Answer :
The adaptive immune system responds to foreign molecules, typically protein components of bacteria, viruses, molds, or parasites, which are collectively known as antigens.
An antigen in immunology is a molecule, molecular structure, foreign particle, pollen grain, or other foreign substance that can attach to a particular antibody or T-cell receptor.
Initially, an agent that produces antibodies was referred to as an antigen. Proteins, peptides (chains of amino acids), polysaccharides (chains of monosaccharides/simple sugars), lipids, and nucleic acids can all serve as antigens.
Antigen receptors, such as antibodies and T-cell receptors, are responsible for recognizing antigens. The majority of the time, an antibody can only react with and bind one particular antigen; occasionally, though, antibodies can cross-react with and bind many antigens.
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