[{"data":1,"prerenderedAt":-1},["ShallowReactive",2],{"$fxLN3MUwXCdr5RPjwZYIDpOj8CHyjOmngWTgoKXPtZbg":3,"$f-IzLnVcxMOI7m9uaI9uwzv-k4AoyWE4xVwqQieuGZgI":32,"$f3Ft0rKFJHppdzE-vuveecxx1BUcg9iOlMLtyzf_MJDg":78},[4,8,12,16,20,24,28],{"title":5,"slug":6,"path":7},"About Microbeonline.com","about-microbeonline-com","\u002Fabout-microbeonline-com\u002F",{"title":9,"slug":10,"path":11},"About Me","about-me","\u002Fabout-microbeonline-com\u002Fabout-me\u002F",{"title":13,"slug":14,"path":15},"Advertise with Us","advertise-us","\u002Fadvertise-us\u002F",{"title":17,"slug":18,"path":19},"Privacy Policy","privacy-policy","\u002Fprivacy-policy\u002F",{"title":21,"slug":22,"path":23},"Abbreviations","abbreviations","\u002Fabbreviations\u002F",{"title":25,"slug":26,"path":27},"Microbes","microbes","\u002Fmicrobes\u002F",{"title":29,"slug":30,"path":31},"Books","recommended-books","\u002Frecommended-books\u002F",{"type":33,"data":34},"blog",{"slug":35,"title":36,"description":36,"seoTitle":37,"seoDescription":37,"author":38,"createdDate":39,"lastUpdatedDate":40,"draft":41,"category":42,"image":37,"body":43,"faq":44,"tags":45,"related":47},"immunoglobulin-structure","Immunoglobulins (Antibodies) Structure and Classes",null,"Acharya Tankeshwar","2020-04-03","2026-07-21",false,"immunology","Immunoglobulins, also known as antibodies, are special types of **glycoprotein** molecules that are secreted by well-differentiated B cells, called **plasma cells**. Antibodies specifically react with the antigens which stimulated their production. They are released from **lymph** **nodes** and **spleen** into the blood where they serve as the effector of **humoral immunity.**\n\nAntibodies are part of the **serum proteins** and are found in the **gamma (γ) globulin fraction** if serum proteins are separated by electrophoresis so also called **immunoglobulins.**\n\n### Basic Structure of an Antibody\n\n![Basic Structure of antibody molecule](https:\u002F\u002Fassets.microbeonline.com\u002Fblogs\u002FBasic-structure-of-an-antibody-molecule.jpg)Figure: Basic Structure of antibody molecule\n\nAntibodies are **Y-shaped tetra-peptide** molecules consisting of two identical heavy (H) chains and two identical light (L) chains, held together by **disulfide bonds.** Each light chain is bound to a heavy chain by a disulfide bond to form a **heterodimer**(H-L). Two identical heavy and light (H-L) chain combinations are also held together by disulfide bridges forming a basic **four-chain (H-L)2 antibody structure, a dimer of dimers.** The exact number and precise locations of these interchain disulfide bonds differ among antibody classes and subclasses.\n\n> Light chains are called light chains because their molecular weight is less i.e. about 25,000. Molecular weight of heavy chains is 50,000 to 70,000 depending upon antibody isotype\u002Fclass.\n\nBasically, an antibody molecule has two functions i.e., antigen binding and effector functions. The binding of an antibody with an antigen is very specific (i.e., a single antibody can not bind with different antigens\u002F[epitopes](\u002Fepitope\u002F)) which is determined by the structural configuration of the **antigen-binding region** of that antibody.\n\n> Immunoglobulins have two Fab (“fragment, antigen-binding”) fragments and one Fc (“fragment, crystallizable) fragment.\n\n![Variable and Constant regions of Immunoglobulins](https:\u002F\u002Fassets.microbeonline.com\u002Fblogs\u002Fvariable-region-and-constant-region.png)Figure: Variable and Constant regions of Immunoglobulins\n\nThis antigen-binding region of an immunoglobulin is formed by the 100-110 **amino acids** located in the **amino-terminal regions** of corresponding heavy and light chains. These amino acids vary greatly among antibodies of different specificity. These segments of the variable sequence are called **V regions**: **VL** in light chains and **VH** in heavy chains.\n\n#### Complementarity-determining regions (CDRs)\n\nWithin the variable regions of heavy chains and light chains, **sequence variations are mostly concentrated in three discrete regions**, known as hypervariable regions. Hypervariable regions form the **antigen-binding site** of the antibody molecule. As these regions are complementary to the structure of the epitope, they are also known as **complementarity determining regions (CDRs)**\n\n#### Constant regions\n\nThe remainder of both the L and H chain contain regions of amino acid sequences which show very little variation among immunoglobulins and is called the **constant region**,**CL** on the light chain and **CH** on the heavy chain. Heavy chain constant regions are also the site for carbohydrate attachment.\n\n> This region of an immunoglobulin, known as Fc fragment, does not have antigen-binding activity but is involved in most of the effector functions.\n\n![Basic structure of an Immunoglobulin - Schematic diagram of Immunoglobulin](https:\u002F\u002Fassets.microbeonline.com\u002Fblogs\u002FBasic-Immunoglobulin-structure.png)Figure: Schematic diagram of Immunoglobulin\n\nThere are five basic sequence patterns that correspond to five different heavy-chain constants (C) regions γ, α, μ, ε, and δ. Each of these five different heavy chains is called an isotype. The heavy chains of a given antibody molecule determine the class of that antibody: IgG (γ), IgA (α), IgM (μ), IgE (ε), and IgD (δ).\n\n![Constant regions determine immunoglobulin class](https:\u002F\u002Fassets.microbeonline.com\u002Fblogs\u002Fconstant-regions-determine-antibody-class.png)Figure: Constant regions determine immunoglobulin class\n\nLight chains have only one constant region whereas heavy chains have many constant regions CH1, CH2, CH3, and sometimes CH4. IgM and IgE have four heavy chain constant regions with approximately 440 amino acids but the remaining immunoglobulins (IgG, IgA, IgD) have only three heavy chain constant regions with approximately 330 amino acids.\n\n#### Hinge regions\n\nThe γ, α, and δ heavy chains contain an extended proline-rich peptide sequence between the CH1 and CH2 domains known as the hinge region. Hinge regions give **segmental flexibility to IgG, IgA, and IgD** classes of antibodies as a result Fab can form different angles with each other while capturing the antigen. Although μ and ε heavy chains lack a hinge region, they have an additional domain of 110 amino acids that have hinge-like features.\n\n## Immunoglobulin classes\n\nThere are 5 classes of immunoglobulins IgG, IgA, IgM, IgE, and IgD as determined by the presence of unique amino acid sequences in the heavy chain constant regions. The basic structural properties of these immunoglobulin classes are discussed here briefly.\n\n![General structure of five major classes of immunoglobulins (antibodies) - General structure of five major classes of immunoglobulins (antibodies)(Image source: Kubay Immunology)](https:\u002F\u002Fassets.microbeonline.com\u002Fblogs\u002Fstructure-of-different-immunoglobulins.png)Figure: General structure of five major classes of immunoglobulins (antibodies)(Image source: Kubay Immunology)\n\n#### Immunoglobulin G (IgG)\n\nThe IgG molecule consists of two γ heavy chains and two κ or two λ light chains. There are four human IgG subclasses, distinguished by differences in γ -chain sequence and numbered according to their decreasing average serum concentrations: IgG1, IgG2, IgG3, and IgG4.\n\n[Find more about Immunoglobulin G (IgG) and its function here.](\u002Figg-antibody-structure-subclasses-functions-and-clinical-significance\u002F)\n\n#### Immunoglobulin A (IgA)\n\nThe IgA molecule consists of two α heavy chains and two κ or two λ light chains.\n\n> The molecular formula of IgA is (α2κ2)n or (α2λ2)n, where n =1, 2, 3 or 4\n\nIgA exists primarily as a monomer in serum but in external secretions, it (secretory IgA) is present as a dimer or tetramer linked by a J-chain polypeptide.\n\n[Find more about immunoglobulin A and its function here.](\u002Fimmunoglobulin-a-iga-structure-and-functions\u002F)\n\n#### Immunoglobulin M (IgM)\n\nThe IgM molecule consists of two μ heavy chains and two κ or two λ light chains. IgM has an “additional” heavy chain constant domain and the absence of a hinge region in the μ-chain.\n\n> The molecular formula of IgM is (μ2κ2)n or (μ2λ2)n, where n =1 or 5\n\nIgM has two forms; monomeric IgM (membrane-bound on B cells) and pentameric IgM (secreted by plasma cells). In pentameric IgM, five monomer units are held together by disulfide bonds that link their carboxyl-terminal heavy chain domains (Cμ4\u002FCμ4) and their (Cμ3\u002FCμ3) domains held together by an Fc-linked polypeptide called the J (joining) chain.\n\n[Find more about immunoglobulin M (IgM) and its functions here.](\u002Figm-antibody-structure-properties-functions-clinical-significance\u002F)\n\n#### Immunoglobulin E (IgE)\n\nThe [IgE molecule](\u002Fimmunoglobulin-e-ige-antibodies\u002F) consists of two ε heavy chains and two κ or two λ light chains. IgE has an “additional” heavy chain constant domain and the absence of a hinge region in the ε-chain. Immunoglobulin E (IgE) is well known for its role in mediating immediate hypersensitivity reactions. More information about IgE will be published later in another blog post.\n\n#### Immunoglobulin D (IgD)\n\nThe IgD molecule consists of two δ heavy chains and two κ or two λ light chains. IgD is typically coexpressed with IgM on the surface of mature B cells.\n\n#### References and further readings\n\n- [Cellular and Molecular Immunology, 9th Edition](https:\u002F\u002Famzn.to\u002F2Ofk9Rv)\n- [Kuby Immunology, 8th Edition](https:\u002F\u002Famzn.to\u002F2TXGUOX)\n- [Roitt’s Essential Immunology, 13th Edition](https:\u002F\u002Famzn.to\u002F2OdRNal)",[],[46],"antibody-mediated-immunity",[48,54,60,66,72],{"slug":49,"title":50,"description":50,"seoTitle":37,"seoDescription":37,"author":38,"createdDate":51,"lastUpdatedDate":40,"draft":41,"category":42,"image":37,"faq":52,"tags":53},"immunoglobulin-e-ige-antibodies","IgE Antibodies: Structure, Properties, and Functions","2021-05-23",[],[46],{"slug":55,"title":56,"description":56,"seoTitle":37,"seoDescription":37,"author":38,"createdDate":57,"lastUpdatedDate":40,"draft":41,"category":42,"image":37,"faq":58,"tags":59},"immunoglobulin-iga-structure-functions","IgA Antibodies: Structure, Properties, and Functions","2020-04-07",[],[46],{"slug":61,"title":62,"description":62,"seoTitle":37,"seoDescription":37,"author":38,"createdDate":63,"lastUpdatedDate":40,"draft":41,"category":42,"image":37,"faq":64,"tags":65},"function-of-antibodies","Function of Antibodies (Immunoglobulins)","2019-04-17",[],[46],{"slug":67,"title":68,"description":68,"seoTitle":37,"seoDescription":37,"author":38,"createdDate":69,"lastUpdatedDate":40,"draft":41,"category":42,"image":37,"faq":70,"tags":71},"igg-antibody-structure-subclasses-functions-and-clinical-significance","IgG Antibodies: Structure, Subclasses, and Functions","2018-09-17",[],[46],{"slug":73,"title":74,"description":74,"seoTitle":37,"seoDescription":37,"author":38,"createdDate":75,"lastUpdatedDate":40,"draft":41,"category":42,"image":37,"faq":76,"tags":77},"igm-antibody-structure-properties-functions-clinical-significance","IgM Antibodies: Structure, Properties, and Functions","2016-04-28",[],[46],[79,85,92,97,101,105,110,115,119,123],{"slug":80,"name":38,"description":81,"image":82,"body":83,"postCount":84},"acharya-tankeshwar","Editor-in-chief","https:\u002F\u002Fassets.microbeonline.com\u002Fauthors\u002Ftankeshwar-acharya-author-microbeonline.jpg","***Tankeshwar Acharya, MSc (Medical Microbiology)***\n\n*Tankeshwar Acharya is an Assistant Professor in the Department of Microbiology at Patan Academy of Health Sciences (PAHS), Nepal, where he has been teaching and practicing clinical microbiology for over 14 years. He is the founder of Microbe Online, one of the leading free microbiology education resources on the web, covering bacteriology, mycology, parasitology, immunology, and clinical laboratory diagnostics written from direct experience in both the classroom and the diagnostic laboratory.*",433,{"slug":86,"name":87,"description":88,"image":89,"body":90,"postCount":91},"ashma-shrestha","Ashma Shrestha","SEO Copywriter and Science Communicator\nKathmandu, Nepal","https:\u002F\u002Fassets.microbeonline.com\u002Fauthors\u002Fashma-shrestha.png","Ashma Shrestha holds a Master of Science in Medical Microbiology from the Institute of Science and Technology (IOST), Tribhuvan University, Nepal, where she developed a strong foundation in virology, molecular biology, and diagnostic microbiology.\n\nShe now works as an SEO Copywriter at Resolution Digital, where she combines her scientific training with research-driven content strategy. She is certified in Google Analytics and Google Business Profile (GBP), and brings a data-informed approach to science communication writing content that is not only accurate but structured to reach and serve the students who need it most.\n\nAt microbeonline, Ashma contributes articles primarily in virology and molecular biology, areas she finds most compelling for their mechanistic depth and their growing clinical relevance. Her writing reflects the same standard the site is built on: factual rigor, clear explanation of the *why* behind microbiology concepts, and content that helps students move from memorization to genuine understanding.\n\nShe is passionate about making complex microbiological concepts accessible without sacrificing accuracy; a skill that sits at the intersection of her scientific training and her professional work in content and SEO.",81,{"slug":93,"name":94,"description":95,"image":37,"body":37,"postCount":96},"sushmita-baniya","Sushmita Baniya","Author \u002F Contributor",32,{"slug":98,"name":99,"description":95,"image":37,"body":37,"postCount":100},"samikshya-acharya","Samikshya Acharya",20,{"slug":102,"name":103,"description":95,"image":37,"body":37,"postCount":104},"alisha-tripathi","Alisha Tripathi",6,{"slug":106,"name":107,"description":108,"image":37,"body":37,"postCount":109},"aastha-shrestha","Aastha Shrestha"," Author \u002F Contributor",10,{"slug":111,"name":112,"description":113,"image":37,"body":37,"postCount":114},"guest-author","Guest Author","Guest Author \u002F Contributor",2,{"slug":116,"name":117,"description":95,"image":37,"body":37,"postCount":118},"srijana-khanal","Srijana Khanal",18,{"slug":120,"name":121,"description":113,"image":37,"body":37,"postCount":122},"dr-poonam-acharya","Dr. Poonam Acharya",1,{"slug":124,"name":125,"description":95,"image":37,"body":126,"postCount":127},"nisha-rijal","Nisha Rijal","**Nisha Rijal** is a microbiologist and quality assurance specialist. She served for nearly 12 years as a microbiologist at the National Public Health Laboratory (NPHL), Nepal's national reference laboratory, and continues to work as a consultant microbiologist in international public health organization. ",51]