Antigen vs Antibody- Definition and 12 Major Differences
Antigens and antibodies are two key components of the immune system that play vital but distinct roles in protecting the body from foreign substances and pathogens. In this article, we will explore the definition and 12 major differences between antigen and antibody, as well as some examples of their types and functions.
An antigen is any substance that stimulates an immune response. Antigens may come from outside the body (foreign antigens) or from within the body (autoantigens). Foreign antigens include pathogens (such as viruses or bacteria), allergens (such as pollen or dust), toxins (such as venom or chemicals), or blood cells from another person. Autoantigens are normally produced by the body and do not trigger an immune response, unless there is an autoimmune disorder that causes the body to attack its own cells and tissues.
An antibody is a Y-shaped protein that is produced by white blood cells called B cells in response to exposure to antigens. Antibodies are highly specific, meaning that each type of antibody can only bind to one type of antigen. When an antibody binds to an antigen, it forms an antigen-antibody complex that activates other immune cells and mechanisms to eliminate the foreign substance. Antibodies also prevent antigens from infecting healthy cells by blocking their entry or neutralizing their effects.
Antigens and antibodies are involved in various types of immune reactions, such as humoral immunity, cellular immunity, allergic reactions, autoimmune diseases, blood transfusions, and vaccinations. Understanding the differences between antigen and antibody can help us appreciate how the immune system works and how it can be manipulated for medical purposes.
Antibodies are proteins that protect you when an unwanted substance enters your body. Produced by your immune system, antibodies bind to these unwanted substances in order to eliminate them from your system. Another word for antibody is immunoglobulin.
An antibody is a protein produced by your immune system to attack and fight off these antigens. An antigen is a foreign substance that enters your body. This can include bacteria, viruses, fungi, allergens, venom and other various toxins.
The reaction between antigens and antibodies involves complementary binding sites on the antibody and on the antigen molecules. The binding of an antibody to a toxin, for example, can neutralize the poison simply by changing its chemical composition; such antibodies are called antitoxins. By attaching themselves to some invading microbes, other antibodies can render such microorganisms immobile or prevent them from penetrating body cells. In other cases the antibody-coated antigen is subject to a chemical chain reaction with complement, which is a series of proteins found in the blood. The complement reaction either can trigger the lysis (bursting) of the invading microbe or can attract microbe-killing scavenger cells that ingest, or phagocytose, the invader.
Antibodies are produced by specialized white blood cells called B lymphocytes (or B cells) . When an antigen comes into contact with a B cell, it causes the B cell to divide and clone. These cloned B cells — or plasma cells — release millions of antibodies into your bloodstream and lymph system. Antibodies are located in various areas of your body, including your skin, lungs, tears, saliva and even breast milk.
Monoclonal antibodies are created in a lab. They mimic your immune system’s natural ability to fight off pathogens. Using monoclonal antibodies to fight infections is a type of immunotherapy.
Antigens are substances that can trigger an immune response in the body. They are usually foreign molecules that are recognized as harmful or non-self by the immune system. Antigens can be derived from various sources, such as bacteria, viruses, fungi, parasites, toxins, allergens, transplanted organs, or cancer cells. Antigens can also be endogenous or autoantigens, which are normally present in the body but can become targets of the immune system under certain conditions.
Antigens have specific regions on their surface called epitopes, which are recognized and bound by antibodies. Each antigen can have multiple epitopes that can elicit different antibody responses. The specificity of the antigen-antibody interaction depends on the molecular structure and shape of the epitope and the paratope (the binding site on the antibody).
Antigens can be classified into different types based on their chemical nature, such as proteins, polysaccharides, lipids, nucleic acids, or glycoproteins. Proteins are the most common and immunogenic type of antigens, as they have a high degree of variability and complexity. Polysaccharides are also important antigens, especially in bacterial cell walls and capsules. Lipids and nucleic acids are usually not very immunogenic by themselves, but they can become antigens when they are combined with proteins or polysaccharides.
Antigens play a crucial role in the adaptive immune system, which is the branch of the immune system that can generate specific and long-lasting responses to foreign invaders. Antigens stimulate the production and activation of B cells and T cells, which are the main types of lymphocytes (white blood cells) involved in adaptive immunity. B cells produce antibodies that can neutralize or eliminate antigens, while T cells help to regulate and coordinate the immune response. Antigens also help to generate immunological memory, which is the ability of the immune system to remember and respond more quickly and effectively to previously encountered antigens.
Antigens and antibodies are two key components of the immune system that help protect the body from foreign invaders and diseases. Antigens are molecules that can trigger an immune response, while antibodies are proteins that can bind to specific antigens and neutralize them. Antigens and antibodies have many differences in their characteristics, molecule type, effect, origin, parts, synonyms, specific binding site, complexity, and source. Here is a summary of the 12 major differences between antigen and antibody:
|Usually proteins, but can also be polysaccharides, lipids, or nucleic acids
|These are molecules that can be recognized by the immune system as foreign or non-self and elicit an immune response
|These are glycoproteins that are secreted by immune cells (plasma cells) in response to a foreign substance (antigen)
|Stimulate the production of antibodies and activate other immune cells
|Protect the body by binding to antigens and facilitating their elimination
|Can be exogenous (from outside the body) or endogenous (from within the body)
|Always endogenous (produced by the body)
|Have one or more epitopes (regions that can bind to antibodies)
|Have two light chains and two heavy chains (four polypeptides) that form a Y-shaped structure with two antigen-binding sites
|Immunogens (if they can induce an immune response) or haptens (if they need to be attached to a carrier molecule to induce an immune response)
|Specific binding site
|Epitope (also called antigenic determinant)
|Paratope (also called antigen-binding site)
|Varies depending on the nature and size of the molecule; can have multiple epitopes that can bind to different antibodies
|Very high; each antibody has a unique amino acid sequence and three-dimensional structure that determines its specificity and affinity for an antigen
|Usually from a foreign substance (such as bacteria, viruses, fungi, parasites, toxins, pollen, etc.) or from abnormal cells (such as cancer cells or transplanted organs)
|Produced by B lymphocytes (a type of white blood cell) that differentiate into plasma cells upon encountering an antigen
|Can be classified into different types based on their origin, structure, function, or immunogenicity; some examples are T-dependent antigens, T-independent antigens, superantigens, allergens, etc.
|There are five basic kinds of antibodies (IgG, IgM, IgA, IgE, and IgD) that differ in their structure, function, distribution, and role in immunity
|These are antigens that originate from within the body and are normally tolerated by the immune system; some examples are blood group antigens and HLA (Histocompatibility Leukocyte antigens) that determine tissue compatibility for transplantation
|These are antibodies that recognize endogenous antigens as foreign and cause autoimmune diseases; some examples are rheumatoid factor (IgM antibody against IgG), anti-nuclear antibodies (ANAs), etc.
|These are antigens that are normally present in the body but become targets of the immune system due to genetic or environmental factors; some examples are nucleoproteins and nucleic acids that are associated with autoimmune diseases such as systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), etc.
|These are antibodies that are produced against autoantigens and contribute to tissue damage and inflammation in autoimmune diseases; some examples are anti-dsDNA antibodies in SLE, anti-citrullinated protein antibodies in RA, etc.
These differences between antigen and antibody help us understand how the immune system works and how it can be modulated for therapeutic purposes. In the following sections, we will explain each difference in more detail and provide examples of different types of antigens and antibodies.
In this section, we will explain each of the 12 major differences between antigen and antibody in more detail.
- Characteristics: Antigens are substances that can trigger an immune response in the body. They are usually foreign molecules, such as proteins, polysaccharides, lipids, or nucleic acids. Antibodies are proteins that are produced by the immune system in response to antigens. They are also called immunoglobulins or Ig.
- Molecule type: Antigens can be any type of molecule, but they are usually large and complex. Antibodies are always proteins, and they have a specific structure with four polypeptide chains arranged in a Y-shape.
- Definition: Antigens are defined as molecules that can bind to specific antibodies or T cell receptors and elicit an immune response. Antibodies are defined as molecules that can bind to specific antigens and neutralize or eliminate them.
- Effect: Antigens can have different effects on the body, depending on their nature and source. Some antigens can cause diseases, such as viruses, bacteria, fungi, or parasites. Some antigens can cause allergies, such as pollen, dust, or animal dander. Some antigens can be beneficial, such as vaccines, which stimulate immunity against pathogens. Antibodies have a protective effect on the body, as they help to fight off infections and diseases. They can also mediate allergic reactions, such as anaphylaxis or asthma.
- Origin: Antigens can originate from outside the body (exogenous) or from inside the body (endogenous). Exogenous antigens include pathogens, toxins, allergens, transplanted organs, or blood transfusions. Endogenous antigens include self-antigens, which are normally tolerated by the immune system, and autoantigens, which are recognized as foreign by the immune system in autoimmune diseases. Antibodies originate from within the body, as they are produced by B cells (a type of white blood cell) in the bone marrow and lymph nodes.
- Parts: Antigens have different parts that can be recognized by antibodies or T cells. The part of the antigen that binds to the antibody is called the epitope or antigenic determinant. The part of the antibody that binds to the antigen is called the paratope or antigen-binding site. Antibodies have four parts: two identical light chains and two identical heavy chains. Each chain has a constant region and a variable region. The variable regions form the antigen-binding sites at the tips of the Y-shaped molecule. The constant regions determine the class and function of the antibody.
- Synonyms: Antigens are also known as immunogens, as they induce an immune response. Antibodies are also known as immunoglobulins (Ig), as they are globular proteins involved in immunity.
- Specific binding site: Antigens have multiple epitopes that can bind to different antibodies or T cells. Each epitope is specific for one type of antibody or T cell receptor. Antibodies have two paratopes that can bind to one antigen or two identical antigens. Each paratope is specific for one type of epitope.
- Complexity: Antigens vary in their complexity and immunogenicity (ability to induce an immune response). Generally, larger and more complex antigens are more immunogenic than smaller and simpler antigens. However, other factors such as dose, route of administration, genetic background, and presence of adjuvants (substances that enhance immunogenicity) also influence the immune response to antigens. Antibodies are highly complex and diverse molecules that can recognize a wide range of antigens. They are generated by a process called somatic recombination, which involves random rearrangement of gene segments encoding the variable regions of the antibody chains. This process creates millions of different antibody molecules with different antigen-binding sites.
- Source: Antigens can come from various sources, such as microorganisms (bacteria, viruses, fungi, parasites), plants (pollen), animals (venom), chemicals (drugs), or human tissues (transplants). Antibodies come from B cells that differentiate into plasma cells (antibody-secreting cells) or memory B cells (long-lived cells that remember previous encounters with antigens) upon activation by antigens.
- Kinds: Antigens can be classified into different kinds based on their origin (exogenous or endogenous), structure (protein or non-protein), size (hapten or carrier), shape (linear or conformational), location (surface or intracellular), or function (toxin or enzyme). Antibodies can be classified into different kinds based on their structure (monomer or polymer), function (neutralization or opsonization), location (blood or mucosa), or class (IgG, IgM, IgA, IgE, IgD).
- Examples: Some examples of antigens are:
- Exogenous antigens: influenza virus, tetanus toxin, ragweed pollen, penicillin, ABO blood group antigens.
- Endogenous antigens: self-antigens, autoantigens, tumor antigens, fetal antigens. Some examples of antibodies are:
- IgG: most abundant antibody in blood; provides long-term protection; crosses placenta; neutralizes toxins; opsonizes bacteria; activates complement system.
- IgM: first antibody produced in primary response; pentamer; activates complement system; agglutinates microbes.
- IgA: most abundant antibody in mucosal secretions; dimer; protects mucosal surfaces; neutralizes toxins; prevents attachment of microbes.
- IgE: least abundant antibody in blood; binds to mast cells and basophils; mediates allergic reactions; protects against parasites.
- IgD: present on surface of naive B cells; functions as B cell receptor; signals B cell activation.
Antibodies are also known as immunoglobulins (Ig), which are specialized proteins produced by the immune system to fight against foreign substances (antigens) such as bacteria, viruses, fungi, allergens, venom and toxins. Antibodies have a Y-shaped structure with two identical heavy chains and two identical light chains. The tips of the Y-shaped arms are called paratopes, which are specific binding sites that recognize and attach to the antigens. The stem of the Y-shaped structure is called the Fc region, which determines the class or isotype of the antibody.
There are five main classes or isotypes of antibodies in humans: IgG, IgM, IgA, IgE and IgD . They differ in their heavy chains, which have different amino acid sequences and constant domains. The constant domains affect the antibody`s function, location and interaction with other immune cells and proteins. The following table summarizes some of the characteristics and functions of each antibody class:
|Blood and tissue fluids
|The most abundant and versatile antibody; protects against viral and bacterial infections; activates complement system; crosses placenta and provides passive immunity to fetus
|Blood and lymph
|The first antibody produced in response to an infection; forms pentamers (five antibodies joined together); activates complement system; agglutinates antigens
|Mucosal tissues, saliva, tears, breast milk and intestinal fluid
|The main antibody in mucosal immunity; protects against ingested and inhaled pathogens; prevents them from adhering to epithelial cells; neutralizes toxins
|Skin, lungs and mucous membranes
|The main antibody involved in allergic reactions; binds to mast cells and basophils; triggers histamine release; protects against parasitic infections
|Surface of B cells
|The least understood antibody; supports B cell maturation and activation; may have a role in allergic reactions
Each antibody class has a different role and function in the immune system. By producing different types of antibodies, the body can mount a specific and effective response to various antigens.
Endogenous antigens are antigens that have been generated within the body. They include xenogenic (heterologous), autologous, idiotypic or allogenic (homologous) antigens and autoantigens . Endogenous antigens are processed by the macrophages which are later accepted by the cytotoxic T-cells. Endogenous antigens exist on cells inside the body and tell the immune system that they are either friendly (“self”) or harmful.
Some examples of endogenous antigens are blood group antigens and HLA (Histocompatibility Leukocyte antigens).
Blood group antigens are molecules on the surface of red blood cells that determine a person`s blood type. They are inherited from the parents and can be A, B, AB or O. Blood group antigens are important for blood transfusion, as incompatible blood types can cause a severe immune reaction.
HLA (Histocompatibility Leukocyte antigens) are molecules on the surface of most cells in the body that encode a person`s genetic identity. They are also inherited from the parents and can be classified into six major groups: HLA-A, HLA-B, HLA-C, HLA-DP, HLA-DQ and HLA-DR. HLA molecules are essential for organ transplantation, as they determine whether a donor organ is compatible with a recipient`s immune system.
Endogenous antigens play a vital role in the immune system`s recognition and response to foreign or harmful substances. However, sometimes the immune system can mistakenly attack endogenous antigens that are normal or beneficial, leading to autoimmune diseases. Therefore, understanding endogenous antigens can help in developing treatments and prevention strategies for various immune disorders.
Autoantigens are antigens that are derived from the body`s own cells or tissues. They are normally tolerated by the immune system and do not elicit an immune response. However, under certain conditions, such as autoimmune diseases, infections, or genetic mutations, autoantigens can become targets of the immune system and cause damage to the host.
One example of autoantigens is nucleoproteins, which are proteins that are associated with nucleic acids (DNA or RNA). Nucleoproteins are found in the nucleus and cytoplasm of cells, as well as in some viruses. Some examples of nucleoproteins are histones, which bind to DNA and form chromatin; ribonucleoproteins, which are involved in RNA processing and transport; and viral capsid proteins, which enclose the viral genome.
Another example of autoantigens is nucleic acids, which are the molecules that store and transmit genetic information. Nucleic acids are composed of nucleotides, which have a nitrogenous base, a sugar, and a phosphate group. The two main types of nucleic acids are DNA and RNA. DNA is the main genetic material in most organisms, while RNA has various functions such as coding for proteins, regulating gene expression, and catalyzing reactions.
Autoantibodies are antibodies that recognize and bind to autoantigens. Autoantibodies can cause inflammation, tissue damage, and organ dysfunction by various mechanisms, such as forming immune complexes, activating complement system, blocking or stimulating receptors, or interfering with cellular functions. Autoantibodies can be detected by various laboratory tests, such as enzyme-linked immunosorbent assay (ELISA), immunofluorescence, or western blot.
Some examples of autoimmune diseases that involve autoantibodies against nucleoproteins and nucleic acids are:
- Systemic lupus erythematosus (SLE): a chronic inflammatory disease that affects multiple organs and tissues. SLE is characterized by the presence of autoantibodies against nuclear antigens, such as double-stranded DNA (dsDNA), single-stranded DNA (ssDNA), histones, ribonucleoproteins, and Smith antigen (Sm).
- Sjögren`s syndrome: a chronic inflammatory disease that affects the exocrine glands, especially the salivary and lacrimal glands. Sjögren`s syndrome is characterized by the presence of autoantibodies against ribonucleoproteins, such as Ro/SS-A and La/SS-B.
- Rheumatoid arthritis (RA): a chronic inflammatory disease that affects the joints and other tissues. RA is characterized by the presence of autoantibodies against citrullinated proteins, which are proteins that have undergone a post-translational modification of arginine residues to citrulline. Citrullinated proteins can be found in various nucleoproteins, such as histones and fibrinogen.
- Mixed connective tissue disease (MCTD): a rare autoimmune disease that has features of SLE, Sjögren`s syndrome, RA, and polymyositis. MCTD is characterized by the presence of autoantibodies against U1 ribonucleoprotein (U1-RNP), which is a component of the spliceosome complex that is involved in RNA splicing.
In summary, autoantigens are antigens that originate from the host`s own cells or tissues. They can include nucleoproteins and nucleic acids, which are important molecules for cellular structure and function. Autoantibodies are antibodies that recognize and bind to autoantigens. They can cause various autoimmune diseases that affect different organs and systems. Autoimmune diseases can be diagnosed by detecting autoantibodies in blood samples using various laboratory tests.
In this article, we have learned about the differences between antigens and antibodies, two important components of the immune system. Antigens are substances that stimulate an immune response, while antibodies are proteins that bind to and neutralize antigens. Antigens can be foreign or self-originated, and antibodies can be classified into five types based on their structure and function. We have also discussed some examples of endogenous antigens and autoantigens, which are involved in blood group compatibility and autoimmune disorders, respectively. By understanding how antigens and antibodies interact, we can appreciate the complexity and diversity of the immune system and its role in protecting us from various diseases.
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