Antigen is a substance which is identified or recognised by the hosts body as foreign and as a result an immune response is stimulated towards it.
Antigens can be foreign or self.
Foreign antigens
Foreign antigens are also known as Heteroantigens.• As the name indicates these substances are not part of human body. They originate from outside the body.
• They can be microbial agents. It's called agents because, these include whole microbes or their parts. For example, capsules, cell walls, flagella, fimbriae, toxins of bacteria, the coats of viruses or any surface component of microbes.
• Foreign antigens can also be non microbial agents. Non microbial agents may arise from animals, plants, other humans. For example, pollen, egg white, serum proteins and blood cells from another individual, transplanted tissues and organs.
Self-antigens
Self-antigens are also known as autoantigens.• Our immune system is able to distinguish between self and non-self. But sometimes our body provokes an immune response against its own cells and molecules.
• Rheumatoid arthritis is an autoimmune disease in which immune system attacks and damages own tissues.
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| Antigen and it's Epitopes |
Epitope
The immune system does not recognise whole pathogen, but portions or parts of these agents. we termed these portions or parts as antigens."Epitopes can be defined as regions or sites of an antigen that are recognised by the immune system and as a result they bind to specific antibodies or T-cell receptors."
• Antigens are large protein molecules in chemical nature, some are polysaccharides in a few are glycoproteins or nucleoproteins.
• Our immune system recognise these antigens by recognising three dimensional (3D) shapes of regions called Epitopes.
• The epitopes are also known as antigenic determinants, because they are the actual part of an antigen that determines an immune response.
• Each antigen can have several epitopes. E.g.
Most antigens are proteins. So when antibodies bind to the antigens, which are protein in chemical composition antibodies do not bind to the whole protein, but they bind to a segment of that protein which is known as epitope.
Now this segment consists of few amino acids. These few amino acids forms the epitopes to which antibodies bind. It means a single protein antigen can actually contain many different epitopes to which antibodies can bind.
