What is an Antigen?

Antigen is a substance that can trigger an immune response in the body. It is typically a foreign substance, such as a microorganism, a toxin, or a particle, that the immune system recognizes as being “non-self” and potentially harmful. When the immune system encounters an antigen, it produces specific proteins called antibodies that can recognize and neutralize the antigen, or stimulate other immune cells to attack and destroy it 

What is Antigen?

By immunological definition, we can say that “an antigen (Ag) is any chemical substance that can stimulate a body’s immune system to produce antibodies”. As they are able to stimulate the body’s immune response thus they are also called immunogens. Basically, they are large protein molecules that are normally found on tumor cells, pathogens like bacteria, viruses, fungi, and even in our normal body cells. Sometimes particulate matter like dust or pollen grains can act as antigens. Other than large molecules of proteins they can be just amino-acid chains or peptides, polysaccharides, nucleic acids, and lipids. They can be their own body-based origin or external environmental origin. In the case of own body origin, the antibodies identify them as “self” and refrain from attacking them whereas external antigens are identified as “non-self” and thus are attacked. The ability of the body to fight against these antigens is called immunity and this immunity can be either innate immunity (inborn i.e. present from birth) or acquired immunity (that we acquire in our lifetime).

Characteristics of Antigen

There are certain properties of an antigen that makes them a good immunogen. These are:

1. With exceptions to auto-antigens (self-antigens), all antigens should be foreign to the host’s body. 
2. Chemically they should be proteins, peptides, polysaccharides, lipids, or nucleic acids only.
3. Their chemical structure should be more complex in nature to be more immunogenic i.e. combination of different types of amino acids, monosaccharides, or even a combination of lipid and nucleic acid with proteins and carbohydrates.  
4. Their molecular size should be greater than 5000 Da to be considered immunogenic. However, partial antigens or haptens have lower molecular weight but they have to be coupled with large-sized carriers to show immunogenicity. 
5. They must possess regions called antigenic determinants which get involved in the antigen-antibody reaction.
6. Antigens must show species specificity.

Structure of Antigen

The antigenic structure differs based on the size, nature, and immunogenicity of the antigen. Its molecular structure is based on its ability to bind to the antigen-binding site of the antibody. On the basis of the molecular structure called antigenic determinants or the epitope, the antibodies differentiate between different antigens. The counterpart of the epitope in antigen is the paratope in antibody which is the exact region that interacts with the antigen. The epitope and paratope combine with each other like a lock and key. The number of antibodies a single antigen can bind with depends on the number of epitopes present in that antigen. Antibodies stimulated by one antigen can even interact with different antigens and this mechanism is called cross-reactivity. 

Antigen Presenting Cells

When an antigen enters a body, it needs to be identified first so that the immune system can be informed about the invasion. This identification of the antigen is done with the help of the Antigen Presenting Cells or APC. When detecting the presence of an unknown substance comes in contact with the substance engulfs it and then digests the substance in smaller fragments and then these fragments are transported to the cell’s surface with the help of MHC class I and MHC class II molecules, where these fragments will be used as an indicator for the other immune cells to inform them about the invasion. By this mechanism, these APCs inform the immune system about the antigen. Examples of APCs are B-cells before they activate and differentiate into antibodies, dendritic cells, and macrophages. 

After the entry of an Antigen, what happens?

When an antigen enters the body the APCs like B-cells, macrophages, or dendritic cells come to inspect it. They engulf the antigen digests it into smaller fragments and with the help of MHC molecules present it on their surface where the T cells inspect them and then alert the whole immune system. Then the B-cells as per the epitopes of the antigen design the exact paratope containing antibodies whose job is then to find other such antigens and destroy them. On the other hand, there is another group of B-cells which after the identification of the exact type of antigen will start developing memory cells for that antigen so that in the future if another encounter occurs with the same type of antigen then antibody formation and immune response can be faster. 

Types of Antigens

Antigens can be classified based on the following categories;

Based on their Immunogenicity

1. Immunogen: These are complete antigens that are capable of induction immunogenic response on their own like bacteria, viruses, etc.
2. Haptens: These are incomplete antigens that on their own are incapable of inducing an immune response but can bind with the antibody-like urushiol toxin of poison ivy, penicillin drug as an allergen, etc.
3. Autoantigens: Sometimes our body fails to determine certain self-proteins as our own and causes immunogenic responses against them like sperm protein, kidney protein, etc.
4. Allo-antigen and Iso-antigen: These are individual-specific antigens that are different from person to person like ABO and Rh antigen of blood cells.
5. Heterophilic antigens: When an antigen stimulates an antibody production but those antibodies can even bind with other antigens. These are also called cross-reacting antigens; antibodies produced for the bacteria Streptococcus pyogens can even act against heart muscle proteins. 
6. Super antigen: The antigens that have the capability to stimulate the production of a large fraction of antibody cells like shock toxins, pyrogenic exotoxins, etc.

Based on their Origin

1. Exogenous antigens: The antigens that originate outside the host’s body and then enters the body causing immunogenic response like pollen grains, bacteria, virus, etc.
2. Endogenous antigens: The antigens that are produced inside own body like own cells, fragments or compounds produced during cellular metabolism, components produced by the body during any infection, etc.
3. Tumor Antigen or Neoantigens: The antigens present on the surface of the tumor cells that stimulate the immune response in the body like MHC-I and MHC-II molecules.
4. Native Antigens: Those antigens that are not yet processed by antigen-identifying cells like APC cells due to which they cannot properly induce a full-blown immune response in the body. 

Difference Between Antigen and Antibody

Antigen	Antibody
Any foreign particle and in some cases own body particles that can induce an immune response in the body is called an antigen.	When the antigen induces an immune response in the body the immune system releases its “magic bullet” against the antigen which is called the antibody.
As they act against the immune system thus are also called immunogens.	They are globulin proteins of the immune system and thus are also called immunoglobulins.
They are mainly proteins and carbohydrates but lipids and nucleic acids can also act as antigens when coupled with proteins or carbohydrates.	They are proteins only.
They have specialized regions called epitopes which interact with the antibodies. interact	They have specialized regions called paratopes which interact with epitopes of antigens.
They are responsible for causing disease or allergic reactions in the body.	They are responsible for protecting the body by destroying the disease or allergy-causing antigen.
Antigens can be divided into multiple classes based on their immunogenicity and origin.	Antibodies are of only five basic classes. These are Immunoglobulin G, A, M, E, and D.

FAQs on Antigen

Q1: How can you define an antigen?

Answer:

An antigen is any foreign substance that comes inside our body or sometimes our own body’s substances that have the capability to induce an immune response in our body.

Q2: What do you call the substance that is produced by our immune system in response to the antigen?

Answer:

The substance that is produced by our body in response to antigenic stimulation is called the antibodies or the magic bullet of our immune system.

Q3: What do you call the study of antigens, antibodies, and the system associated with them?

Answer:

The branch of biology that deals with the study of antigens, antibodies, and the whole system of immune system is called immunology.

Q4: Give one example of acquired immunity that is used to fight antigens or pathogens.

Answer:

Acquired immunity is something that is acquired throughout our life like vaccines.

Q5: What do you understand by the term immunogenicity?

Answer:

Immunogenicity can be defined as the ability of an antigen to induce a specific immune response in the body of a vertebrate host.