Antigen

An 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 recognises as being "non-self" and potentially harmful. When the immune system encounters an antigen, it produces specific proteins called antibodies that can recognise and neutralise the antigen or stimulate other immune cells to attack and destroy it. 

Features of the structure of Antigens are given below:

• 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.
• Based on 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 like a lock and key.
• The number of antibodies a single antigen can bind to 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. 

Characteristics of Antigen

There are certain properties of an antigen that make it 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 a lower molecular weight, but they have to be coupled with large-sized carriers to show immunogenicity. 
5. They must possess regions called antigenic determinants that are involved in the antigen-antibody reaction.
6. Antigens must show species specificity.

Antigen Presenting Cells

When an antigen enters the 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.

Features of Antigen Presenting Cells (APC) are given below:

• When detecting the presence of an unknown substance comes in contact with the substance, engulfs it and then digests the substance into smaller fragments.
• 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, digest it into smaller fragments and with the help of MHC molecules, present it on their surface where the T cells inspect it and then alert the whole immune system.

• 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

There are six types of Antigens based on their Immunogenicity:

1. Immunogen: These are complete antigens that are capable of induction an 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 causing 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 the ABO and Rh antigens 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 pyogenes can even act against heart muscle proteins. 
6. Super antigen: The antigens that can stimulate the production of a large fraction of antibody cells, like shock toxins, pyrogenic exotoxins, etc.

Based on their Origin

There are four types of Antigens based on their Origin:

1. Exogenous antigens: The antigens that originate outside the host’s body and then enter the body, causing an immunogenic response, like pollen grains, bacteria, viruses, 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. Tumour Antigen or Neoantigens: The antigens present on the surface of the tumour 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. 

Antigen vs Antibody

Major differences between antigens and antibodies are given below:

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