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CD4, a single chain transmembrane glycoprotein, is found on a T cell subset (helper/inducer) representing 45% of peripheral blood lymphocytes. It is also present on 80% of thymocytes and at a lower level on monocytes. It is involved in recognition of antigen presented along with MHC class II by APCs. It serves as receptor for HIV. Antibody to CD4 recognizes T-helper cells required for recognition of class II antigens. It reacts with ~60% of peripheral blood E rosette-positive (E+) cells while showing negligible reactivity with E- cells, monocytes, granulocytes, EBV-transformed B cell lines, and mouse splenocytes.

Endoglin is a type I membrane glycoprotein located on cell surfaces and is part of the TGF beta receptor complex.
The Endoglin protein consists of a homodimer of 180 kDA with disulfide links. Endoglin has been found on endothelial cells, activated macrophages, fibroblasts, and smooth muscle cells. Furthermore, Endoglin has been found to be part of the TGF-beta1 receptor complex. Endoglin thus may be involved in the binding of TGF-beta1, TGF-beta3, activin-A, BMP-2, and BMP-7. Beside TGF-beta signaling endoglin may have other functions. It has been postulated that endoglin is involved in the cytoskeletal organization affecting cell morphology and migration. Endoglin has a role in the development of the cardiovascular system and in vascular remodeling. Endoglin expression is regulated during heart development . Experimental mice without the endoglin gene die due to cardiovascular abnormalities.

CD207 (C-type lectin domain family 4 member K) is expressed in Langerhans cells which are immature dendritic cells of the epidermis and mucosa. Moreover, CD207 is expressed in several other dendritic cell types including dermal CD103+ DCs and splenic CD8+ DCs. Langerin is localized in the Birbeck granules, the organelles present in the cytoplasm of Langerhans cells and comprised of superimposed and zippered membranes. CD207 is a C-type lectin with mannose binding specificity, and it has been suggested that mannose binding by the CD207 protein leads to internalization of antigen into Birbeck granules thus providing access to a nonclassical antigen-processing pathway.

CD93, is a receptor or else an element of a larger receptor complex for C1q, MBL2-mannose-binding lectin and SPA-pulmonary surfactant protein A. CD93 mediates the enhancement of phagocytosis in monocytes as well as macrophages upon interaction with soluble defense collagens. CD93 takes part in the intercellular adhesion. Furthermore, CD93 was expressed on (pre) plasmablasts/plasma cells, including long-lived plasma cells which demonstrated decreased cell cycle activity, high levels of isotype-switched Ig secretion, as well as modification of the transcriptional network. CD93 is vital for the maintenance of plasma cells in bone marrow niches.

HCV is a small 50nm, enveloped, single-stranded, positive sense RNAvirus in the family Flaviviridae.
HCV has a high rate of replication with approximately one trillion particles produced each day in an infected individual. Due to lack of proofreading by the HCV RNA polymerase, the HCV has an exceptionally high mutation rate, a factor that may help it elude the host's immune response. Hepatitis C virus is classified into six genotypes(1-6) with several subtypes within each genotype. The preponderance and distribution of HCV genotypes varies globally. Genotype is clinically important in determining potential response to interferon-based therapy and the required duration of such therapy. Genotypes 1 and 4 are less responsive to interferon-based treatment than are the other genotypes (2, 3, 5 and 6).

BTB/POZ domain-containing protein KCTD5 (KCTD5), is a 234 amino acid protein which localizes mainly in the cytoplasm however translocates to the nucleus on interaction with REP proteins. The expression of KCTD5 is up regulated post-transcriptionally in peripheral blood lymphocytes stimulated via the T-cell receptor. KCTD5 interacts particularly with cullin3, attaches ubiquitinated proteins, and created oligomers through its BTB domain.

CD28 co-stimulation is necessary for CD4 positive T-cell proliferation & survival, interleukin-2 production, and T-helper type-2 development. Human post-thymic regulatory T cells require CD28 co-stimulation to expand and maintain potent suppressive function in vivo. Apoptosis plays a key role in the age-related decline of CD28 expression and in immunosenescence.

CD1D, also known as antigen-presenting glycoprotein CD1d, is a transmembrane glycoprotein which belongs to the CD1 family of glycolipid antigen-presenting MHC-like molecules. CD1d-presented lipid antigens activate a special class of T cells, familiar as natural killer T (NKT) cells, during the interaction with the T-cell receptor present on NKT membranes. Once activated, NKT cells rapidly produce Th1 &Th2 cytokines, usually represented by interleukin 4 production.

CD86, also known as B7-2 or Cluster of Differentiation 86 is a protein necessary for T cell activation and survival, it is expressed on cells that are antigen-presenting cells and acts as costimulatory signals for T cells. CD86 is the ligand for CD28 (for autoregulation and intercellular association) and CTLA-4 (for attenuation of regulation and cellular disassociation), two different proteins.

CD99, or MIC2 gene product, or E2 antigen is expressed on the cell membrane of some lymphocytes, cortical thymocytes, and granulosa cells of the ovary. The antigen is also expressed by most pancreatic islet cells, Sertoli cells of the testis, and some endothelial cells. Mature granulocytes express very little or no CD99. MIC2 is strongly expressed on Ewing’s sarcoma cells and primitive peripheral neuroectodermal tumors.

CD72 antigen is a member of the type II integral membrane glycoproteins which includes other related cell surface moleculaes such as the asialoglycoprotein receptors, CD23 and the Kupffer cell recpeotr. The function of CD72 is unknown but the exposure of B cells to CD72 antibodies activates a variety of signaling pathways and can induce MHC class II expression and B cell proliferation. CD72 antigen is expressed on all cells of B cell lineage with the exception of plasma cells and weakly on human tissue macrophages.

CD22, also known as Sialic Acid-Binding Ig-Like Lectin 2, is part of the immunoglobulin(Ig)superfamily. CD22 mediates B-cell to B-cell interactions and is implicated in the localization of B-cells in lymphoid tissues. CD22 acts as positive regulator via interaction with the Src family tyrosine kinases, in addition CD22 preform as an inhibitory receptor by recruiting cytoplasmic phosphatases.

CD79B is a 36 kDa trans membrane glycoprotein belonging to the immunoglobulin superfamily. Heterodimers of CD79A and CD79B connect with a membrane attached immunoglobulin on the B cell surface to create the B cell antigen receptor complex (BCR). CD79A and CD79B are essential for BCR-mediated signaling and subsequently for the development and activation of B lineage cells.

CD209 (a member of the C-typelectin family) is Pathogen-recognition receptor expressed on the surface of immature dendritic cells (DCs) andtakes part in initiation of primary immune response.CD209has a high affinity for theICAM3 molecule and binds various microorganisms by recognizing high-mannose-containing glycoproteins ontheir envelopes and especially functions as receptor for several viruses such as HIV and Hepatitis C. Along with other C-typelectins, CD209is involved in recognition of tumors by dendritic cells.CD209also initiatesnatural immunity bymodulating toll-like receptors.

CD200 is a type-1 membrane glycoprotein which is a part of the immunoglobulin superfamily due to its 2 immunoglobulin domains. CD200 regulates myeloid cell activity and delivers an inhibitory signal for the macrophage lineage in various tissues. CD200 is also Costimulates T-cell proliferation.

CD3-epsilonpolypeptide (CD3E) along with CD3-gamma, -delta and -zeta, and the T-cell receptor alpha/beta and gamma/delta heterodimers, creates the T-cell receptor-CD3 complex. This complex has an imperative role in coupling antigen recognition to a number of intracellular signal-transduction pathways. The genes encoding the epsilon, gamma delta polypeptides are found in the same cluster on chromosome 11. The epsilon polypeptide plays a vital role in T-cell development. CD3E gene defects cause immunodeficiency. CD3E is also linked to a susceptibility to type I diabetes in women.

CD2 which is also known as E-rosette receptor, T11 and lymphocytefunction antigen-2 (LFA-2) is expressed on T cells, thymocytes, and subset of natural killer cells. Human CD2 functions as the receptor for sheep erythrocytes, human CD58 (LFA-3), and CD15s (Sialyl Lewis X). p56lck, p59fyn, CD3eta and CD3epsilon are tyrosine-phosphorylated after CD2 stimulation. CD2 plays a role in T cell activation, T- or NK-cell-mediated cytolysis, apoptosis in activated peripheral T cells, and regulation of T cell anergy.

CD43 is a key sialoglycoprotein located in the surface of T lymphocytes, monocytes, granulocytes, thymocytes and some B lymphocytes. CD43 is a part of a physiologic ligand-receptor complex which participates in T-cell activation. Throughout T-cell activation, CD43 is actively taken out from the T-cell-APC (antigen-presenting cell) contact site, signifying CD43 has a negative regulatory role in adaptive immune response.

CD40 belongs to the TNF-receptor super family. CD40 has been found to be vital in mediating a wide range of immune and inflammatory responses including T cell-dependent immunoglobulin classswitching, memory B cell development, and germinal center formation. AT-hook transcription factor AKNA isaccounted to coordinately regulate the expression of CD40 and its ligand, which is significant forhomotypic cell interactions. Adaptor protein TNFR2 interacts with CD40 and functions as a mediator of thesignal transduction. The interaction of CD40 and its ligand is found to be essential for amyloid-beta-inducedmicroglial activation, and therefore is considered to be an early event in Alzheimer disease pathogenesis.

CD8 protein is a glycoprotein that is located on the cell surface of cytotoxic T lymphocytes the interferes interactions between cells of the immune system. This protein is a coreceptor that helps detect antigens shown by the APC (antigen presenting cells) as part of the class I MHC proteins. It can be a homodimer build from 2 alpha chains orheterodimer build from alpha and beta chains.

CD9 which is found on the surface of exosomes is a cell surface glycoprotein which is interacts with integrins and various transmembrane superfamily proteins. CD9 is involved in platelet activation and aggregation and also regulates paranodal junction formation. CD9 takes part in cell adhesion and migration and also promotes muscle cell fusion. CD9 is necessary for the egg-sperm fusion during mammalian fertilization.

CD27 belongs to the TNF-receptor superfamily. CD27 is necessary for initiation and long-term maintenance of T cell immunity. CD27 binds to ligand CD70, and has a crucial role in regulating B-cell activation and immunoglobulin synthesis. The CD27 receptor transduces signals which result in the activation of NF-kappaB and MAPK8/JNK. Adaptor proteins TRAF2 and TRAF5 mediate the signaling process of CD27. CD27-binding protein (SIVA), which is a proapoptotic protein, can bind to the CD27 receptor and is believed to have a significant role in the apoptosis induced by CD27.

Cytochrome b5 reductase 1 (CYB5R1) is a part of the flavoprotein pyridine nucleotide cytochrome reductase family. CYB5R1 is engaged in desaturation and elongation of fatty acids, cholesterol biosynthesis, drug metabolism, and, in erythrocyte, methemoglobin decrease.

CD44 is a cell-surface glycoprotein that plays a role in cell-cell interactions, cell adhesion and migration. CD44 is a receptor for hyaluronic acid and also interacts with other ligands, such as osteopontin, collagens, and matrix metalloproteinases. CD44 participates in a wide variety of cellular functions such as lymphocyte activation, recirculation and homing, hematopoiesis, and tumor metastasis.