--- title: "Antibody - Structure, Types, Functions and Properties" description: "Get Free Biology Notes" url: https://biologynotesonline.com/antibody-structure-types-functions-and-properties/ --- # Antibody – Structure, Types, Functions and Properties Antibody or immunoglobulin (Ig) is a specific glycoprotein produced by B-lymphocytes and plasma cells. It is produced in response to entry of foreign substance in the body. These foreign substances are called antigens. Antibody is an important part of humoral immunity. It is used to identify and bind with bacteria, viruses, toxins and other harmful ... [Read more](https://biologynotesonline.com/antibody-structure-types-functions-and-properties/) **Antibody** or **immunoglobulin (Ig)** is a specific **glycoprotein** produced by **B-lymphocytes** and **plasma cells**. It is produced in response to entry of foreign substance in the body. These foreign substances are called **antigens**. Antibody is an important part of **humoral immunity**. It is used to identify and bind with **bacteria**, **viruses**, **toxins** and other harmful particles. After binding with antigen, it helps in neutralization and removal of antigen from the body. Antibody reacts with antigen in a specific manner. The antigenic part which is recognized by antibody is called **epitope**. The binding part of antibody which binds with epitope is called **paratope**. The structure of antibody is **Y-shaped**. It is formed of four **polypeptide chains**. There are two similar **heavy chains** and two similar **light chains**. These chains are joined with each other by **disulfide bonds**. The tip portion of Y-shaped antibody is called **variable region**. This region binds with specific antigen. The stem portion is called **constant region**. It decides the class of antibody and also helps in reaction with other immune cells and complement proteins. In human body, antibodies are mainly of five types. These are **IgG**, **IgA**, **IgM**, **IgE** and **IgD**. **IgG** is the most abundant antibody in blood. It gives long term immunity. **IgA** is mainly present in mucosal secretion and protects respiratory tract and gastrointestinal tract. **IgM** is the first antibody formed during a new infection. It is important in early immune response. **IgE** is involved in allergic reaction and defense against parasites. **IgD** is present mainly on surface of **B cells** and helps in activation of B cells. ## Characteristics of Antibody The following are the characteristics of **antibody**– - **Antibody** is **glycoprotein** in nature. It is also known as **immunoglobulin (Ig)**. - It is produced by **B-lymphocytes** and **plasma cells**. It is formed during immune reaction against foreign particles. - The foreign particle which stimulates antibody formation is called **antigen**. It may be **bacteria**, **virus**, **toxin** etc. - Antibody molecule is generally **Y-shaped**. Its molecular weight is about **150 kDa**. - Antibody is formed of four **polypeptide chains**. These are two same **heavy chains** and two same **light chains**. - The **heavy chains** are larger chain. Each heavy chain is about **50 kDa**. - The **light chains** are smaller chain. Each light chain is about **25 kDa**. - The chains are joined by **disulfide bonds**. Non-covalent bonds also helps in holding the chains. - The tip part of antibody is **variable region**. It has the site for binding with antigen. - The binding site present on antibody is called **paratope**. It combines with the antigenic part called **epitope**. - **Complementarity determining regions (CDRs)** are present in variable region. These are hypervariable loops and gives specific nature to antibody. - The stem part of antibody is **constant region**. This region determines the class of antibody. - The **constant region** also takes part in immune reaction. It can bind with immune cell receptors and activates **complement system**. - A flexible **hinge region** is present in most antibody. It allows movement of two arms of antibody. - Antibody is generally **bivalent**. It has two same antigen binding sites. - Antibodies are classified into five classes. These are **IgG**, **IgA**, **IgM**, **IgE** and **IgD**. - The classification is based on difference in **heavy chains**. Each class performs different role in immune response. ## Structure of Immunoglobulins/Antibody The structure of **immunoglobulin** or **antibody** is as follows- - **Antibody** molecule is generally **Y-shaped** in structure. It is made up of two arms and one stem like region. - It is formed of four **polypeptide chains**, which include two identical **heavy chains (H-chains)** and two identical **light chains (L-chains)**. - The **heavy chains** are larger in size and forms the main part of the antibody molecule. The **light chains** are smaller and attached with the heavy chains. - The chains are connected with each other by **disulfide bonds**. These bonds help in maintaining the stable **Y-shaped** structure of antibody. - The terminal part of the two arms are called **variable region (V-region)**. This region is not same in all antibodies and it is concerned with antigen binding. - The **variable region** contains the **antigen binding site**. This site binds with specific part of antigen called **epitope**. - The lower stem part of the antibody is called **constant region (C-region)**. This region is nearly same in antibodies of same class. - The **constant region** determines the class of antibody such as **IgG**, **IgA**, **IgM**, **IgE** and **IgD**. It also takes part in biological functions of antibody. - The two upper arms of antibody are called **Fab region** or **fragment antigen binding region**. This part is responsible for binding with antigen. - The stem portion is called **Fc region** or **fragment crystallizable region**. It does not bind with antigen, but it binds with immune cell receptors and helps in immune reaction. - A flexible **hinge region** is present between the **Fab** and **Fc** portion in many antibodies. This region allows movement of two arms, so antigen binding becomes easy. - The **heavy chains** and **light chains** are made up of repeated domains. Each domain contains about **110 amino acids**. - Each domain folds into compact structure called **immunoglobulin fold**. It is made up of two **beta-sheets** which are held by intra-chain **disulfide bond**. ![Structure of Immunoglobulins/Antibody](https://biologynotesonline.com/wp-content/uploads/2026/06/Structure-of-ImmunoglobulinsAntibody-1024x768.png)Structure of Immunoglobulins/Antibody ## Immunoglobulin Classes/Types of Antibody The following are the main classes of **immunoglobulin**– - **IgG (Immunoglobulin G)**– **IgG** is the most abundant antibody in serum. It forms about **70-75%** of total serum immunoglobulin. It is present as **monomer**. It gives long time immunity against infection. It neutralizes **toxins** and **viruses** and also helps in **opsonization**. It can activate **complement system**. It is the only immunoglobulin which can pass through **placenta**, so it gives passive immunity to foetus. The subclasses are **IgG1**, **IgG2**, **IgG3** and **IgG4**. - **IgA (Immunoglobulin A)**– **IgA** is mainly found in mucosal secretion. It is present in **saliva**, **tears**, **mucus**, **colostrum** and **breast milk**. It forms about **10-15%** of serum immunoglobulin. In serum it is **monomer**, but in secretion it is **dimer** and called **secretory IgA**. It protects the mucosal surface of respiratory tract and intestinal tract. It prevents attachment of pathogen with mucosal surface. - **IgM (Immunoglobulin M)**– **IgM** is the largest immunoglobulin. It forms about **5-10%** of serum immunoglobulin. It is generally present as **pentamer**. Five antibody units are joined together in it. It is the first antibody formed during primary immune response. It has ten antigen binding sites, so it shows high binding power. It is important in **agglutination** reaction and strong activation of **complement system**. - **IgE (Immunoglobulin E)**– **IgE** is present in very small amount in serum. It is **monomer** in structure. It is mainly concerned with **allergic reaction**. It takes part in asthma, hay fever and other allergic condition. It also gives protection against **parasitic infection**. It remains attached with **mast cells** and **basophils**. After contact with allergen, these cells release **histamine** and other chemicals. - **IgD (Immunoglobulin D)**– **IgD** is present in very low amount in blood. It is less than **1%** of serum immunoglobulin. It is **monomer** in structure. It is mainly present on the surface of immature **B-cells**. It acts as receptor for antigen. It helps in selection, maturation and activation of **B-cells**. Its exact function is not fully clear. ![Types of Antibody](https://biologynotesonline.com/wp-content/uploads/2026/06/Types-of-Antibody-1024x819.png)Types of Antibody ## IgG (Immunoglobulin G) **IgG** or **Immunoglobulin G** is the most abundant antibody present in human serum. It is also found in extracellular fluid. It forms about **70-75%** of total serum immunoglobulin. - **IgG** is monomer in nature. It contains one **Y-shaped** antibody molecule. - The molecule is formed of two **gamma (γ) heavy chains** and two **light chains**. The chains are joined together and gives the typical antibody structure. - The molecular weight of **IgG** is about **150 kDa**. It is smaller than **IgM**, so it can pass easily from blood into tissue spaces. - **IgG** is the main antibody of secondary immune response. It is produced in large amount after repeated exposure of same antigen. - It gives long lasting immunity. So it remains important after infection and also after vaccination. - **IgG** combines with **viruses** and **bacterial toxins**. It blocks their attachment with host cell and their harmful action becomes neutralized. - It acts as an important **opsonin**. It coats the surface of pathogen, so **macrophages** and **neutrophils** can recognize and engulf the pathogen easily. - **IgG** also activates **complement system**. This helps in lysis of pathogen and also helps in inflammatory reaction. - It is the only immunoglobulin which can cross the **placenta**. It passes from mother to foetus and gives natural passive immunity to the newborn. - **IgG** remains for longer time in circulation. The half life of most **IgG** is about **21 days**. - The long half life is due to **neonatal Fc receptor (FcRn)**. This receptor protects **IgG** from breakdown and returns it again into blood. - In human, **IgG** has four subclasses. These are **IgG1**, **IgG2**, **IgG3** and **IgG4**. **IgG1** is the major subclass. It forms about **60-70%** of total **IgG** and mainly reacts with protein antigens. - **IgG2** mainly reacts with bacterial capsular **polysaccharide antigens**. It is important against encapsulated bacteria. - **IgG3** is more inflammatory type. It has long flexible **hinge region**. Its half life is short, about **7 days**. - **IgG4** is formed during chronic antigen exposure. It is more regulatory type and shows **Fab-arm exchange**. - The level of **IgG** is tested to know past infection and immunity after vaccination. It is also used to know immune status of a patient. - **IgG** is used in many **monoclonal antibody** drugs. It is also the main antibody present in **intravenous immunoglobulin (IVIG)** preparation. ![Structure of IgG (Immunoglobulin G)](https://biologynotesonline.com/wp-content/uploads/2026/06/Structure-of-IgG-Immunoglobulin-G-1024x1024.png)Structure of IgG (Immunoglobulin G) ![Immunoglobulin G (IgG)](https://biologynotesonline.com/wp-content/uploads/2024/04/image-1615.png)Immunoglobulin G (IgG) ## IgA (Immunoglobulin A) **IgA** or **Immunoglobulin A** is an important antibody of mucosal immunity. It is present in serum but the main amount is found in mucosal secretions. It protects the body surface which is continuously exposed to outside environment. - **IgA** forms about **10-15%** of total serum immunoglobulin and it is second most abundant antibody in blood. But in the whole body, **IgA** is produced in highest amount than other antibody classes. It is mainly present in **saliva**, **tears**, **mucus**, **colostrum**, **breast milk**, respiratory tract, gastrointestinal tract and urogenital tract. - In serum, **IgA** is generally present as **monomer**. It is formed of two **alpha (α) heavy chains** and two **light chains**. In mucosal secretion, it is present mostly as **dimer** and called **secretory IgA (sIgA)**. In this form two **IgA** molecules are joined by **J-chain**. - **Secretory IgA** has a **secretory component** attached with it. This component protects **IgA** from the action of enzymes present in mucosal secretion. Due to this, **IgA** can remain stable in mucosal surface and perform its protective function. - The main function of **IgA** is protection of mucosal surface. It prevents attachment of **bacteria**, **viruses** and toxins with epithelial cells. This process is referred to as **immune exclusion**. In this process antigen is blocked before it enters into the tissue. - **IgA** also neutralizes toxins and pathogens in secretion. It helps to control infection at the entry site itself. It also helps in maintaining normal microbial flora, so useful microorganisms remain in balanced condition. - **IgA** is important for newborn baby. It is present in high amount in **colostrum** and **breast milk**. The **IgA** received from mother gives passive immunity to infant and mainly protects the intestine and mucosal surface. - In human, **IgA** has two subclasses, **IgA1** and **IgA2**. **IgA1** is the major form in serum and forms about **85%** of serum **IgA**. It has long flexible hinge region, but it can be cleaved by some bacterial proteases. - **IgA2** is more common in mucosal secretion. It is found in higher amount in lower gastrointestinal tract and female reproductive tract. It has shorter hinge region, so it is more resistant to bacterial proteases and useful in mucosal defence. - **Selective IgA deficiency** is the common primary immunodeficiency. In this condition **IgA** is very low or absent. The patient may suffer from repeated respiratory infection, gastrointestinal infection, allergy and asthma. - Abnormal **IgA** is related with **IgA nephropathy**. In this disease **IgA immune complex** gets deposited in kidney and causes inflammation. In **celiac disease**, **IgA anti-tissue transglutaminase antibody** and **IgA antiendomysial antibody** are used as diagnostic markers. ![Structure of IgA (Immunoglobulin A)](https://biologynotesonline.com/wp-content/uploads/2026/06/Structure-of-IgA-Immunoglobulin-A-1024x768.png)Structure of IgA (Immunoglobulin A) ![Structure and formation of secretory IgA](https://biologynotesonline.com/wp-content/uploads/2026/06/Structure-and-formation-of-secretory-IgA-931x1024.png)Structure and formation of secretory IgA ## IgM (Immunoglobulin M) **IgM** or **Immunoglobulin M** is the largest antibody present in serum. It is the first antibody produced during primary immune response. It is important in early protection before other antibodies are produced in large amount. - **IgM** forms about **5-10%** of total serum immunoglobulin. Due to its large size, it mainly remains inside blood vessels. It is not able to pass easily into tissues like **IgG**. - **IgM** may also be present in mucosal surface as **secretory IgM**. In this form it is transported through epithelial cells and helps in protection of mucosal region. - In serum, **IgM** is generally present as **pentamer**. It means five basic **Y-shaped** antibody units are joined together. These units are connected by **disulfide bonds** and one **J-chain**. - **IgM** has molecular weight about **900 kDa**. So it is much heavier than other immunoglobulins. Each antibody unit contains two **mu (μ) heavy chains** and two **light chains**. - The **mu (μ) heavy chain** is larger than heavy chain of many other antibodies. It contains four constant domains, while many other heavy chains have three constant domains. - **IgM** is also present as **monomer** on the surface of **B-cells**. In this condition it acts as **B-cell receptor (BCR)**. It helps the B-cell to recognize specific antigen. - **IgM** is the first antibody secreted after new antigen enters into body. So its presence mainly indicates early immune response. Later, other antibody like **IgG** becomes more important for long term immunity. - **IgM** has total ten antigen binding sites because of its pentameric structure. Each binding site may not be very strong in early stage, but all binding sites together gives very high binding power. This is called high **avidity**. - **IgM** is very effective in **agglutination** reaction. It can bind many antigen particles at the same time and forms large clumps. These clumps are then easily removed by phagocytic cells. - **IgM** is a strong activator of **classical complement pathway**. After binding with antigen, it activates complement proteins and helps in destruction of pathogen. - **IgM** is important in diagnosis of recent infection. When specific **IgM** is found in blood, it usually shows acute, early or recent infection. - During immune response, **IgM** level first rises and then gradually decreases. After this, **IgG** level increases and gives longer protection. So **IgM** is mainly the antibody of early phase of infection. ![Structure of IgM (Immunoglobulin M)](https://biologynotesonline.com/wp-content/uploads/2026/06/Structure-of-IgM-Immunoglobulin-M-1024x1024.png)Structure of IgM (Immunoglobulin M) ## IgE (Immunoglobulin E) **IgE** or **Immunoglobulin E** is an antibody mainly related with allergic reaction and parasitic infection. It is present in very small amount in serum, but its action is very important in tissue level. It is mostly attached with **mast cells** and **basophils**. - **IgE** is the least abundant antibody in blood. It forms only about **0.002-0.05%** of total immunoglobulins. So it is present only in trace amount in serum. - **IgE** is mainly found in tissues. It remains tightly attached with **mast cells** and **basophils** by its **Fc region**. It may also be present in **saliva** and **nasal secretion**. - **IgE** is monomer in structure. It is formed of two **epsilon (ϵ) heavy chains** and two **light chains**. Its molecular weight is about **200,000 Da**. - The **epsilon heavy chain** is larger than many other antibody heavy chains. It has four constant domains, **CH1**, **CH2**, **CH3** and **CH4**. - **IgE** has very short half life in serum. It remains in blood only for about **2 days**. But when it is attached with mast cells and basophils, it can remain for longer time. - The main function of **IgE** is in **type I hypersensitivity reaction**. It takes part in allergic conditions like **asthma**, **hay fever**, **eczema** and **anaphylaxis**. - **IgE** also gives protection against **parasitic** and **helminthic infections**. It helps in activation of cells which can act against large parasites. - **IgE** binds with high affinity **Fc receptor** present on **mast cells** and **basophils**. This receptor is called **FcϵRI**. - When allergen enters the body, it binds with **IgE** molecules present on mast cell surface. The allergen cross-links the **IgE** molecules and then mast cell becomes activated. - After activation, **mast cells** and **basophils** show **degranulation**. They release **histamine** and other inflammatory mediators. Due to this itching, swelling, mucus secretion, bronchoconstriction and other allergic symptoms are produced. - **IgE** reaction also helps in recruitment of **eosinophils**. These cells are important in killing of parasites and also take part in allergic inflammation. - Increased **IgE** level is seen in allergic diseases and parasitic infections. It is also found very high in rare immune disease called **hyper-IgE syndrome**. - **IgE** level is used clinically to know allergic condition of patient. Specific **IgE** test can show sensitivity against particular allergen. - Severe allergic reactions may be controlled by **allergen immunotherapy**. It changes the immune response against allergen slowly and reduces **IgE** mediated reaction. - Some monoclonal antibody drugs are made against **IgE**. Example is **omalizumab**, which binds with free **IgE** and prevents its attachment with mast cells and basophils. ![IgE (Immunoglobulin E)](https://biologynotesonline.com/wp-content/uploads/2026/06/IgE-Immunoglobulin-E-1024x1024.png)IgE (Immunoglobulin E) ## IgD (Immunoglobulin D) **IgD** or **Immunoglobulin D** is one of the antibody class present in very small amount in serum. It is mainly present on the surface of immature **B-cells**. It acts mostly as **B-cell receptor (BCR)** and helps in regulation of **B-cell** activity. - **IgD** is present in very low concentration in blood. It forms about **0.03-0.1%** of total serum immunoglobulins. So it is not a major circulating antibody like **IgG** or **IgA**. - Most of the **IgD** remains attached on the membrane of **B-cells**. It is found mainly on immature and mature naive **B-cells** along with **IgM**. Some amount of **IgD** may also be present in mucosal areas. - **IgD** is monomer in structure. It is formed of two **delta (δ) heavy chains** and two **light chains**. The light chain may be **kappa (κ)** or **lambda (λ)** type. - **IgD** does not have subclasses. This is different from **IgG** and **IgA**, where subclasses are present. - The half life of **IgD** in serum is short. It remains about **3 days** in blood. Due to this its free serum amount is very less. - The main function of **IgD** is to act as antigen receptor on **B-cells**. When antigen binds with membrane bound **IgD**, it helps in activation of **B-cell**. - **IgD** takes part in **B-cell** selection, maturation and differentiation. It helps the **B-cell** to respond against proper antigen. - It also helps in maintenance of peripheral tolerance and immune homeostasis. So unnecessary activation of **B-cells** can be controlled. - The exact function of **IgD** is not fully clear. But it is known that it has important role in regulation of **B-cell** response. - In routine diagnosis, **IgD** has less clinical importance than other antibodies. It is not commonly measured like **IgG**, **IgM** or **IgE**. - Increased level of serum **IgD** may be seen in **hyper-IgD syndrome**. This is an autoinflammatory condition where recurrent fever and inflammation may occur. - Recent studies also show that **IgD** may act through a receptor called **FcδR**. This pathway may be useful as therapeutic target in some autoimmune and blood related diseases. ![IgD (Immunoglobulin D)](https://biologynotesonline.com/wp-content/uploads/2026/06/IgD-Immunoglobulin-D-1024x1024.png)IgD (Immunoglobulin D) ## Antigenic Determinants (Epitopes) on Immunoglobulins **Immunoglobulins** are protein molecule. So they can also behave as **antigen** in some condition. The antigenic sites present on immunoglobulin molecule are called **antigenic determinants** or **epitopes**. These determinants are mainly present due to difference in structure of **heavy chain**, **light chain** and **variable region**. The following are the antigenic determinants of **immunoglobulins**– - **Isotypes**– **Isotypes** are the antigenic determinants present in the **constant region** of immunoglobulin. These are same in all normal individuals of a species. It forms the main classes of antibody. The type of **heavy chain** decides the class of immunoglobulin. **IgG** has **γ-chain**, **IgA** has **α-chain**, **IgM** has **μ-chain**, **IgD** has **δ-chain** and **IgE** has **ε-chain**. The **light chain** also has two isotypes, **kappa (κ)** and **lambda (λ)**. - **Allotypes**– **Allotypes** are the small antigenic differences present in immunoglobulin of different individuals of same species. These differences are due to genetic variation. It is present mainly in **constant region** of **heavy chain** or **light chain**. Allotype does not change the main class of antibody, but it gives individual difference in antibody molecule. - **Idiotypes**– **Idiotypes** are the antigenic determinants present in the **variable region** of antibody. It is mainly present near the **antigen binding site** or **paratope**. These determinants are different in different antibody molecules. So it is related with the specificity of antibody. Antibodies having same **idiotype** can bind with same **epitope** of antigen. **Isotype**, **allotype** and **idiotype** are the three antigenic determinants of **immunoglobulin**. **Isotype** is related with class difference. **Allotype** is related with individual genetic difference. **Idiotype** is related with antigen binding specificity. ![Immunoglobulin Antigen Determinants](https://biologynotesonline.com/wp-content/uploads/2024/04/image-1613.png)Immunoglobulin Antigen Determinants ## Antigen Binding Sites (B-cell Receptors) The following are the important points of **antigen binding sites** or **B-cell receptors**– - **Antigen binding site** is the specific region of **antibody** or **B-cell receptor (BCR)** which combines with **antigen**. It recognizes the particular part of antigen called **epitope**. - The antigen binding site is also called **paratope**. It is complementary with the **epitope** of antigen. So antigen and antibody binding is specific type. - In antibody molecule, the antigen binding sites are present at the tip of two arms of **Y-shaped** structure. So one antibody monomer has two antigen binding sites. - Before secretion, the antibody molecules are present on the surface of **B-cells**. These membrane bound antibody molecules act as **B-cell receptors**. - A newly formed **B-cell** contains many **B-cell receptors** on its surface. When proper antigen binds with these receptors, the **B-cell** becomes activated. - After activation, **B-cell** divides and forms many cells. Some cells become **plasma cells** and produces soluble **antibodies** in large amount. - Each antigen binding site is formed by the variable part of **heavy chain (VH)** and variable part of **light chain (VL)**. These two variable domains come together and forms the binding surface. - The actual binding part contains **complementarity determining regions (CDRs)**. These are also called **hypervariable regions**. - There are six **CDR loops** in one antigen binding site. Three are from **heavy chain** and three are from **light chain**. - Only few amino acids of these **CDR loops** directly touches the antigen. Usually about **15-20 amino acids** make contact with antigen. - Antigen binding is not by permanent chemical bond. It occurs by weak non-covalent forces like **hydrogen bond**, **Van der Waals force**, **electrostatic interaction** and **hydrophobic interaction**. - These weak bonds are reversible. But when many bonds act together, the binding becomes strong and stable. - The shape of antigen binding site is different in different antibodies. It may be pocket like, groove like, flat surface or projection like structure. - This different shape is due to variation in amino acid sequence of **CDR loops**. Because of this variation, antibody can recognize large number of antigens. - Sometimes the binding site changes slightly after contact with antigen. This is called **induced fit**. It helps in better fitting between **paratope** and **epitope**. - A simple antibody monomer is **bivalent**. It means it has two same antigen binding sites and can bind with two same antigenic determinants. - Due to bivalency, antibody can cross-link antigens. This helps in formation of larger antigen-antibody complex for removal by immune system. ## Biosynthesis of Antibody The following are the steps of **antibody biosynthesis**– **Step 1- Development of B-cell in bone marrow** The formation of antibody starts from developing **B-cells** in **bone marrow**. In this stage the **B-cell** undergoes genetic rearrangement. This process is called **V(D)J recombination**. **Step 2- Gene rearrangement** During **V(D)J recombination**, the **V**, **D** and **J** gene segments are joined in different combination. This gives a different antigen binding site to each **B-cell**. So each **B-cell** becomes specific for one type of antigen. **Step 3- Formation of B-cell receptor** After gene rearrangement, the mature naive **B-cell** forms surface antibodies. These antibodies are mainly **IgM** and **IgD**. They remain attached on the plasma membrane and act as **B-cell receptors (BCRs)**. **Step 4- Movement of naive B-cell** The naive **B-cells** then move from bone marrow to peripheral lymphoid organs. These organs include **lymph node**, **spleen** and other lymphoid tissues. Here the **B-cells** wait for the entry of suitable antigen. **Step 5- Antigen binding** When the specific **antigen** comes, it binds with matching **B-cell receptor**. The binding occurs between **epitope** of antigen and **paratope** of B-cell receptor. This is the first signal for activation of **B-cell**. **Step 6- Activation of B-cell** The **B-cell** becomes fully activated after antigen binding. In most cases help from **helper T-cells** is also needed. These **T-cells** give stimulatory signals to the **B-cell**. **Step 7- Multiplication of B-cell** The activated **B-cell** divides rapidly and forms many similar cells. This is called **clonal proliferation**. All these cells produce antibody against the same antigen. **Step 8- Somatic hypermutation** In this step, small changes occur in the antibody gene of activated **B-cells**. This is called **somatic hypermutation**. It helps to improve the binding strength of antibody with antigen. **Step 9- Class switching** Some activated **B-cells** change the class of antibody. At first mainly **IgM** is produced. Later it may change to **IgG**, **IgA** or **IgE** according to immune need. This process is called **class switching**. **Step 10- Differentiation into plasma cell** The activated **B-cells** then differentiate into **plasma cells**. Plasma cells are antibody secreting cells. They are specialized for producing large amount of antibody. **Step 11- Secretion of antibody** The **plasma cells** produce and secrete soluble antibodies into blood and body fluids. A plasma cell can release very large number of antibody molecules, about **2000 molecules per second**. These antibodies then bind with antigen and helps in removal of infection. ## Mechanism of action of Antibody The mechanism of action of **antibody** takes place in the following steps- ![Mechanism of action of Antibody](https://biologynotesonline.com/wp-content/uploads/2026/06/Mechanism-of-action-of-Antibody-1024x768.png) Mechanism of action of Antibody **Step 1- Antigen recognition** In this step **antibody** first recognizes the foreign antigen. The antigen may be present on **bacteria**, **virus**, toxin or any abnormal cell. The binding part of antibody is called **paratope**. It combines with specific part of antigen called **epitope**. **Step 2- Formation of antigen-antibody complex** After recognition, **antibody** binds with the antigen. This binding is specific type. As a result **antigen-antibody complex** is formed. This complex then starts different immune reactions in the body. **Step 3- Neutralization** In this process antibody directly binds with pathogen or toxin. The active site of pathogen becomes blocked. So the pathogen cannot attach with host cell and cannot enter into the cell. The toxic effect is also neutralized. **Step 4- Agglutination** A normal antibody has two antigen binding sites. So one antibody can bind with two same antigens at a time. In this way many antigen particles are joined together and form clumps. This clumping process is called **agglutination**. **Step 5- Opsonization** In this step antibody coats the surface of pathogen. The pathogen becomes marked for destruction. The **Fc region** of antibody is recognized by **phagocytic cells** like **macrophages** and **neutrophils**. Then these cells engulf and digest the pathogen. **Step 6- Complement activation** The **antigen-antibody complex** can activate the **classical complement pathway**. The **Fc region** of antibody binds with complement protein like **C1q**. Then a series of complement reactions are started. **Step 7- Formation of membrane attack complex** After complement activation, **membrane attack complex (MAC)** is formed on the pathogen membrane. It makes pores in the membrane. Due to this the pathogen cell swells and finally lysis occurs. **Step 8- Antibody dependent cellular cytotoxicity (ADCC)** In this process antibody binds with virus infected cell or tumour cell. The exposed **Fc region** is recognized by **Natural Killer (NK) cells**. Then **NK cells** release **perforin**, **granzymes** and other lytic substances. **Step 9- Destruction of target cell** The lytic substances enter into the abnormal target cell. They damage the cell and cause cell death. Thus infected cell or tumour cell is removed from the body. **Step 10- Removal of antigen** Finally the antigen is removed by **phagocytosis**, **complement lysis**, **neutralization** or **ADCC**. In this way antibody protects the body from infection and harmful foreign substances. ## The Immunoglobulin Superfamily **Immunoglobulin superfamily** is a large group of proteins which have **immunoglobulin domain** or **immunoglobulin like domain** in their structure. These proteins may be present in immune cells and also in many non-immune cells. They are mainly involved in recognition, binding, adhesion and cell communication. - The main character of **immunoglobulin superfamily** is the presence of **Ig domain**. This domain is a repeated structural unit present in many proteins. Due to this common structural unit, these proteins are placed in one superfamily. - The **Ig domain** usually contains about **110 amino acids**. It folds into a compact structure which is called **immunoglobulin fold**. This fold is the basic structural pattern of this whole superfamily. - In **immunoglobulin fold**, two **anti-parallel beta sheets** are arranged face to face. These beta sheets form a sandwich like structure. The structure is stabilized by internal **disulfide bond**. - It is believed that the members of **immunoglobulin superfamily** developed from one ancestral gene. Repeated gene duplication and modification produced many different proteins having same basic **Ig fold**. - **Antibodies** or **immunoglobulins** are the main members of this superfamily. These include **IgG**, **IgA**, **IgM**, **IgE** and **IgD**. They are Y-shaped proteins and take part in humoral immune response. - **T-cell receptors (TCRs)** are also important members of **immunoglobulin superfamily**. These receptors are present on **T-cells** and bind with antigen presented by **MHC molecules**. Their antigen binding part is similar in arrangement with antibody binding region. - Many **cell adhesion molecules** are also included in this superfamily. These molecules help in cell to cell recognition and attachment. They are important in nervous system, immune system and different tissues. - Other immune cell surface proteins and receptors also contain **Ig-like domains**. These proteins are present on **lymphocytes** and other cells. They help in immune recognition, signaling and interaction between cells. - As many proteins of this superfamily have similar structural framework, special numbering systems are used for their comparison. **IMGT** and **Honegger (AHo)** numbering systems are used to align and study variable domains of these proteins. - **immunoglobulin superfamily** includes many different proteins, but their basic structural plan is same. The presence of **Ig fold** makes them related, although their functions may be different in immune and non-immune tissues. ![Immunoglobulin Superfamily](https://biologynotesonline.com/wp-content/uploads/2024/04/image-1620.png)Immunoglobulin Superfamily ![Some members of the immunoglobulin superfamily, a group of structurally related, usually membrane-bound glycoproteins. In all cases shown here except for 2-microglobulin, the carboxyl-terminal end of the molecule is anchored in the membrane.](https://biologynotesonline.com/wp-content/uploads/2024/04/image-1621.png)Some members of the immunoglobulin superfamily, a group of structurally related, usually membrane-bound glycoproteins. In all cases shown here except for 2-microglobulin, the carboxyl-terminal end of the molecule is anchored in the membrane. ## Diseases related with Antibody The following are the diseases related with abnormal **antibody** level and function- ### A. Diseases due to low antibody level - **Hypogammaglobulinemia**– It is a condition where **immunoglobulin** level becomes low in blood. One or more class of antibody may be decreased. Due to this the person suffer from repeated infections. - **Selective IgA deficiency**– In this disease **IgA** is very low or absent. It is one of the common primary immunodeficiency. The patient mainly gets repeated respiratory infection, chest infection and allergic diseases. - **X-linked agammaglobulinemia**– It is a genetic immunodeficiency disease. Here **B-cells** do not mature properly. So antibody production becomes very less and bacterial infections occur again and again. - **Secondary immunodeficiency**– It is caused by other external or acquired causes. It may occur in **HIV infection**, malnutrition, bone marrow failure and after **chemotherapy**. In this condition antibody formation is reduced. ### B. Autoimmune diseases related with antibody - **Systemic lupus erythematosus (SLE)**– It is an autoimmune disease. In this disease antibodies are formed against own body tissues. Increased **IgG** and different autoantibodies are found. - **Rheumatoid arthritis**– It is an autoimmune disease of joints. Abnormal antibody like **rheumatoid factor** is present. It causes pain, swelling and stiffness of joints. - **Immune thrombocytopenia (ITP)**– It is a blood autoimmune disease. Antibody is formed against platelets. Platelets are destroyed and bleeding spots or purpura may occur. - **Guillain-Barre syndrome**– It is an autoimmune disease of peripheral nerves. Abnormal antibody reaction damages the nerves. **IVIG** is used in treatment of this disease. - **Chronic inflammatory demyelinating polyneuropathy (CIDP)**– It is a chronic nerve disease. Immune reaction damages the myelin sheath of nerve. **Intravenous immunoglobulin (IVIG)** is used as treatment. - **Celiac disease**– It is an autoimmune disease related with gluten. In this disease abnormal **IgA** response is present. **IgA anti-tissue transglutaminase** and **antiendomysial antibodies** are used for diagnosis. - **Kawasaki disease**– It is an inflammatory disease of children. Blood vessels are affected in this disease. **IVIG** is given to reduce the chance of coronary artery aneurysm. ### C. Allergic diseases - **Allergic diseases**– These are mainly related with increased **IgE**. It causes type I hypersensitivity reaction. The diseases include **asthma**, **eczema**, **hay fever**, food allergy and **anaphylaxis**. ### D. Diseases due to excess antibody production - **Multiple myeloma**– It is a cancer of **plasma cells**. In this disease one type of monoclonal immunoglobulin is produced in very high amount. Mostly **IgG** or **IgA** is increased. - **Hypergammaglobulinemia**– It is a condition with excess **immunoglobulin** in blood. It usually occurs in chronic infection and chronic inflammatory disease. - **Hyper-IgE syndrome**– It is a rare immune disorder. **IgE** level becomes very high in blood. Recurrent infection and allergic manifestations are seen. - **Hyper-IgD syndrome**– It is an autoinflammatory disease. Serum **IgD** level is increased. Recurrent fever and inflammatory symptoms are present. ### E. IgA related diseases - **IgA nephropathy**– It is a kidney disease. Abnormal **IgA** immune complex gets deposited in glomeruli of kidney. It causes inflammation and may lead to kidney damage. - **Henoch-Schonlein purpura (HSP)**– It is a systemic vasculitis of children. It occurs due to deposition of **IgA** and complement in small blood vessels. Skin, joint, gastrointestinal tract and kidney are affected. - **Linear IgA bullous dermatosis**– It is an autoimmune blistering disease of skin. It occurs due to **IgA** deposition in skin. Sometimes it is produced by drug like **vancomycin**. - **IgA pemphigus**– It is also an autoimmune blistering skin disease. **IgA antibodies** act against the skin cell surface. Due to this blisters are formed. ## Functions of Antibody The following are the main functions of **antibody**– - **Neutralization**– In this process **antibody** combines with **virus**, **bacteria** or **toxin**. The active part of antigen becomes blocked. So the antigen cannot attach with host cell and its harmful effect is neutralized. - **Opsonization**– **Antibody** covers the surface of pathogen. This makes the pathogen more easy for recognition by **macrophages** and **neutrophils**. Then these cells engulf the pathogen by **phagocytosis**. - **Complement activation**– The **antigen-antibody complex** can activate **complement system**. After activation, complement proteins act on the pathogen surface. This may cause lysis of pathogen and inflammation at the site of infection. - **Agglutination**– **Antibody** has two antigen binding sites. It can bind with more than one antigen at same time. Due to this, many antigen particles are joined together and forms clumps. This is called **agglutination**. - **Antibody dependent cellular cytotoxicity (ADCC)**– In this reaction **antibody** binds with infected cell or tumour cell. The **Fc region** of antibody is recognized by **Natural Killer (NK) cells**. Then the target cell is killed by lytic substances. - **B-cell activation**– Some antibodies are present on the surface of **B-cells**. These antibodies act as **B-cell receptors (BCRs)**. Mainly **IgD** and **IgM** are present in this form. After binding with antigen, **B-cell** becomes activated. - **Defence against parasites**– **IgE** is important against **parasitic infections**. It is attached with **mast cells** and **basophils**. After contact with parasite antigen, inflammatory chemicals are released. - **Allergic reaction**– **IgE** also takes part in **allergic reactions**. It causes release of **histamine** from mast cells. This produces allergy like **asthma**, **hay fever** and other hypersensitivity reactions. ## Clinical applications of Antibody The following are the clinical applications of **antibody**– ### A. Diagnostic applications - **Disease diagnosis**– **Antibodies** are used in different diagnostic tests. It is used in **ELISA**, **lateral flow test**, **rapid test** and other immunological test for detection of disease. - **Detection of infection**– **IgM** and **IgG** antibodies are measured to know the stage of infection. **IgM** usually shows recent or acute infection. **IgG** shows past infection or immunity. - **Detection of antigen**– Specific antibody is used to detect antigen of **bacteria**, **virus**, toxin and other pathogen. This is used in many laboratory diagnosis. - **Biomarker detection**– Antibodies are used to detect different **biomarkers** in blood or tissue. It helps in diagnosis of cancer, degenerative disease and other abnormal conditions. - **Cancer cell detection**– **Monoclonal antibodies** are used to identify specific marker present on cancer cells. So it helps in diagnosis and classification of tumour. ### B. Therapeutic applications - **Monoclonal antibody therapy**– **Monoclonal antibodies** are used as targeted treatment. It is used in cancer, autoimmune disease, inflammatory disease and some infectious diseases. - **Cancer treatment**– Antibodies bind with specific antigen present on tumour cell. Then the tumour cell is destroyed by immune mechanism or by attached drug. - **Autoimmune disease treatment**– Some antibodies are used to block inflammatory mediators. It is used in diseases like **rheumatoid arthritis**, inflammatory bowel disease and other autoimmune conditions. - **Intravenous immunoglobulin (IVIG)**– **IVIG** is prepared from plasma of healthy donors. It is given to patients with antibody deficiency and some autoimmune diseases. - **Use of IVIG in diseases**– **IVIG** is used in primary immunodeficiency, **Guillain-Barre syndrome**, **Kawasaki disease**, **ITP** and some neurological immune disorders. - **Allergy treatment**– Some antibody treatment is used to control **IgE** mediated allergic reaction. It reduces severe allergy, asthma and other hypersensitivity condition. - **Immune stimulation**– Some antibodies are made to stimulate immune cells. These antibodies activate **T-cells** or antigen presenting cells and helps in anti-tumour immunity. ### C. Advanced antibody applications - **Antibody drug conjugate (ADC)**– In this method antibody is joined with cytotoxic drug. The antibody carries the drug to tumour cell and kills the malignant cell with less damage to normal cell. - **Antibody radionuclide conjugate (ARC)**– In this method antibody is joined with radioactive isotope. It carries radiation directly to tumour tissue. It is used in some lymphoma and other cancer treatment. - **Immunotoxin**– In this method antibody or antibody fragment is joined with toxin. It enters into target cell and inhibits protein synthesis. Finally cell death occurs. - **Antibody directed enzyme prodrug therapy (ADEPT)**– In this method antibody carrying enzyme reaches tumour site. Then a non-toxic prodrug is changed into toxic drug near cancer cells. - **Bispecific antibody**– These antibodies can bind two different targets. One side binds with tumour cell and other side binds with immune cell. In this way immune cell is brought near tumour cell for destruction. - **CAR T-cell therapy**– In this method **T-cells** are modified with receptor made from antibody fragment. These modified **T-cells** recognize tumour antigen and kill the tumour cell. ### D. Nanobody applications - **Nanobody**– **Nanobodies** are small single domain antibody fragments. 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