic biological product to refer to engineered macromolecular products like protein- and nucleic acid-based drugs, distinguishing them from products like blood, blood components, or vaccines, which are extracted directly from a biological source.
Specialty drugs, a recent classification of pharmaceuticals, are high-cost drugs that are biologics. The European Medicines Agency uses the term advanced therapy medicinal products for medicines for human use that are "based on genes, cells, or tissue engineering", including gene therapy medicines, somatic-cell therapy medicines, tissue-engineered medicines, combinations thereof. Within EMA contexts, the term advanced therapies refers to ATMPs, although that term is rather nonspecific outside those contexts. Gene-based and cellular biologics, for example are at the forefront of biomedical research, may be used to treat a variety of medical conditions for which no other treatments are available. In some jurisdictions, biologics are regulated via different pathways than other small molecule drugs and medical devices; the term biopharmacology is sometimes used to describe the branch of pharmacology that studies biopharmaceuticals. Some of the oldest forms of biologics are extracted from the bodies of animals, other humans especially.
Important biologics include: Whole blood and other blood components Organs and tissue transplants Stem cell therapy Antibodies for passive immunization Human breast milk Fecal microbiota Human reproductive cellsSome biologics that were extracted from animals, such as insulin, are now more produced by recombinant DNA. As indicated the term "biologics" can be used to refer to a wide range of biological products in medicine. However, in most cases, the term "biologics" is used more restrictively for a class of therapeutics that are produced by means of biological processes involving recombinant DNA technology; these medications are one of three types: Substances that are identical to the body's own key signalling proteins. Examples are the blood-production stimulating protein erythropoetin, or the growth-stimulating hormone named "growth hormone" or biosynthetic human insulin and its analogues. Monoclonal antibodies; these are similar to the antibodies that the human immune system uses to fight off bacteria and viruses, but they are "custom-designed" and can therefore be made to counteract or block any given substance in the body, or to target any specific cell type.
Receptor constructs based on a occurring receptor linked to the immunoglobulin frame. In this case, the receptor provides the construct with detailed specificity, whereas the immunoglobulin-structure imparts stability and other useful features in terms of pharmacology; some examples are listed in the table below. Biologics as a class of medications in this narrower sense have had a profound impact on many medical fields rheumatology and oncology, but cardiology, gastroenterology and others. In most of these disciplines, biologics have added major therapeutic options for the treatment of many diseases, including some for which no effective therapies were available, others where existing therapies were inadequate. However, the advent of biologic therapeutics has raised complex regulatory issues, significant pharmacoeconomic concerns, because the cost for biologic therapies has been higher than for conventional medications; this factor has been relevant since many biological medications are used for the treatment of chronic diseases, such as rheumatoid arthritis or inflammatory bowel disease, or for the treatment of otherwise untreatable cancer during the remainder of life.
The cost of treatment with a typical monoclonal antibody therapy for common indications is in the range of €7,000–14,000 per patient per year. Older patients who receive biologic therapy for diseases such as rheumatoid arthritis, psoriatic arthritis, or ankylosing spondylitis are at increased risk for life-threatening infection, adverse cardiovascular events, malignancy; the first such substance approved for therapeutic use was biosynthetic "human" insulin made via recombinant DNA. Sometimes referred to as rHI, under the trade name Humulin, was developed by Genentech, but licensed to Eli Lilly and Company, who manufactured and marketed it starting in 1982. Major kinds of biopharmaceuticals include: Blood factors Thrombolytic agents Hormones (insulin, growth hormone, gonadotr
Glycosaminoglycans or mucopolysaccharides are long unbranched polysaccharides consisting of a repeating disaccharide unit. The repeating unit consists of an amino sugar along with a uronic galactose. Glycosaminoglycans are polar and attract water, they are therefore useful to the body as a shock absorber. Mucopolysaccharidoses are a group of metabolic disorders in which abnormal accumulations of glycosaminoglycans occur because of enzyme deficiencies. Glycosaminoglycans have high degrees of heterogeneity with regards to molecular mass, disaccharide construction, sulfation due to the fact that GAG synthesis, unlike proteins or nucleic acids, is not template driven, dynamically modulated by processing enzymes. Based on core disaccharide structures, GAGs are classified into four groups. Heparin/heparan sulfate and chondroitin sulfate/dermatan sulfate are synthesized in the Golgi apparatus, where protein cores made in the rough endoplasmic reticulum are posttranslationally modified with O-linked glycosylations by glycosyltransferases forming proteoglycans.
Keratan sulfate may modify core proteins through N-linked glycosylation or O-linked glycosylation of the proteoglycan. The fourth class of GAG, hyaluronic acid, is not synthesized by the Golgi, but rather by integral membrane synthases which secrete the dynamically elongated disaccharide chain. HSGAG and CSGAG modified proteoglycans first begin with a consensus Ser-Gly/Ala-X-Gly motif in the core protein. Construction of a tetrasaccharide linker that consists of -GlcAβ1–3Galβ1–3Galβ1–4Xylβ1-O--, where xylosyltransferase, β4-galactosyl transferase,β3-galactosyl transferase, β3-GlcA transferase transfer the four monosaccharides, begins synthesis of the GAG modified protein; the first modification of the tetrasaccharide linker determines whether the HSGAGs or CSGAGs will be added. Addition of a GlcNAc promotes the addition of HSGAGs while addition of GalNAc to the tetrasaccharide linker promotes CSGAG development. GlcNAcT-I transfers GlcNAc to the tetrasaccahride linker, distinct from glycosyltransferase GlcNAcT-II, the enzyme, utilized to build HSGAGs.
EXTL2 and EXTL3, two genes in the EXT tumor suppressor family, have been shown to have GlcNAcT-I activity. Conversely, GalNAc is transferred to the linker by the enzyme GalNAcT to initiate synthesis of CSGAGs, an enzyme which may or may not have distinct activity compared to the GalNAc transferase activity of chondroitin synthase. With regard to HSGAGs, a multimeric enzyme encoded by EXT1 and EXT2 of the EXT family of genes, transfers both GlcNAc and GlcA for HSGAG chain elongation. While elongating, the HSGAG is dynamically modified, first by N-deacetylase, N-sulfotransferase, a bifunctional enzyme that cleaves the N-acetyl group from GlcNAc and subsequently sulfates the N-position. Next, C-5 uronyl epimerase coverts d-GlcA to l-IdoA followed by 2-O sulfation of the uronic acid sugar by 2-O sulfotransferase; the 6-O and 3-O positions of GlcNAc moities are sulfated by 6-O and 3-O sulfotransferases. Chondroitin sulfate and dermatan sulfate, which comprise CSGAGs, are differentiated from each other by the presence of GlcA and IdoA epimers respectively.
Similar to the production of HSGAGs, C-5 uronyl epimerase converts d-GlcA to l-IdoA to synthesize dermatan sulfate. Three sulfation events of the CSGAG chains occur: 4-O and/or 6-O sulfation of GalNAc and 2-O sulfation of uronic acid. Four isoforms of the 4-O GalNAc sulfotransferases and three isoforms of the GalNAc 6-O sulfotransferases are responsible for the sulfation of GalNAc. Unlike HSGAGs and CSGAGs, the third class of GAGs, those belonging to keratan sulfate types, are driven towards biosynthesis through particular protein sequence motifs. For example, in the cornea and cartilage, the keratan sulfate domain of aggrecan consists of a series of tandemly repeated hexapeptides with a consensus sequence of EPFPS. Additionally, for three other keratan sulfated proteoglycans, lumican and mimecan, the consensus sequence NX along with protein secondary structure was determined to be involved in N-linked oligosaccharide extension with keratan sulfate. Keratan sulfate elongation begins at the nonreducing ends of three linkage oligosaccharides, which define the three classes of keratan sulfate.
Keratan sulfate. Keratan sulfate II and keratan sulfate III are O-linked, with KSII linkages identical to that of mucin core structure, KSIII linked to a 2-O mannose. Elongation of the keratan sulfate polymer occurs through the glycosyltransferase addition of Gal and GlcNAc. Galactose addition occurs through the β-1,4-galactosyltransferase enzyme while the enzymes responsible for β-3-Nacetylglucosamine have not been identified. Sulfation of the polymer occurs at the 6-position of both sugar residues; the enzyme KS-Gal6ST transfers sulfate groups to galactose while N-acetylglucosaminyl-6-sulfotransferase transfers sulfate groups to terminal GlcNAc in keratan sulfate. The fourth class of GAG, hyaluronan, is not sulfated and is synthesized by three transmembrane synthase proteins HAS1, HAS2, HAS3. HA, a linear polysaccharide, is composed of repeating disaccharide units of →4)GlcAβGlcNAcβ(1→ and has a high molecular mass, ranging from 105 to 107 Da; each HAS enzyme is capable of transglycosylation when supplied with UDP-G
A T cell, or T lymphocyte, is a type of lymphocyte that plays a central role in cell-mediated immunity. T cells can be distinguished from other lymphocytes, such as B cells and natural killer cells, by the presence of a T-cell receptor on the cell surface, they are called T cells. The several subsets of T cells each have a distinct function; the majority of human T cells, termed alpha beta T cells, rearrange their alpha and beta chains on the cell receptor and are part of the adaptive immune system. Specialized gamma delta T cells, have invariant T-cell receptors with limited diversity, that can present antigens to other T cells and are considered to be part of the innate immune system. Effector cells are the superset of all the various T cell types that respond to a stimulus, such as co-stimulation; this includes helper, killer and other T cell types. Memory cells are their opposite counterpart that are longer lived to target future infections as necessary. T helper cells assist other white blood cells in immunologic processes, including maturation of B cells into plasma cells and memory B cells, activation of cytotoxic T cells and macrophages.
These cells are known as CD4+ T cells because they express the CD4 glycoprotein on their surfaces. Helper T cells become activated when they are presented with peptide antigens by MHC class II molecules, which are expressed on the surface of antigen-presenting cells. Once activated, they divide and secrete small proteins called cytokines that regulate or assist in the active immune response; these cells can differentiate into one of several subtypes, including TH1, TH2, TH3, TH17, TH9, or TFH, which secrete different cytokines to facilitate different types of immune responses. Signalling from the APC directs T cells into particular subtypes. Cytotoxic T cells destroy virus-infected cells and tumor cells, are implicated in transplant rejection; these cells are known as CD8+ T cells since they express the CD8 glycoprotein at their surfaces. These cells recognize their targets by binding to antigen associated with MHC class I molecules, which are present on the surface of all nucleated cells. Through IL-10, other molecules secreted by regulatory T cells, the CD8+ cells can be inactivated to an anergic state, which prevents autoimmune diseases.
Antigen-naïve T cells expand and differentiate into memory and effector T cells after they encounter their cognate antigen within the context of an MHC molecule on the surface of a professional antigen presenting cell. Appropriate co-stimulation must be present at the time of antigen encounter for this process to occur. Memory T cells were thought to belong to either the effector or central memory subtypes, each with their own distinguishing set of cell surface markers. Subsequently, numerous new populations of memory T cells were discovered including tissue-resident memory T cells, stem memory TSCM cells, virtual memory T cells; the single unifying theme for all memory T cell subtypes is that they are long-lived and can expand to large numbers of effector T cells upon re-exposure to their cognate antigen. By this mechanism they provide the immune system with "memory" against encountered pathogens. Memory T cells may be either CD4+ or CD8+ and express CD45RO. Memory T cell subtypes: Central memory T cells express CD45RO, C-C chemokine receptor type 7, L-selectin.
Central memory T cells have intermediate to high expression of CD44. This memory subpopulation is found in the lymph nodes and in the peripheral circulation.. Effector memory T cells lack expression of CCR7 and L-selectin, they have intermediate to high expression of CD44. These memory T cells lack lymph node-homing receptors and are thus found in the peripheral circulation and tissues. TEMRA stands for terminally differentiated effector memory cells re-expressing CD45RA, a marker found on naive T cells. Tissue resident memory T cells occupy tissues without recirculating. One cell surface marker, associated with TRM is the integrin αeβ7. Virtual memory T cells differ from the other memory subsets in that they do not originate following a strong clonal expansion event. Thus, although this population as a whole is abundant within the peripheral circulation, individual virtual memory T cell clones reside at low frequencies. One theory is. Although CD8 virtual memory T cells were the first to be described, it is now known that CD4 virtual memory cells exist.
Regulatory T cells are crucial for the maintenance of immunological tolerance. Their major role is to shut down T cell-mediated immunity toward the end of an immune reaction and to suppress autoreactive T cells that escaped the process of negative selection in the thymus. Suppressor T cells along with Helper T cells can collectively be called Regulatory T cells due to their regulatory functions. Two major classes of CD4 + Treg cells have been described -- FOXP3 − Treg cells. Regulatory T cells can develop either during normal development in the thymus, are known as thymic Treg cells, or can be induced peripherally and are called peripherally derived Treg cel
International Standard Serial Number
An International Standard Serial Number is an eight-digit serial number used to uniquely identify a serial publication, such as a magazine. The ISSN is helpful in distinguishing between serials with the same title. ISSN are used in ordering, interlibrary loans, other practices in connection with serial literature; the ISSN system was first drafted as an International Organization for Standardization international standard in 1971 and published as ISO 3297 in 1975. ISO subcommittee TC 46/SC 9 is responsible for maintaining the standard; when a serial with the same content is published in more than one media type, a different ISSN is assigned to each media type. For example, many serials are published both in electronic media; the ISSN system refers to these types as electronic ISSN, respectively. Conversely, as defined in ISO 3297:2007, every serial in the ISSN system is assigned a linking ISSN the same as the ISSN assigned to the serial in its first published medium, which links together all ISSNs assigned to the serial in every medium.
The format of the ISSN is an eight digit code, divided by a hyphen into two four-digit numbers. As an integer number, it can be represented by the first seven digits; the last code digit, which may be 0-9 or an X, is a check digit. Formally, the general form of the ISSN code can be expressed as follows: NNNN-NNNC where N is in the set, a digit character, C is in; the ISSN of the journal Hearing Research, for example, is 0378-5955, where the final 5 is the check digit, C=5. To calculate the check digit, the following algorithm may be used: Calculate the sum of the first seven digits of the ISSN multiplied by its position in the number, counting from the right—that is, 8, 7, 6, 5, 4, 3, 2, respectively: 0 ⋅ 8 + 3 ⋅ 7 + 7 ⋅ 6 + 8 ⋅ 5 + 5 ⋅ 4 + 9 ⋅ 3 + 5 ⋅ 2 = 0 + 21 + 42 + 40 + 20 + 27 + 10 = 160 The modulus 11 of this sum is calculated. For calculations, an upper case X in the check digit position indicates a check digit of 10. To confirm the check digit, calculate the sum of all eight digits of the ISSN multiplied by its position in the number, counting from the right.
The modulus 11 of the sum must be 0. There is an online ISSN checker. ISSN codes are assigned by a network of ISSN National Centres located at national libraries and coordinated by the ISSN International Centre based in Paris; the International Centre is an intergovernmental organization created in 1974 through an agreement between UNESCO and the French government. The International Centre maintains a database of all ISSNs assigned worldwide, the ISDS Register otherwise known as the ISSN Register. At the end of 2016, the ISSN Register contained records for 1,943,572 items. ISSN and ISBN codes are similar in concept. An ISBN might be assigned for particular issues of a serial, in addition to the ISSN code for the serial as a whole. An ISSN, unlike the ISBN code, is an anonymous identifier associated with a serial title, containing no information as to the publisher or its location. For this reason a new ISSN is assigned to a serial each time it undergoes a major title change. Since the ISSN applies to an entire serial a new identifier, the Serial Item and Contribution Identifier, was built on top of it to allow references to specific volumes, articles, or other identifiable components.
Separate ISSNs are needed for serials in different media. Thus, the print and electronic media versions of a serial need separate ISSNs. A CD-ROM version and a web version of a serial require different ISSNs since two different media are involved. However, the same ISSN can be used for different file formats of the same online serial; this "media-oriented identification" of serials made sense in the 1970s. In the 1990s and onward, with personal computers, better screens, the Web, it makes sense to consider only content, independent of media; this "content-oriented identification" of serials was a repressed demand during a decade, but no ISSN update or initiative occurred. A natural extension for ISSN, the unique-identification of the articles in the serials, was the main demand application. An alternative serials' contents model arrived with the indecs Content Model and its application, the digital object identifier, as ISSN-independent initiative, consolidated in the 2000s. Only in 2007, ISSN-L was defined in the