Immunoglobulin variable domain (IgV); The members here are composed of the immunoglobulin variable domain (IgV). The IgV family contains the standard Ig superfamily V-set AGFCC'C"/DEB domain topology, and are components of immunoglobulin (Ig) and T cell receptors. The basic structure of Ig molecules is a tetramer of two light chains and two heavy chains linked by disulfide bonds. In Ig, each chain is composed of one variable domain (IgV) and one or more constant domains (IgC); these names reflect the fact that the variability in sequences is higher in the variable domain than in the constant domain. Within the variable domain, there are regions of even more variability called the hypervariable or complementarity-determining regions (CDRs) which are responsible for antigen binding. A predominant feature of most Ig domains is the disulfide bridge connecting 2 beta-sheets with a tryptophan residue packed against the disulfide bond. Ig superfamily (IgSF) domains can be divided into 4 main classes based on their structures and sequences: the Variable (V), Constant 1 (C1), Constant 2 (C2), and Intermediate (I) sets. Typically, the V-set domains have A, B, E and, D strands in one sheet and A', G, F, C, C', and C" strands in the other.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1207170594  21 VTQSPV-ITVSKGQTAQLDCNL--GTVTNYAAWYKQTPAEVPQYVLYNYhgwSSPEYGSGFSAPKFTSTCSSKSDCSLII 97
Cdd:cd00099     1 VTQSPRsLSVQEGESVTLSCEVssSFSSTYIYWYRQKPGQGPEFLIYLS---SSKGKTKGGVPGRFSGSRDGTSSFSLTI 77

                          90       100       110
                  ....*....|....*....|....*....|....
gi 1207170594  98 SNVDVSDSAVYYCQTWDSS-VSEYVFGQGTKLIV 130
Cdd:cd00099    78 SNLQPEDSGTYYCAVSESGgTDKLTFGSGTRLTV 111

Immunoglobulin variable domain (IgV); The members here are composed of the immunoglobulin variable domain (IgV). The IgV family contains the standard Ig superfamily V-set AGFCC'C"/DEB domain topology, and are components of immunoglobulin (Ig) and T cell receptors. The basic structure of Ig molecules is a tetramer of two light chains and two heavy chains linked by disulfide bonds. In Ig, each chain is composed of one variable domain (IgV) and one or more constant domains (IgC); these names reflect the fact that the variability in sequences is higher in the variable domain than in the constant domain. Within the variable domain, there are regions of even more variability called the hypervariable or complementarity-determining regions (CDRs) which are responsible for antigen binding. A predominant feature of most Ig domains is the disulfide bridge connecting 2 beta-sheets with a tryptophan residue packed against the disulfide bond. Ig superfamily (IgSF) domains can be divided into 4 main classes based on their structures and sequences: the Variable (V), Constant 1 (C1), Constant 2 (C2), and Intermediate (I) sets. Typically, the V-set domains have A, B, E and, D strands in one sheet and A', G, F, C, C', and C" strands in the other.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1207170594  21 VTQSPV-ITVSKGQTAQLDCNL--GTVTNYAAWYKQTPAEVPQYVLYNYhgwSSPEYGSGFSAPKFTSTCSSKSDCSLII 97
Cdd:cd00099     1 VTQSPRsLSVQEGESVTLSCEVssSFSSTYIYWYRQKPGQGPEFLIYLS---SSKGKTKGGVPGRFSGSRDGTSSFSLTI 77

                          90       100       110
                  ....*....|....*....|....*....|....
gi 1207170594  98 SNVDVSDSAVYYCQTWDSS-VSEYVFGQGTKLIV 130
Cdd:cd00099    78 SNLQPEDSGTYYCAVSESGgTDKLTFGSGTRLTV 111

Immunoglobulin C1-set domain;

                          10        20        30        40        50        60        70
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*...
gi 1207170594 145 PSSREELSSsKVTLLCLINHMSVAFADVRWLVNGNSVTEGVFTGSAEQQPDHKFKSSSSLTIQRSEWDKDRELTCEAT 222
Cdd:pfam07654   5 PPSPEELGK-PNTLTCLVTGFYPPDITVTWLKNGQEVTEGVKTTPPSPNSDWTYQLSSYLTVTPSDWESGDEYTCRVE 81

Immunoglobulin V-Type;

                           10        20        30        40        50        60        70        80
                   ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1207170594   34 TAQLDCNLGTVTN---YAAWYKQTPAEVPQYVLYnyHGWSSPEYGSGFSAPKFT-STCSSKSDCSLIISNVDVSDSAVYY 109
Cdd:smart00406   1 SVTLSCKFSGSTFssyYVSWVRQPPGKGLEWLGY--IGSNGSSYYQESYKGRFTiSKDTSKNDVSLTISNLRVEDTGTYY 78

                   ...
gi 1207170594  110 CQT 112
Cdd:smart00406  79 CAV 81

Immunoglobulin variable domain (IgV); The members here are composed of the immunoglobulin variable domain (IgV). The IgV family contains the standard Ig superfamily V-set AGFCC'C"/DEB domain topology, and are components of immunoglobulin (Ig) and T cell receptors. The basic structure of Ig molecules is a tetramer of two light chains and two heavy chains linked by disulfide bonds. In Ig, each chain is composed of one variable domain (IgV) and one or more constant domains (IgC); these names reflect the fact that the variability in sequences is higher in the variable domain than in the constant domain. Within the variable domain, there are regions of even more variability called the hypervariable or complementarity-determining regions (CDRs) which are responsible for antigen binding. A predominant feature of most Ig domains is the disulfide bridge connecting 2 beta-sheets with a tryptophan residue packed against the disulfide bond. Ig superfamily (IgSF) domains can be divided into 4 main classes based on their structures and sequences: the Variable (V), Constant 1 (C1), Constant 2 (C2), and Intermediate (I) sets. Typically, the V-set domains have A, B, E and, D strands in one sheet and A', G, F, C, C', and C" strands in the other.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1207170594  21 VTQSPV-ITVSKGQTAQLDCNL--GTVTNYAAWYKQTPAEVPQYVLYNYhgwSSPEYGSGFSAPKFTSTCSSKSDCSLII 97
Cdd:cd00099     1 VTQSPRsLSVQEGESVTLSCEVssSFSSTYIYWYRQKPGQGPEFLIYLS---SSKGKTKGGVPGRFSGSRDGTSSFSLTI 77

                          90       100       110
                  ....*....|....*....|....*....|....
gi 1207170594  98 SNVDVSDSAVYYCQTWDSS-VSEYVFGQGTKLIV 130
Cdd:cd00099    78 SNLQPEDSGTYYCAVSESGgTDKLTFGSGTRLTV 111

Immunoglobulin (Ig) variable (V) domain of T-cell receptor (TCR) alpha chain and similar proteins; The members here are composed of the immunoglobulin (Ig) variable domain of the alpha chain of alpha/beta T-cell antigen receptors (TCRs). TCRs mediate antigen recognition by T lymphocytes, and are composed of alpha and beta, or gamma and delta polypeptide chains with variable (V) and constant (C) regions. This group represents the variable domain of the alpha chain of TCRs and also includes the variable domain of delta chains of TCRs. Alpha/beta TCRs recognize antigen as peptide fragments presented by major histocompatibility complex (MHC) molecules. The variable domain of TCRs is responsible for antigen recognition, and is located at the N-terminus of the receptor. Gamma/delta TCRs recognize intact protein antigens directly without antigen processing and recognize MHC independently of the bound peptide. Members of this group contain standard Ig superfamily V-set AGFCC'C"/DEB domain topology.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1207170594  21 VTQSP-VITVSKGQTAQLDCNLGTVTNYAA-WYKQTPAEVPQYVLYNYhGWSSPEYGSGFSApkfTSTCSSKSdCSLIIS 98
Cdd:cd04983     1 VTQSPqSLSVQEGENVTLNCNYSTSTFYYLfWYRQYPGQGPQFLIYIS-SDSGNKKKGRFSA---TLDKSRKS-SSLHIS 75

                          90       100       110
                  ....*....|....*....|....*....|....
gi 1207170594  99 NVDVSDSAVYYCQTWDSSVSE-YVFGQGTKLIVT 131
Cdd:cd04983    76 AAQLSDSAVYFCALSESGGTGkLTFGKGTRLTVE 109

Immunoglobulin (Ig) variable (V) domain of T-cell receptor (TCR) beta chain; The members here are composed of the immunoglobulin (Ig) variable domain of the beta chain of alpha/beta T-cell antigen receptors (TCRs). TCRs mediate antigen recognition by T lymphocytes, and are composed of alpha and beta, or gamma and delta, polypeptide chains with variable (V) and constant (C) regions. This group includes the variable domain of the alpha chain of alpha/beta TCRs. Alpha/beta TCRs recognize antigen as peptide fragments presented by major histocompatibility complex (MHC) molecules. The variable domain of TCRs is responsible for antigen recognition, and is located at the N-terminus of the receptor. Gamma/delta TCRs recognize intact protein antigens directly without antigen processing and recognize MHC independently of the bound peptide. Members of this group contain standard Ig superfamily V-set AGFCC'C"/DEB domain topology.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1207170594  21 VTQSPVITVSK-GQTAQLDCNLGTVTNYAAWYKQTPAEVPQYVLYNYHGwSSPEYGsGFSAPKFTSTCSSKSDCSLIISN 99
Cdd:cd05899     1 VTQSPRYLIKRrGQSVTLRCSQKSGHDNMYWYRQDPGKGLQLLFYSYGG-GLNEEG-DLPGDRFSASRPSLTRSSLTIKS 78

                          90       100       110
                  ....*....|....*....|....*....|..
gi 1207170594 100 VDVSDSAVYYC-QTWDSSVSEYVFGQGTKLIV 130
Cdd:cd05899    79 AEPEDSAVYLCaSSLGGGADEAYFGPGTRLTV 110

Immunoglobulin (Ig) light chain, kappa type, variable (V) domain; The members here are composed of the immunoglobulin (Ig) light chain, kappa type, variable (V) domain. This group contains the standard Ig superfamily V-set AGFCC'C"/DEB domain topology. The basic structure of Ig molecules is a tetramer of two light chains and two heavy chains linked by disulfide bonds. There are two types of light chains: kappa and lambda, each composed of a constant domain (CL) and a variable domain (VL). There are five types of heavy chains (alpha, gamma, delta, epsilon, and mu), which determines the type of immunoglobulin formed: IgA, IgG, IgD, IgE, and IgM, respectively. In higher vertebrates, there are two types of light chain, designated kappa and lambda, which seem to be functionally identical, and can associate with any of the heavy chains.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1207170594  19 TVVTQSPV-ITVSKGQTAQLDC--NLGTVTNYAAWYKQTPAEVPQYVLYnyhgWSSPEYgSGFSApKFTSTcSSKSDCSL 95
Cdd:cd04980     1 IVMTQSPAsLSVSPGERVTISCkaSQSISSNYLAWYQQKPGQAPKLLIY----YASTLH-SGVPS-RFSGS-GSGTDFTL 73

                          90       100       110
                  ....*....|....*....|....*....|...
gi 1207170594  96 IISNVDVSDSAVYYCQTWDSSVseYVFGQGTKL 128
Cdd:cd04980    74 TISSVEPEDAAVYYCQQGYTFP--YTFGGGTKL 104

T cell receptor (TCR) beta chain constant immunoglobulin domain; member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the T cell receptor (TCR) beta chain constant immunoglobulin domain. TCRs mediate antigen recognition by T lymphocytes, and are composed of alpha and beta, or gamma and delta, polypeptide chains with variable (V) and constant (C) regions. This group includes the variable domain of the beta chain. Alpha/beta TCRs recognize antigen as peptide fragments presented by major histocompatibility complex (MHC) molecules. The antigen binding site is formed by the variable domains of the alpha and beta chains, located at the N-terminus of each chain. Alpha/beta TCRs recognize antigens differently from gamma/delta TCRs.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1207170594 137 PPVLTILRPSSREELSSSKVTLLCLinhMSVAFAD---VRWLVNGNSVTEGVFTGSAEQQPDHKFKS-SSSLTIQRSEWD 212
Cdd:cd05769     2 PPTVALFPPSEAEIRNKRKATLVCL---ATGFYPDhvsLSWKVNGKEVKDGVATDPQALRENTSTYSlSSRLRVSATEWF 78

                          90       100
                  ....*....|....*....|....*
gi 1207170594 213 K-DRELTCEATVASKTSRASIRKSE 236
Cdd:cd05769    79 NpRNTFTCIVKFYGGTDTDTWTQGI 103

CH2 domain (second constant Ig domain of the heavy chain) in immunoglobulin mu chain; member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the second immunoglobulin constant domain (IgC) of mu heavy chains. This domain is found on the Fc fragment. The basic structure of Ig molecules is a tetramer of two light chains and two heavy chains linked by disulfide bonds. There are two types of light chains: kappa and lambda; each is composed of a constant domain and a variable domain. There are five types of heavy chains: alpha, delta, epsilon, gamma, and mu, all consisting of a variable domain (VH) with three (alpha, delta and gamma) or four (epsilon and mu) constant domains (CH1 to CH4). Ig molecules are modular proteins, in which the variable and constant domains have clear, conserved sequence patterns.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1207170594 137 PPVLTILRPSSREELSSSKVTLLCLINHMSVAFADVRWLVNGNSVTEGVFTGSAEQQPDHK--FKSSSSLTIQRSEWDKD 214
Cdd:cd16093     1 PPTVSLHAPSREEFLGNRTATFVCLATGFSPKTISFKWLRNGKEVTSSTGAVVEEPKEDGKtlYSATSFLTITESEWKSQ 80

                  ....*.
gi 1207170594 215 RELTCE 220
Cdd:cd16093    81 TEFTCE 86

Immunoglobulin C-Type;

                           10        20        30        40        50        60
                   ....*....|....*....|....*....|....*....|....*....|....*....|....*...
gi 1207170594  155 KVTLLCLINHMSVAFADVRWLVNGNSVTEGVFTGSAEQQPDHKFKSSSSLTIQRSEWDKDRELTCEAT 222
Cdd:smart00407   1 KATLVCLVSGFYPPDITVTWLRNGQEVTEGVSTTDPLKNSDGTYFLSSYLTVPASTWESGDVYTCQVT 68

CH3 domain (third constant Ig domain of the heavy chain) in immunoglobulin heavy alpha, gamma, and delta chains, and CH4 domain (fourth constant Ig domain of the heavy chain) in immunoglobulin heavy alpha, epsilon, and mu chains; member of the C1-set of I; The members here are composed of the third and fourth immunoglobulin constant domain (IgC) of alpha, delta, gamma and alpha, epsilon, and mu heavy chains, respectively. This domain is found on the Fc fragment. The basic structure of Ig molecules is a tetramer of two light chains and two heavy chains linked by disulfide bonds. There are two types of light chains: kappa and lambda; each is composed of a constant domain and a variable domain. There are five types of heavy chains: alpha, delta, epsilon, gamma, and mu, all consisting of a variable domain (VH) with three (alpha, delta and gamma) or four (epsilon and mu) constant domains (CH1 to CH4). Ig molecules are modular proteins, in which the variable and constant domains have clear, conserved sequence patterns.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1207170594 138 PVLTILRPSSREELSSSKVTLLCLINHMSVAFADVRWLVNGNSVTEGVFTGSAEQQ-PDHKFKSSSSLTIQRSEWDKDRE 216
Cdd:cd05768     1 PSVYLLPPPEEELSLNETVTLTCLVKGFYPEDIFVSWLQNGEPLPSADYKTTAPVPeSDGSFFVYSKLNVSTADWNSGDV 80

                          90       100
                  ....*....|....*....|...
gi 1207170594 217 LTC----EATVASKTSRaSIRKS 235
Cdd:cd05768    81 FSCvvghEALPLQFTQK-SIDKS 102

HLA class II histocompatibility antigen DO alpha; member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the nonclassical MHC class II (MHCII) protein, HLA-DO, which binds HLA-DM and influences the repertoire of peptides presented by MHCII proteins. In complex with HLA-DM, HLA-DO adopts a classical MHCII structure, with alterations near the a subunit's 310 helix. HLA-DO binds to HLA-DM at the same sites implicated in MHCII interaction, and kinetic analysis showed that HLA-DO acts as a competitive inhibitor by acting as a substrate mimic. Though more remains to be elucidated about the function of HLA-DO, its unique distribution in the mammalian body namely, the exclusive expression of HLA-DO in B cells, thymic medullary epithelial cells, and dendritic cells indicate that it may be of physiological importance and has inspired further research. Class I MHC proteins bind antigenic peptide fragments and present them to CD8+ T lymphocytes. Class I molecules consist of a transmembrane alpha chain and a small chain called the beta-2-microglobulin. The alpha chain contains three extracellular domains, two of which fold together to form the peptide-binding cleft (alpha1 and alpha2), and one which has an Ig fold (alpha3). Peptide binding to class I molecules occurs in the endoplasmic reticulum (ER) and involves both chaperones and dedicated factors to assist in peptide loading. Class I MHC molecules are expressed on most nucleated cells.

                          10        20        30        40        50        60
                  ....*....|....*....|....*....|....*....|....*....|....*....|...
gi 1207170594 137 PPVLTILrPSSREELSSSKVtLLCLINHMSVAFADVRWLVNGNSVTEGVFTGSAEQQPDHKFK 199
Cdd:cd21004     2 PPRVTVL-PKSRVELGQPNI-LICIVDNIFPPVINITWLRNGQTVTEGVAQTSFYSQPDHLFR 62

Immunoglobulin (Ig) heavy chain (H), variable (V) domain; The members here are composed of the immunoglobulin (Ig) heavy chain (H), variable (V) domain. This group contains the standard Ig superfamily V-set AGFCC'C"/DEB domain topology. The basic structure of Ig molecules is a tetramer of two light chains and two heavy chains linked by disulfide bonds. In Ig, each chain is composed of one variable domain (IgV) and one or more constant domains (IgC); these names reflect the fact that the variability in sequences is higher in the variable domain than in the constant domain. There are five types of heavy chains (alpha, gamma, delta, epsilon, and mu), which determines the type of immunoglobulin formed: IgA, IgG, IgD, IgE, and IgM, respectively. In higher vertebrates, there are two types of light chain, designated kappa and lambda, which can associate with any of the heavy chains. This family includes alpha, gamma, delta, epsilon, and mu heavy chains.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1207170594  23 QSPVITVSKGQTAQLDCNL--GTVTNYA-AWYKQTPAEVPQYVLYNYHGWSSPEYGSGFSApKFT-STCSSKSDCSLIIS 98
Cdd:cd04981     4 ESGPGLVKPGQSLKLSCKAsgFTFTSYGmGWVRQAPGKGLEWIGLIYPGGGDTYYADSFKG-RFTiTRDTSKSTAYLQLN 82

                          90       100       110
                  ....*....|....*....|....*....|....*.
gi 1207170594  99 NVDVSDSAVYYCQ---TWDSSVSEYVFGQGTKLIVT 131
Cdd:cd04981    83 SLTSEDTAVYYCArglGGYGYSYFDYWGQGTTVTVS 118

Gammadelta T-cell antigen receptor, variable (V) domain; The members here are composed of the immunoglobulin (Ig) variable (V) domain of the gamma/delta T-cell receptors (TCRs). TCRs mediate antigen recognition by T lymphocytes, and are heterodimers consisting of alpha and beta chains or gamma and delta chains. Each chain contains a variable (V) and a constant (C) region. The majority of T cells contain alpha/beta TCRs, but a small subset contain gamma/delta TCRs. Alpha/beta TCRs recognize antigen as peptide fragments presented by major histocompatibility complex (MHC) molecules. Gamma/delta TCRs recognize intact protein antigens; they recognize protein antigens directly and without antigen processing, and MHC independently of the bound peptide. Gamma/delta T cells can also be stimulated by non-peptide antigens such as small phosphate- or amine-containing compounds. The variable domain of gamma/delta TCRs is responsible for antigen recognition and is located at the N-terminus of the receptor. Members of this group contain standard Ig superfamily V-set AGFCC'C"/DEB domain topology.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1207170594  20 VVTQSPVITVSKGQTAQLDCNL--GTVTN-YAAWYKQTPAEVPQYVlYNYHGwsspEYGSGFSAPKFTSTCSSKSDCSLI 96
Cdd:cd20988     1 LVPEHQTVTVSVGKPVTLKCSMkgEAISNyYINWYRKTQGNTMTFI-YREGG----IYGPGFKDNFRGDIDSSNNLAVLK 75

                          90       100       110
                  ....*....|....*....|....*....|....*...
gi 1207170594  97 ISNVDVSDSAVYYCQTWDSSVSEY----VFGQGTKLIV 130
Cdd:cd20988    76 ILEASERDEGSYYCASDTPGGGREydplIFGKGTYLTV 113

Class II major histocompatibility complex (MHC) alpha chain immunoglobulin domain; member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the immunoglobulin (Ig) domain of the major histocompatibility complex (MHC) class II alpha chain. MHC class II molecules play a key role in the initiation of the antigen-specific immune reponse. These molecules have been shown to be expressed constitutively on the cell surface of professional antigen-presenting cells (APCs), including B-lymphocytes, monocytes, and macrophages in both humans and mice. The expression of these molecules has been shown to be induced in nonprofessional APCs such as keratinocyctes, and they are also expressed on the surface of activated human T cells and on T cells from other species. The MHC II molecules present antigenic peptides to CD4(+) T-lymphocytes. These peptides derive mostly from proteolytic processing via the endocytic pathway, of antigens internalized by the APC. These peptides bind to the MHC class II molecules in the endosome before they are transported to the cell surface. MHC class II molecules are heterodimers, comprised of two similarly-sized membrane-spanning chains, alpha and beta. Each chain had two globular domains (N- and C-terminal), and a membrane-anchoring transmembrane segment. The two chains form a compact four-domain structure. The peptide-binding site is a cleft in the structure.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1207170594 137 PPVLTILrPSSREELSSSKvTLLCLINHMSVAFADVRWLVNGNSVTEGVFTGSAEQQPDHKFKSSSSLTIQRSEWDkdrE 216
Cdd:cd05767     2 PPEVTVF-PKSPVELGEPN-TLICFVDNFFPPVINVTWLRNGQPVTDGVSETVFLPREDHSFRKFSYLPFTPSEGD---I 76

                  ....*.
gi 1207170594 217 LTCEAT 222
Cdd:cd05767    77 YDCRVE 82

Class II major histocompatibility complex (MHC) alpha chain immunoglobulin domain of histocompatibility antigen (HLA) I-E; member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the immunoglobulin (Ig) domain of major histocompatibility complex (MHC) class II alpha chain of histocompatibility antigen (HLA) I-E. MHC class II molecules play a key role in the initiation of the antigen-specific immune reponse. These molecules have been shown to be expressed constitutively on the cell surface of professional antigen-presenting cells (APCs), including B-lymphocytes, monocytes, and macrophages in both humans and mice. The expression of these molecules has been shown to be induced in nonprofessional APCs such as keratinocyctes, and they are expressed on the surface of activated human T cells and on T cells from other species. The MHC II molecules present antigenic peptides to CD4(+) T-lymphocytes. These peptides derive mostly from proteolytic processing via the endocytic pathway, of antigens internalized by the APC. These peptides bind to the MHC class II molecules in the endosome before they are transported to the cell surface. MHC class II molecules are heterodimers, comprised of two similarly-sized membrane-spanning chains, alpha and beta. Each chain had two globular domains (N- and C-terminal), and a membrane-anchoring transmembrane segment. The two chains form a compact four-domain structure. The peptide-binding site is a cleft in the structure.

                          10        20        30        40        50
                  ....*....|....*....|....*....|....*....|....*....|....*
gi 1207170594 158 LLCLINHMSVAFADVRWLVNGNSVTEGVFTGSAEQQPDHKFKSSSSLTIQRSEWD 212
Cdd:cd21005    21 LICFIDKFSPPVVNVTWLRNGRPVTEGVSETVFLPRDDHLFRKFHYLTFLPSTDD 75

CH1 domain (first constant Ig domain of the heavy chain) in immunoglobulin heavy epsilon and gamma chain; member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the first immunoglobulin constant-1 set domain of epsilon and gamma chains. It belongs to a family composed of the first immunoglobulin constant-1 set domain of alpha, delta, epsilon, gamma, and mu heavy chains. This domain is found on the Fab antigen-binding fragment. The basic structure of Ig molecules is a tetramer of two light chains and two heavy chains linked by disulfide bonds. There are two types of light chains: kappa and lambda; each is composed of a constant domain and a variable domain. There are five types of heavy chains: alpha, delta, epsilon, gamma, and mu, all consisting of a variable domain (VH) with three (alpha, delta and gamma) or four (epsilon and mu) constant domains (CH1 to CH4). Ig molecules are modular proteins, in which the variable and constant domains have clear, conserved sequence patterns. This group belongs to the C1-set of IgSF domains, which are classical Ig-like domains resembling the antibody constant domain. C1-set domains are found almost exclusively in molecules involved in the immune system, such as in immunoglobulin light and heavy chains, in the major histocompatibility complex (MHC) class I and II complex molecules, and in various T-cell receptors.

                          10        20        30        40        50        60        70
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*
gi 1207170594 137 PPVLTILRPSSREElSSSKVTLLCLINHMSVAFADVRWlvNGNSVTEGVFTGSAEQQPDHKFKSSSSLTIQRSEW 211
Cdd:cd21817     1 APSVFPLAPCCKST-NGSSVTLGCLVTGYFPEPVTVTW--NSGSLTSGVKTFPAVLQSSGLYTTSSQVTVPSSSW 72