Question

What is an antigen?

Answer 1

As we know, Antigen is a foreign substance or toxin which induces an immune response in the body especially the production of antibodies.
Antigens that are found in the body's own cells are known as self- antigens, and the immune system normally does not attack them.
The membrane of red blood cells contains millions of antigens.

Complete answer:
We discussed that, any substance that causes the body to elicit an immune response against that substance. It includes pathogens like- viruses, bacteria, chemicals, toxins that invade into our body.
Function- Antigen binds to the human body are used to neutralize any harmful foreign substance in the bloodstream.
There are various types of antigens like- Exogenous, endogenous, autoantigens.
Exogenous are external antigen that enter the body from outside. example- inhalation, injection.
Endogenous are cytosols of human cells such as viral proteins, protein from intracellular bacteria.
Autoantigens are produced by the host.
There are two types of antigens in the body – Self antigens and Non-self antigens.
The antigens on your own cells are known as self- antigens whereas those that do not originate in your body are called non-self antigens.
haptens, immunogens are also antigens present to detect and produce antibodies respectively.
Antigen receptor, a surface protein located on B cells and T cells, binds to antigens and initiates acquired immune responses. The antigen receptors on B cells are called B cell receptors or membrane immunoglobulins and the antigen receptors on T cells are called T cell receptors.
As we discussed in human blood the lettering of different blood types is designated by the specific antigen present in an individual's blood cells.
While all the types contain the oligosaccharide (O) antigen, the A and B blood types are defined by having N-acetyl galactose (A) or galactose (B) monosaccharide.
Likewise, the AB blood group has both A and B antigens.
The structures of the enzymes that bind to the antigen are similar and very slightly different, demonstrating antigen specificity.
Applications-
Besides being used in the indirect detection of presence of antibodies, antigens can also be used to prepare vaccines to establish or to improve immunity to a particular disease.
When pathogens get into the blood and lymph, antigens on the surface of the pathogens stimulate lymphocytes to produce specific antibodies which kill invaders by lysis, enhanced phagocytosis, clumping the pathogen together or neutralizing the toxins from pathogens.
There are three types of antigen based vaccines namely – purified, recombinant and synthetic.
Purified antigen vaccines called subunit vaccines composed of small fragments of molecules directly from pathogens that generate a protective immune response.
Recombinants are immunogenic proteins produced by genetic engineering.
Synthetic antigen vaccines are peptide antigens synthesized by automated machines.

Note:
Without antigens antibodies formation towards that specific foreign substance or invader is nearly impossible. So it's vitally essential for our immune system.
Antigens are small proteins which are embedded in membranes of all the cells in your body.
Antigens may either be proteins or polysaccharides.
Sometimes polysaccharide can generate a specific immune response. The specific binding between the antigen and antibody as we know is similar to that of the lock and key binding model.