Antigens and Antibodies: In the human immune system, antigens and antibodies both play essential but separate roles. One tries to wreak havoc on health, while the other struggles to protect it from invading pathogens. Antigens are mostly bacteria or viruses, but they can also be foreign substances that evoke an immune response when introduced into the body. Immunoglobulins, or \({\rm{lg}}\), are another name for antibodies. They’re \({\rm{Y}}\)-shaped proteins produced by \({\rm{B}}\) lymphocytes or \({\rm{B}}\) cells in our immune system. \({\rm{B}}\) cells attack and eliminate viruses and other toxins outside the cell. They do this by developing antibodies that are specific to a single antigen. Please continue reading to learn more about the structure, types, functions of antigens and antibodies.

Definition of Antigen

Antigens are substances that induce the formation of antibodies when they are introduced into the body. The conjugated proteins, such as lipoproteins, glycoproteins, and nucleoproteins, are the most common antigens.

Structure of Antigen

Antigenic determinants are antigen components, often known as epitopes (Gk. epi – upon, topos – place). Each antigen contains a large number of epitopes. At least two binding sites on each \({\rm{Y}}\)-shaped antibody molecule can attach to a specific epitope on an antigen. An antibody can also bind to the same epitopes on two separate cells simultaneously, causing neighbouring cells to clump together. Antigens interact with antibodies. The combination is similar to that of a lock and key.

Characteristics of Antigen

1. Immunogen: Immunogen is an antigen that, when introduced into a vertebrate host, causes a specific immunological response. They have the ability to activate the immune system and trigger an immunological response.
2. Incomplete antigen: The term “incomplete antigen” or “hapten” refers to a material that can bind to certain antibodies but cannot trigger an immune response on its own. These are small non-antigenic foreign molecules that must be combined with carrier molecules to be antigenic. Antibodies will recognise haptens once they have been produced.
3. Antigenicity: It is the capacity of a chemical or foreign molecule to interact precisely with antibodies.
4. Immunogenicity: It is the molecule’s capability to trigger an immunological response. Foreignness, molecular size, chemical composition, heterogenicity, and capacity to be processed and displayed on the surface of antigen presentation cells are the four qualities that characterise an antigen’s immunogenicity \(\left( {{\rm{APCs}}} \right)\).

Antigenic Determinant

1. An immunologically active region of an immunogen (or antigen) that binds to antigen-specific membrane receptors on lymphocytes or to the Fab region of released antibodies is known as a functional binding area. It is also termed antigenic determinants or Epitopes.
2. Paratopes are the antibody’s corresponding binding region to antigen epitopes (Fab region).
3. Antigens are usually multivalent. The number of epitopes present in an antigen molecule is referred to as the antigen’s valency.

Factors Determining Immunogenicity

1. Foreignness
a. A molecule must be foreign to the host in order to induce an immunological response. Antibodies are not generated in response to self-antigen.
b. The immunogenicity of an antigen increases as the degree of foreignness increases. In general, the larger the phylogenetic gap between two species, the stronger the immunological response.

2. Molecular size
a. There is a link between antigen molecular size and immunogenicity.
b. The majority of active immunogens have a molecular weight of \(100,000\,{\rm{ Da}}\) or above.
c. The immunogenicity of compounds having a molecular mass of less than \(5,000\,{\rm{ Da}}\) is generally low.

3. Chemical composition and heterogenicity
Proteins are the most effective immunogens, followed by carbohydrates. In contrast, unless they are complexed with protein or carbohydrates, lipids and nucleic acids do not behave as antigens (immunogens).

4. Susceptibility to antigen processing and presentation
\({\rm{Y}}\)-cell interaction with the antigen that has been processed and displayed on the surface of Antigen presentation cells \(\left( {{\rm{APCs}}} \right)\) coupled with MHC molecules is required for the development of both antibody mediated immunity \(\left( {{\rm{AMI}}} \right)\), and cell mediated immunity \(\left( {{\rm{CMIs}}} \right)\).

Types of Antigen

I. On the Basis of Immunogenicity

1. Complete Antigen or Immunogen: Complete antigens or immunogens are molecules that induce a specific immune response by themselves. E.g., Bacteria, fungi, viruses, etc.
2. Incomplete Antigen or Haptens: Incomplete antigens, also known as haptens, are antigens that cannot induce an immune response on their own but can attach to a specific antibody.
3. Autoantigens: Lens proteins, sperm proteins, myolin basic protein, thyroglobulin, kidney protein, and certain heart muscle protein are all sequestered from blood circulation throughout life. As a result, \({\rm{T}}\) and \({\rm{B}}\) cells detect these proteins as foreign, triggering an immunological response.
4. Allo-antigen or Iso-antigen: These antigens are individual antigens that are present in one person but not in another. E.g., Blood grouping antigen.
5. Heterophilic or Cross Reacting Antigen: When an antigen produced against one antigen binds to another antigen, the antigen is referred to as heterophilic or cross-reacting. Antibodies against Rickettsia, for example, bind to several Proteus species. Antibodies raised against the \({\rm{M}}\) protein of Streptococcus pyogenes also react with human cardiac muscle protein.
6. Superantigen: Antigens that activate a substantial percentage of \({\rm{T}}\) cells (up to \(25\%\)). For example, enterotoxins from Staphylococcus, shock toxins, exfoliation toxin, and Pyrogenic exotoxins.
7. Tolerogen: An antigen that causes immunogenic tolerance, or the inability to respond to an antigen as a result of prior exposure to that antigen.
8. Allergen: Antigen that causes anaphylaxis (severe acute hypersensitivity reaction) due to fast activation of generalised mast cells is called Allergen.

II. On the Basis of Needing the Help of T cells

1. Thymus Dependent Antigen: These antigens trigger both \({\rm{AMI}}\) and \({\rm{CMI}}\). Proteins, for example.
2. Thymus Independent Antigen: These antigens trigger only \({\rm{AMI}}\). E.g., Polysaccharides

III. On the Basis of Origin

1. Exogenous antigens are antigens that come from outside the body and are not natural to it. These antigens enter the body via inhalation, ingestion, or injection and circulate in body fluids, where they are captured by \({\rm{APCs}}\) (Antigen processing cells such as macrophages, dendritic cells etc.) Phagocytosis is the primary mechanism by which \({\rm{APCs}}\) absorb these foreign antigens. Bacteria, fungi, viruses, and other microorganisms are examples.
2. Endogenous antigens are those that originate from within the body. Antigens are the body’s own cells, subfragments, molecules, or antigenic products formed as a result of normal cell metabolism or viral or intracellular bacterial infection. Macrophages digest endogenous antigens and present them to \({\rm{CD8 T}}\) cells (cytotoxic \({\rm{T}}\) cells). E.g., Antigens of blood groups, \({\rm{HLA}}\) (Histocompatibility Leukocyte Antigens), and so on.

Antigen Presenting Cells (APCs)

Antigen-presenting cells are cells that can engulf antigen and display extracellular antigen to \({\rm{T}}\) cells (APCs). Antigen-presenting cells in the body are divided into three categories: macrophages, dendritic cells, and \({\rm{B}}\) cells.

1. Macrophages: The majority of macrophages are found in a dormant state. When they are stimulated to become activated macrophages, their phagocytic activities are considerably enhanced. In most lymphoid tissues, macrophages coexist with lymphocytes, such as monocytes as blood macrophages and histiocytes as tissue macrophages.

2. Dendritic Cells: Long cytoplasmic processes characterise these cells. Their major function is to act as very efficient antigen-trapping and antigen-presenting cells. These cells are not phagocytic in nature. Lymph nodes, spleen, thymus, and skin, are sites where they can be found.

The different types of dendritic cells are:

(i) In the epidermis of the skin, dendritic cells in Langerhan trap organisms that come into touch with the body surface.
(ii) Dendritic cells in the spleen, which trap antigen in the bloodstream.
(iii) In lymph nodes, follicular dendritic cells capture antigen in the lymph. As a result, macrophages and dendritic cells play a crucial role in capturing and presenting antigens to \({\rm{T}}\) and \({\rm{B}}\) cells, which kicks off the immunological response.

3. B-cells

\({\rm{B}}\)-cells have intra-membrane immunoglobulin \(\left( {{\rm{Ig}}} \right)\) molecules on their surfaces that act as cell antigen receptors. Because all of the receptors on a single cell are the same, each cell can only bind one antigen. This distinguishes them from macrophages, which must absorb any foreign substance that comes their way.

Antibodies

Antibodies, the immune system’s magic bullets, are glycoproteins that develop in response to antigenic stimulation and neutralise antigens with a high degree of specificity. Antibodies, also known as immunoglobulins, are found in the serum part of the blood (Ig). G.M. Edelman and R.M. Porter, who received the Nobel Prize in Physiology and Medicine in \(1972\) for this discovery, discovered the chemical composition and structure of antibodies.

Structure of Antibodies

The molecular weights of antibodies (immunoglobulins) range from \(150,000\) to \(900,000\) daltons. According to electron microscopy, antibody molecules resemble the letter \({\rm{“T”}}\) before they mix with antigens, but they resemble the letter \({\rm{“Y”}}\) after they combine with antigens. The antigen-antibody association is thought to promote a rearrangement in the \({\rm{T}}\)-shape structure of the antibody molecule, resulting in a \({\rm{Y}}\)-shape, allowing additional exposure to the heavy-chain complement binding site for further reactions. Four polypeptide chains make up an immunoglobulin (antibody) molecule. Because of the increased number of amino acids (about \(440\) amino acids in each chain) and consequently high molecular weight, two of the four chains are identical to each other. They are referred to as heavy \(\left( {\rm{H}} \right)\) chains (approximately 50,000 daltons).

Types of Antibodies

There are five types of antibodies viz:

1. IgG: These are the antibodies that are found in the highest concentration in your plasma. They cleanse the body of undesirable toxins while also providing long-term protection. \({\rm{lpG}}\) makes up \(80\%\) of all antibodies in the body.
2. IgM: The earliest antibodies produced by \({\rm{B}}\) cells in response to antigens are these.
3. IgA: Antibodies bind to antigens and transport them out of your body through mucus or other bodily fluids.
4. IgE: Allergies are triggered by these antibodies, which also guard against parasites. Skin, lungs, and mucosal membranes contain trace quantities.
5. IgD: These antibodies attach to \({\rm{B}}\) cells and trigger the production of \({\rm{lpM}}\) antibodies.

Antibody Structure

\({\rm{lpG}}\) has been widely investigated and is used as a model for a fundamental structural unit of all Igs. The parts of an antibody molecule are as follows.

(i) Heavy and Light Chains
The four peptide chains that make up an antibody molecule are two short light chains and two longer heavy chains. As a result, an antibody is denoted as \({\rm{H}}2{\rm{L}}2\). The heavy chain has more amino acids than the light chain, whereas the light chain has fewer amino acids. Lambda or Kappa chains can be found in both heavy and light chains.

(ii) Constant and Variable Regions
Each antibody chain has two separate regions: a constant region and a variable region.

(iii) Disulfide Bonds and Hinge Region
A disulfide bond connects two chains that are light and heavy. The two heavy chains are additionally linked by two disulfide bonds. The hinge region of the antibody is characterised by a high degree of flexibility. Because the antibody’s “arms” can shift slightly as the hinge region bends, the molecule can take on a \({\rm{Y}}\) shape.

(iv) Fragment Antigen Binding (Fab) and Fragment Crystallisable (Fc)
Fragment-antigen binding refers to two identical pieces of a \({\rm{Y}}\)-shaped molecule that have antigen-binding sites (Fab). In a lock-and-key configuration, the antigen-binding sites bind to specific antigens, generating an antigen-antibody combination. Fragment crystallisable refers to the third fragment that cannot bind to the antigen but can be crystallised \(\left( {{\rm{Fc}}} \right)\).

The Fc region of the Y-shaped antibody monomer is so termed because it was a fragment \(\left( {{\rm{F}}} \right)\) that crystallised \(\left( {{\rm{c}}} \right)\) in cold storage when antibody structure was first discovered.

Characteristics and Functions of Immunoglobulins (Igs) or Antibodies

Antibodies have the characteristics listed below and perform a variety of functions.

(i) IgG

1. This is the most common type of \({\rm{lg}}\) in the body, accounting for approximately \(80\%\) of all Igs. It can be detected in the bloodstream, lymph nodes, and intestine.
2. It enhances phagocytosis, neutralises toxins, and complements activation to guard against bacteria and viruses.
3. It is the only antibody type that can traverse the placenta from mother to foetus, providing significant immunological protection to infants.
4. Responsible for Rh-factor in the blood.

(ii)  IgA

1. It is the second most common class, accounting for around \(10\%\) to \(15\%\) of serum antibodies. Sweat, tears, saliva, mucus, colostrum (a mother’s first milk) and gastrointestinal secretions are the most major sources.
2. Blood and lymph contain smaller amounts of the substance. A \({\rm{J}}\)-(joining) chain and secretory components are additional polypeptides and proteins found in \({\rm{lgA}}\).
3. During times of stress, levels drop, reducing infection resistance. It provides localised protection against germs and viruses in external secretions (tears, digestive secretions, etc.).
4. Also found in colostrum, i.e. breast milk for newborns immune protection.

(iii) IgM

1. Third most common antibody in our body.
2. IgM accounts for roughly \(5\%\) to \(10\%\) of all antibodies in the blood.
3. It’s also found in the lymphatic system. It is the biggest \({\rm{Ylg}}\) that the plasma cells secrete first.
4. Its name comes from the fact that it is a macroglobulin that is at least five times larger than \({\rm{lpG}}\). The immunoglobulin class \({\rm{lgM}}\) is the oldest. It causes the \({\rm{B}}\) cells to become active.
5. \({\rm{lgM}}\) has a J chain, and each monomer contains a polypeptide called a secretory component, making it the earliest immunoglobulin synthesised by the foetus.
6. It is unable to pass through the placental barrier. \({\rm{lgM}}\) is \(500 – 1000\) times more efficient than \({\rm{lgG}}\) in opsonisation, bacterial action, and bacterial agglutination. However, it is less effective than \({\rm{lgY}}\) at neutralising toxins and viruses. It assists in the activation of the complement system.

(iv) IgE

1. It makes up to \(0.002\%\) of all antibodies in the blood and is found on mast cells and basophils, which release histamine.
2. It has unusual features, including heat lability (inactivated in one hour at \({56^{\rm{o}}}{\rm{C}}\)). Type \({\rm{F}}\) hypersensitivity is mediated by \({\rm{lgE}}\). (anaphylaxis).

(v) IgD

It is mostly located as antigen receptors on the surfaces of \({\rm{B}}\) cells, where it stimulates cells for antigen detection.

Summary

Antigens cause your immune system to launch an antibody response. Antibodies are proteins that recognise specific antigens. This indicates that each antibody is unique to a certain antigen. Antibodies are proteins that recognise and bind to antigens, neutralising them. This information is stored in our immune system’s long-term memory and also remembers what antibodies were released in response to that pathogen so that, the next time it enters, a similar procedure is followed by the body to eliminate it.

Antigens and antibodies serve a variety of purposes in developing tests and vaccines that aid in the diagnosis and treatment of disease and illness. Antigen-presenting cells are cells that can absorb antigen and deliver fragments to \({\rm{T}}\) cells \(\left( {{\rm{APCs}}} \right)\). Antigen-presenting cells in the body are divided into three categories: macrophages, dendritic cells, and B cells. Human antibodies are divided into five isotypes (\({{\rm{lgM,}}\,{\rm{lgD,}}\,\,{\rm{lgG,}}\,{\rm{lgA}}}\), and \(\left. {{\rm{lgE}}} \right)\)) based on their H chains, which provide each isotype-specific properties and function. In the blood (plasma), \({\rm{lpG}}\) is the most common antibody isotype, accounting for \(70 – 75\%\) of all human immunoglobulins (antibodies).

FAQs

Q.1: What is an antigen and antibody in blood?
Ans: White blood cells produce antibodies, which the immune system uses to recognise and kill foreign substances in the body. The immune system ignores blood type antigens, which are present on the surface of red blood cells. On the other hand, antigens from a different blood type will be recognised as foreign substances and attacked by antibodies.

Q.2: What is the function of antigen?
Ans. The antigen is a substance that stimulates the immune system by activating lymphocytes, which are the body’s infection-fighting white blood cells.

Q.3: What happens after antibodies attach to antigens?
Ans: Antibodies bind to a specific antigen, making it easier for immune cells to eliminate it. T lymphocytes are immune cells that assault antigens directly and assist in the management of the immune response. They also release molecules called cytokines, which regulate the immune response overall.

Q.4: What is antigen and examples of antigens?
Ans: Antigen is a term used to describe a variety of compounds that, when recognised by the immune system as non-self, cause an immunological response. Allergens, blood group antigens, substances on the surface of foreign cells, and toxins are some examples.

Q.5: What are antibodies and their types?
Ans: Human antibodies are divided into five isotypes (\({{\rm{lgM,}}\,{\rm{lgD,}}\,\,{\rm{lgG,}}\,{\rm{lgA}}}\)) based on their H chains, which provide each isotype-specific properties and function. In the blood (plasma), \({{\rm{lgG}}}\) is the most common antibody isotype, accounting for \(70 – 75\%\) of all human immunoglobulins (antibodies).

Study About Blood Groups and Its Types Here

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