The term ‘antigen’ refers to any molecule that is capable of being recognised by the immune system. Such molecules range from simple chemical compounds to complex macromolecules and are specifically recognised by one or more constituents of the innate and adaptive immune systems. In the innate immune system, the pattern recognition receptors recognise the pathogen-associated specific molecular patterns. In the adaptive immune system, immunoglobulin and T cell-receptor recognise either specific conformation on the antigen or the amino acid sequence in the peptide, respectively. Although antigens can be recognised by these receptors, all antigens do not necessarily elicit antigen-specific immune responses; antigens that elicit an immune response are termed immunogens. The capacity to be recognised by these receptors expressed by the cells of the immune system is called antigenicity, whereas the ability to induce an immune response is immunogenicity. Thus all immunogens are antigens but all antigens are not immunogens.

Key Concepts:

• Antigens are those molecules that are specifically recognised by the receptors of the innate and adaptive immune systems.
  
• Immunogens are those molecules that are able to elicit immune responses with negative or positive effects such that immune responses are suppressed or activated, respectively.
  
• Haptens are small molecules that are able to bind antibodies but are unable to evoke an immune response.
  
• All immunogens are antigens but all antigens are not immunogens.
  
• Toll-like receptors in the innate immune system recognise many patterns on antigens whereas recognition with finer specificity is executed by immunoglobulin and T-cell receptors, the constituents of the adaptive immune system.
  
• The ability to rearrange the genes of the receptors (B- and T-cell receptor) in the adaptive immune system creates a huge repertoire of antigen-specificity and memory whereas such a feature is not available with the receptors of the innate immune system.
  
• B-cell receptors recognise virtually any antigens in a conformation-dependent manner and do not need any processing of the antigen for its recognition whereas the T-cell receptor recognition of an antigen depends primarily on the sequence of amino acids in the peptide.
  
• T-cell receptors recognise primarily peptide antigens in association with major histocompatibility complex molecules that necessitates processing of the antigens before the recognition.