Antibody- Definition, Structure, Properties, Types, Classes, Applications

Antibodies are proteins that are produced by the immune system to identify and neutralize foreign substances, such as bacteria, viruses, or toxins, that enter the body . They have a Y-shaped structure with two tips that can bind to specific parts of the foreign substance, called antigens . By binding to antigens, antibodies mark them for destruction by other immune cells or mechanisms . Antibodies are made by a type of white blood cell called B cell, which divides and matures into a clone of identical cells when it encounters an antigen. Antibodies are a protective and essential part of the immune response, while antigens can cause disease or allergic reactions .

Another word for antibody is immunoglobulin (Ig), which is an abbreviation of the term "immune globulin". Immunoglobulins are classified into five major classes according to their structure and function: IgG, IgM, IgA, IgD, and IgE . Each class of immunoglobulin has a different role in the immune system and can be found in different locations in the body. For example, IgG is the most abundant and versatile immunoglobulin, which can cross the placenta and provide passive immunity to the fetus . IgM is the first immunoglobulin to appear in response to an initial exposure to an antigen and can activate the complement system . IgA is the main immunoglobulin found in mucosal secretions, such as saliva, tears, and breast milk, and protects against ingested and inhaled pathogens . IgD is mainly found on the surface of B cells and helps in their activation and maturation . IgE is involved in allergic reactions and parasitic infections and binds to mast cells and basophils .

Antibodies are important tools for medical diagnosis, treatment, and research. They can be used to detect the presence of specific antigens or antibodies in blood samples or other biological fluids. They can also be used to treat various diseases by providing passive immunity or by targeting specific antigens on cancer cells or pathogens. Moreover, they can be used to study the structure and function of different antigens and their interactions with the immune system. Monoclonal antibodies are a type of antibodies that are produced in a laboratory by cloning a single B cell that recognizes a specific antigen. Monoclonal antibodies have high specificity and affinity for their target antigen and can be used for various therapeutic and diagnostic purposes .

In summary, antibodies are proteins that recognize and bind to antigens and trigger their elimination from the body. They are produced by B cells as a result of interaction with antigens. They belong to different classes that have different structures and functions. They are also useful for medical applications as they can detect, treat, or study various antigens.

B lymphocytes, or B cells, are a type of white blood cell that plays a key role in the humoral immunity component of the adaptive immune system. B cells produce antibody molecules, which are also known as immunoglobulins, that can either be secreted into the blood and other body fluids or inserted into the plasma membrane where they serve as a part of B-cell receptors (BCRs) .

Antibodies are Y-shaped proteins that have two identical antigen-binding sites at the tips of the Y. These sites can recognize and bind to specific antigens, which are molecules that trigger an immune response. Antigens can be found on the surface of pathogens, such as bacteria and viruses, or on foreign substances, such as toxins and allergens. When antibodies bind to antigens, they can neutralize them by blocking their activity or by tagging them for destruction by other immune cells .

B cells are produced in the bone marrow from hematopoietic stem cells and undergo a process of maturation and selection before they enter the circulation. During maturation, B cells rearrange segments of their antibody-encoding genes to generate a diverse repertoire of potential antigen-binding specificities. Each mature B cell expresses a unique BCR on its surface that can bind to a specific antigen .

B cells can be activated by two different mechanisms: T-dependent and T-independent. T-dependent activation involves the help of helper T cells, which recognize the same antigen as the B cell and provide signals that stimulate B cell proliferation and differentiation. T-independent activation does not require helper T cells, but relies on repeated or strong stimulation by antigens that can cross-link multiple BCRs on the same B cell .

Once activated, B cells differentiate into two main types of cells: plasma cells and memory cells. Plasma cells are antibody-producing factories that secrete large amounts of antibodies into the blood and other body fluids. Plasma cells have a short lifespan and are mainly involved in the primary immune response, which is the first response to a new antigen. Memory cells are long-lived cells that retain the memory of a specific antigen and can quickly mount a secondary immune response if the same antigen is encountered again. Memory cells provide long-term immunity and are responsible for the effectiveness of vaccines .

B cells are essential for protecting the body from various infections and diseases by producing antibodies that target specific antigens. However, B cells can also cause problems when they produce antibodies against self-antigens or harmless antigens, leading to autoimmune disorders or allergic reactions .

Antibodies are globular proteins that belong to the immunoglobulin superfamily. They have a basic Y-shaped structure with four polypeptide chains: two identical heavy chains and two identical light chains. The chains are linked together by disulfide bonds, which form bridges between the sulfur atoms of cysteine residues. The chains are also folded into domains, which are regions of stable three-dimensional structure.

The two arms of the Y-shaped antibody are called the Fab (fragment antigen-binding) regions, and they contain the variable domains of the heavy and light chains. The variable domains are responsible for recognizing and binding to specific antigens, which are foreign molecules that elicit an immune response. The variable domains have a high degree of diversity, as they can be generated by different combinations of gene segments, random mutations, and somatic recombination. The tips of the Fab regions are called the paratopes, and they interact with the epitopes, which are specific parts of the antigen.

The stem of the Y-shaped antibody is called the Fc (fragment crystallizable) region, and it contains the constant domains of the heavy chains. The constant domains are relatively conserved among different antibodies, and they determine the class or isotype of the antibody. There are five main classes of antibodies in humans: IgA, IgD, IgE, IgG, and IgM. Each class has a different type of heavy chain: α, δ, ε, γ, and µ, respectively. The constant domains also mediate the effector functions of antibodies, such as complement activation, opsonization, antibody-dependent cellular cytotoxicity (ADCC), and placental transfer.

Some antibodies have additional components that modify their structure and function. For example, some antibodies have a J chain, which is a polypeptide that links two or more antibody monomers together to form dimers or pentamers. The J chain also facilitates the transport of antibodies across epithelial cells and protects them from degradation by proteases. Another example is the secretory component, which is a fragment of a receptor that binds to polymeric antibodies and helps them to be secreted into mucosal fluids.

The structure of antibodies is crucial for their function in the immune system. By having a flexible hinge region between the Fab and Fc regions, antibodies can adjust their angle and distance to bind to multiple antigens or different parts of the same antigen. By having different classes and subclasses of antibodies, the immune system can tailor its response to different types of pathogens and antigens. By having different forms and modifications of antibodies, such as monomers, dimers, pentamers, J chains, and secretory components, antibodies can perform various roles in different tissues and fluids.

Immunoglobulin G (IgG) is the most abundant and versatile antibody class in human serum, accounting for about 80% of the total serum antibodies. It is found in blood and extracellular fluid and can bind to various pathogens such as viruses, bacteria and fungi. It protects the body from infection by immobilizing, ingesting, neutralizing, or eliminating the pathogens through different mechanisms. It is the only antibody that can cross the placenta and provide immunity to the newborn. It also works with IgA to defend the respiratory tract and the alveoli from inhaled substances.

Structure of IgG

The basic structure of IgG is composed of a Y-shaped protein where the Fab arms are linked to the Fc arms by an extended region of polypeptide chain called the hinge. The region is exposed and sensitive to attack by proteases that cleave the molecule into distinct functional units arranged in a four-chain structure.

An IgG molecule consists of two identical γ heavy chains, usually of the size 50 kDa. The light chains in IgG exist in two forms; κ and λ, where the κ form is more prevalent than λ, in the case of humans. The Fc regions of the molecule have a highly conserved N-glycosylation site in the heavy chain.

The heavy and light chain in an immunoglobulin molecule consists of an amino-terminal variable region with 100-110 amino acids. Each light chain has one variable domain (VL) and one constant domain (CL). The heavy chain, in turn, consists of one variable domain (VH) and three constant domains (CH1, CH2, and CH3).

The variable region varies between clones and is involved in antigen recognition. The constant region is conserved among clones and is required for the structural integrity and effector functions.

Properties of IgG

The IgG antibodies exist in the serum in the monomeric form, and these can cross the placenta from the mother to the fetus. Each IgG antibody has two paratopes that bind to two different epitopes on different antigens.

IgG has four subclasses classified on the basis of the subclasses of the γ heavy chains. These are named in order of their abundance in serum, with IgG1 being the most abundant.

IgG antibodies participate predominantly in secondary immune response as these are generated as a result of class switching and maturation of the response.

Subclasses of IgG

IgG antibodies have been classified into four subclasses; IgG1, IgG2, IgG3, and IgG4. These are named in order of their abundance in serum, with IgG1 being the most abundant.

IgG1

IgG1 is the most abundant subclass of IgG antibodies with γ1 heavy chains. It accounts for about 60-65% of the total serum IgG. It is primarily induced by soluble protein antigen and membrane proteins but is often accompanied by lower levels of the other subclasses. It is also involved in opsonization and activation of the complement cascade. A deficiency of IgG1 can lead to a decreased total IgG level as it is the most abundant subclass.

IgG2

IgG2 is the second most abundant IgG in human serum, and it is composed of γ2 heavy chains. It accounts for about 20-25% of total serum IgG. It is almost entirely responsible for the response against bacterial capsular polysaccharide antigens. It is also less efficient than IgG1 in activating complement and binding to Fc receptors on phagocytic cells. IgG2 is the only subclass of IgG antibodies that cannot cross the placenta during pregnancy. The deficiency of IgG2 can result in a weak defense against pathogenic microorganisms.

IgG3

IgG3 is the third most abundant IgG occurring in human serum with γ3 heavy chains. It accounts for about 8-10% of total serum IgG. It has a longer hinge region than other subclasses which makes it more flexible but also more susceptible to proteolysis. It is particularly effective in inducing effector functions. It is a potent proinflammatory antibody with a shorter half-life than other subclasses. It is also the most effective complement activator and has a high affinity for Fc receptors on phagocytic cells, aiding in opsonization.

IgG4

IgG4 is the least abundant IgG antibody in human serum, which consists of γ4 subclasses of heavy chains. It accounts for about 4-7% of total serum IgG. It has a unique property of exchanging half-molecules with other molecules resulting in bispecific antibodies that can bind to two different antigens simultaneously. This process is called Fab arm exchange or FAE. It also has a low affinity for Fc receptors and complement activation which makes it less inflammatory than other subclasses. It is induced by allergens, parasites, chronic infections, and autoantigens. It can cross the placenta and transfer from mother to fetus. It also plays a role in immune regulation by blocking other subclasses from binding to antigens or receptors. Increased levels of IgG4 have been associated with some autoimmune diseases such as pemphigus vulgaris, systemic lupus erythematosus, rheumatoid arthritis, etc.

Functions of IgG

IgG antibodies provide long-term protection against various agents like bacteria, viruses, toxins, etc. They fight infections by different mechanisms such as:

• Neutralization: The IgG molecule binds with the pathogen or toxin preventing them from interacting with host cells or tissues.
• Opsonization: The antibody-coated pathogen is easy to recognize and ingested by phagocytic immune cells such as macrophages or neutrophils.
• Complement activation: The binding of two or more adjacent Fc regions on an antigen activates the classical pathway of complement system resulting in lysis or inflammation.
• Antibody-dependent cellular cytotoxicity (ADCC): The Fc region on an antibody-bound target cell binds to Fc receptors on natural killer (NK) cells or cytotoxic T cells triggering them to release cytotoxic granules that kill the target cell.
• Antibody-dependent cellular phagocytosis (ADCP): Similar to ADCC but involves phagocytic cells instead of cytotoxic cells.
• Immune complex formation: The aggregation of antigens and antibodies forms immune complexes that can be cleared by phagocytes or complement system.

Immunoglobulin M (IgM) is one of the five classes of antibodies that are produced by B lymphocytes as part of the adaptive immune system. IgM is the first antibody to appear in response to an initial exposure to an antigen and provides short-term protection against infections. IgM is also involved in activating the complement system and agglutinating pathogens.

Structure of IgM

IgM has a basic four-chain structure composed of two identical µ heavy chains and two identical light chains (either κ or λ). Each chain has a variable region that binds to a specific antigen and a constant region that determines the antibody class. The heavy chain has one variable domain and four constant domains, while the light chain has one variable domain and one constant domain.

IgM exists in two forms: a monomeric form that is expressed on the surface of B cells as an antigen receptor, and a pentameric form that is secreted into the blood and other body fluids. The pentameric form consists of five monomers linked together by a J chain, a polypeptide that facilitates the polymerization and transport of IgM. The pentameric form has a molecular weight of about 900 kDa and 10 antigen-binding sites, making it the largest and most multivalent antibody.

Properties of IgM

IgM is the third most abundant immunoglobulin in human serum, accounting for about 10% of the total serum immunoglobulins. It has a concentration of about 1.5 mg/ml and a half-life of about 5 days. IgM is also the first immunoglobulin to be synthesized by the fetus, starting from about 20 weeks of gestation.

IgM is mainly found in the intravascular space, i.e., in the bloodstream and lymph fluid, due to its large size that prevents it from diffusing into the tissues. IgM is also present in low concentrations in some mucosal secretions, such as saliva, tears, and breast milk.

IgM is predominantly involved in the primary immune response, i.e., the immune response that occurs upon the first encounter with an antigen. IgM production is stimulated by T-independent antigens, such as polysaccharides and lipopolysaccharides, that can directly activate B cells without the help of T cells. IgM can also be produced by class switching from IgD or IgG in response to T-dependent antigens, such as proteins, that require T cell cooperation.

Functions of IgM

IgM has several functions in the immune system, such as:

• Neutralizing pathogens: IgM can bind to antigens on the surface of bacteria, viruses, or toxins and prevent them from infecting or harming host cells.
• Agglutinating pathogens: IgM can cross-link multiple antigens together and form large complexes that can be easily removed by phagocytic cells or trapped in blood vessels.
• Activating complement: IgM can initiate the classical pathway of complement activation by binding to C1q, a component of the complement system. This leads to a cascade of reactions that result in the formation of membrane attack complexes (MACs) that lyse pathogens or opsonins that enhance phagocytosis.
• Enhancing phagocytosis: IgM can bind to Fc receptors on phagocytic cells, such as macrophages and neutrophils, and facilitate their ingestion and destruction of pathogens.
• Providing feedback regulation: IgM can bind to B cell receptors (BCRs) on B cells and modulate their activation and differentiation. Depending on the signal strength and duration, IgM can either enhance or inhibit B cell responses.

Immunoglobulin A (IgA) is the main immunoglobulin found in the mucous membranes and secretions of the body, such as saliva, tears, milk, and sweat . It accounts for about 10-15% of the total serum immunoglobulins, but it is the most abundant antibody in external secretions . IgA plays a key role in defending the mucosal surfaces against attack by infectious microorganisms and maintaining immune homeostasis with the microbiota .

Structure of IgA

The basic structure of IgA is composed of a four-chain monomeric unit with two identical α heavy chains and two identical light chains . The heavy chains have three constant domains (CH1, CH2, and CH3) and one variable domain (VH), while the light chains have one constant domain (CL) and one variable domain (VL) . The variable domains of the heavy and light chains form the antigen-binding sites of IgA .

IgA can exist in different forms depending on its location and function. In serum, IgA is mostly monomeric with a molecular weight of 160 kDa . However, in secretions, IgA is mostly dimeric or polymeric with two or more monomeric units linked together by a joining peptide (J chain) . The J chain is a polypeptide of 15 kDa that facilitates the polymerization and transport of IgA across epithelial cells .

In addition to the J chain, secretory IgA (sIgA) has another component called the secretory component (SC) or transport piece (TP) . The SC is a polypeptide of 75 kDa that is derived from the proteolytic cleavage of the polymeric immunoglobulin receptor (pIgR) on the basolateral surface of epithelial cells . The SC protects sIgA from proteolytic degradation by enzymes in the secretions and enhances its binding to mucins and other glycoproteins on the mucosal surface .

The dimeric form of sIgA has a molecular weight of 385 kDa, while the polymeric form can reach up to 900 kDa . Each monomeric unit of sIgA has two antigen-binding sites, resulting in four or more binding domains in a single molecule .

Properties of IgA

IgA has several properties that make it suitable for its role in mucosal immunity. Some of these properties are:

• IgA is resistant to pH changes and proteolytic enzymes in the secretions due to its SC and J chain .
• IgA can cross the epithelial layer and enter into body secretions by binding to pIgR on the basolateral surface of epithelial cells and being transported to the apical surface where it is released with SC .
• IgA can bind to antigens with high specificity and affinity due to its variable regions .
• IgA can neutralize antigens by blocking their attachment to epithelial cells or by interfering with their biological activity [^3^.
• IgA can activate complement by alternative pathways by binding to properdin or factor Bb[^4^.
• IgA can interact with specific receptors and immune mediators on various cells, such as FcαRI (CD89) on phagocytes, polymeric immunoglobulin receptor (pIgR) on epithelial cells, CD71 on enterocytes, asialoglycoprotein receptor on hepatocytes, and Fcα/μR on B cells[^5^.

Subclasses of IgA

IgA can be classified into two subclasses: IgA1 and IgA2. These subclasses differ in their heavy chain structure, distribution, function, and susceptibility to bacterial proteases[^2^.

IgA1

IgA1 is the predominant subclass of IgA in serum (about 80%) and is produced mainly in the bone marrow and released into circulation[^2^. It is also found in some mucosal secretions, such as saliva, tears, nasal fluid, bronchial fluid, colostrum, and milk[^2^.

The main structural difference between IgA1 and IgA2 is the hinge region between CH1 and CH2 domains. IgA1 has an extended hinge region with 13 amino acids that are rich in proline, serine, and threonine residues[^2^. This region provides flexibility to IgA1 but also makes it vulnerable to cleavage by bacterial proteases from Streptococcus pneumoniae , Haemophilus influenzae , Neisseria meningitidis , Neisseria gonorrhoeae , and some anaerobic bacteria[^2^. The cleavage results in loss of antigen-binding activity and effector functions of IgA1[^2^.

The function of IgA1 is mainly to neutralize antigens in serum and some mucosal secretions. It can also activate complement by alternative pathways and bind to FcαRI on phagocytes[^4^.

IgA2

IgA2 is the predominant subclass of IgA in mucosal secretions (about 60%) and is produced mainly by plasma cells located in subepithelial regions of mucosal tissues[^2^. It is also found in low concentrations in serum (about 20%)[.

The main structural difference between IgA2 and IgA1 is the hinge region between CH1 and CH2 domains. IgA2 has a shorter hinge region with only four amino acids that are devoid of proline residues[^2^. This region provides stability to IgA2 but also reduces its flexibility. However, it also makes it resistant to cleavage by bacterial proteases that target IgA1[^2^.

The function of IgA2 is mainly to protect mucosal surfaces from microbial invasion by neutralizing antigens in secretions. It can also activate complement by alternative pathways and bind to FcαRI on phagocytes[^4^.

Immunoglobulin D (IgD) is a type of antibody that is mainly found on the surface of mature B lymphocytes, where it functions as an antigen receptor. IgD is also produced in a secreted form that is present in very small amounts in the blood serum. The exact role of IgD in the immune system is not well understood, but it may be involved in regulating B cell activation and differentiation.

Structure of IgD

IgD has a typical antibody structure with two identical light chains and two identical heavy chains of the delta (δ) class. The light and heavy chains are linked by disulfide bonds and have variable and constant regions. The heavy chains have four constant domains (CH1, CH2, CH3, and CH4) and one variable domain (VH), while the light chains have one constant domain (CL) and one variable domain (VL). The variable regions form the antigen-binding sites, while the constant regions determine the effector functions.

IgD has a long hinge region between the CH1 and CH2 domains, which makes the molecule flexible and susceptible to proteolytic cleavage. The hinge region also contains extra amino acids at the C-terminal end for anchoring to the membrane of B cells. IgD associates with two other proteins, Ig-alpha and Ig-beta, which are involved in signal transduction.

Secreted IgD is a monomeric antibody with a molecular weight of 185 kDa and a half-life of 2.8 days. It accounts for about 0.25% of the total serum immunoglobulins.

Properties of IgD

IgD is mainly expressed on the surface of mature B cells, where it coexists with another antibody, IgM. Both IgM and IgD are specific for the same antigen and are produced by alternative splicing of the same mRNA transcript. IgD expression begins when the B cell leaves the bone marrow and migrates to peripheral lymphoid tissues.

The function of IgD on B cells is not clear, but it may act as a sensor for antigen stimulation and trigger B cell activation. When IgM and IgD bind to an antigen, they induce internalization of the antigen and presentation to helper T cells, which stimulate B cell proliferation and differentiation into plasma cells or memory cells.

Secreted IgD is found in very low concentrations in blood serum and mucosal secretions. It does not bind complement or Fc receptors on other cells. It may have some role in allergic reactions or respiratory immune defense by binding to basophils and mast cells and activating them to produce antimicrobial factors.

Immunoglobulin E (IgE) is a type of antibody that is found only in mammals and is mainly involved in allergic reactions and parasitic infections. IgE is produced by plasma cells in response to specific antigens and binds to receptors on mast cells and basophils, triggering the release of inflammatory mediators. IgE also plays a role in immunity against certain protozoan parasites and venoms.

Structure of IgE

IgE has a typical antibody structure with two light chains and two heavy chains of the epsilon (ε) class. The ε heavy chain has four constant domains (Cε1–Cε4) and one variable domain (VH), while the light chain has one constant domain (CL) and one variable domain (VL). The light and heavy chains are linked by disulfide bonds, forming a Y-shaped molecule with two antigen-binding sites.

The ε heavy chain has a high carbohydrate content (about 12%) and an extended hinge region between Cε1 and Cε2 domains, which increases the flexibility of the molecule but also makes it susceptible to proteolytic cleavage. The hinge region also contains cysteine residues that can form intermolecular disulfide bonds, allowing IgE to form dimers or tetramers.

IgE can exist in two forms: membrane-bound IgE and secreted IgE. Membrane-bound IgE is expressed on the surface of B cells as a monomer, where it serves as an antigen receptor. Secreted IgE is released into the circulation or mucosal secretions as a monomer or a dimer, where it binds to high-affinity receptors (FcεRI) on mast cells and basophils.

Functions of IgE

IgE is mainly involved in type I hypersensitivity reactions, which are mediated by mast cells and basophils. When IgE binds to an antigen, it cross-links FcεRI receptors on these cells, triggering their degranulation and the release of various inflammatory mediators, such as histamine, leukotrienes, prostaglandins, cytokines, and chemokines. These mediators cause vasodilation, increased vascular permeability, smooth muscle contraction, mucus secretion, and recruitment of other inflammatory cells, leading to the symptoms of allergic reactions, such as itching, sneezing, wheezing, swelling, and anaphylaxis.

IgE also plays a role in immunity against certain parasitic infections, such as helminths and protozoa. IgE can bind to antigens on the surface of these parasites and activate eosinophils, which can kill them by releasing cytotoxic granules. IgE can also activate complement via the classical pathway and enhance phagocytosis by macrophages and neutrophils.

IgE may have evolved as a defense mechanism against venoms from snakes, spiders, bees, and other animals. IgE can neutralize venom components by binding to them and preventing their interaction with cellular receptors or enzymes. IgE can also activate mast cells and basophils to release mediators that counteract the effects of venom, such as vasodilation, anticoagulation, and pain relief.

Antibodies are not only essential for our normal immune response, but they also provide powerful tools for various applications in medicine and research. The high specificity and affinity of antibodies make them very useful for detecting and quantifying a wide range of targets, from drugs to serum proteins to microorganisms. Some of the applications of antibodies are:

• Therapeutic applications: Antibodies can be used to treat immune deficiencies as a means of passive immunity. For example, intravenous immunoglobulin (IVIG) is a preparation of pooled human IgG antibodies that can be administered to patients with primary or secondary immunodeficiencies. Antibodies can also be used to treat some types of cancer by targeting specific antigens on tumor cells and triggering their destruction by immune cells or complement activation. For example, rituximab is a monoclonal antibody that binds to CD20 antigen on B cells and is used to treat non-Hodgkin`s lymphoma and chronic lymphocytic leukemia. The development of monoclonal antibodies has also enabled the production of chimeric and humanized antibodies that are less immunogenic and more effective than murine antibodies. For example, trastuzumab is a humanized antibody that binds to HER2 receptor on breast cancer cells and inhibits their growth and survival. Antibodies can also be used to modulate immune responses in autoimmune diseases or inflammatory conditions by blocking cytokines or receptors involved in inflammation. For example, adalimumab is a human monoclonal antibody that binds to tumor necrosis factor alpha (TNF-α) and is used to treat rheumatoid arthritis, psoriasis, and Crohn`s disease .

• Diagnostic applications: Antibodies are widely used in medical diagnostics as they allow the detection of specific antigens or antibodies in biological samples. Many biochemical assays rely on the use of antibodies for the diagnosis of diseases. For example, enzyme-linked immunosorbent assays (ELISAs) use antibodies coated on a solid surface to capture antigens from a sample and then use another antibody conjugated with an enzyme to detect the bound antigens by producing a colorimetric signal. ELISAs can be used to measure the levels of hormones, cytokines, drugs, or antibodies in serum or other fluids. Another example is immunofluorescence assays (IFAs) that use antibodies labeled with fluorescent dyes to visualize antigens on cells or tissues under a microscope. IFAs can be used to identify pathogens, such as bacteria or viruses, or markers of cellular differentiation, such as CD4 or CD8 on T cells .

• Research applications: Antibodies are indispensable in a wide range of research applications as they enable the study of the structure and function of different antigens and their interactions with the host. Antibodies are key components in many techniques used in molecular biology, cell biology, immunology, and biochemistry. For example, Western blotting uses antibodies to detect specific proteins separated by gel electrophoresis and transferred to a membrane. Western blotting can be used to analyze the expression, modification, or interaction of proteins in different samples. Another example is immunoprecipitation (IP) that uses antibodies to isolate specific proteins or protein complexes from cell lysates by binding them to beads coated with antibodies. IP can be used to study the interaction or activity of proteins involved in signaling pathways or gene regulation. Another example is immunohistochemistry (IHC) or immunocytochemistry (ICC) that use antibodies to localize specific antigens on tissue sections or cell cultures by binding them to antibodies conjugated with enzymes or fluorescent dyes. IHC or ICC can be used to study the distribution, localization, or expression of antigens in normal or diseased tissues or cells .

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