An antigen is also called an immunogen in some references or Textbook. It is a substance that binds to a specific antibody or is any molecule or molecular fragment that can be bound by a major histocompatibility complex (MHC) and presented to a TCR or T-cell receptor.
Two features; antigenicity and immunogenicity, are generally used to describe each antigen. Immunogenicity is the ability to induce a humoral or cell-mediated immune response.
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Antigenicity is the ability to specifically combine with the final products of the immune response (i.e., secreted antibodies & / or surface receptors on T-cells). Although all molecules that are immunogenic are also antigenic, the reverse is not true.
If we carefully contemplate these two characteristics that are used to define an antigen, we discover that immunogenicity and antigenicity are tightly related and are always duplicated. Antibodies are produced as the result of immune induction, not from thin air. And antigens cannot trigger an immune response unless they bind with their corresponding antibodies or receptors. The above two concepts simply and repeatedly describe a single generality of all antigens, yet this alone does not allow us to completely characterize various antigens and cannot help us to understand antigens in various guises.
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However, different antigens produce different immune responses as they encounter their antibodies or receptors. Using this feature, antigens can be more accurately defined by the difference in the type of immune responses they induce.
According to the classical definition of immunology, the major function of the immune system, as in the integrated anatomic system and other systems, is to avoid disease in the human body. The immune system has its own mechanisms for maintaining a general physiological balance in life by co-operating with other systems of the body. Here, we attempt to redefine and differentiate antigenicity into immunogenicity and allergenicity. We refer to antigenicity as the ability of an antigen to induce an immunological response when it is encountered by the human body. Antigenicity involves two types of immune characteristics, immunogenicity, and allergenicity.
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Immunogenicity refers to the ability of an antigen to trigger normal and protective immune responses after being encountered by the human body. We describe the immunogenicity of an antigen using the following three aspects:
(1) The ability to defend the immune system (immunological defense), which is the ability to repel an exogenous antigen and to fight against infection.
(2) The ability to keep the immune system stable (immunological homeostasis), which is the ability of the body to recognize and eliminate damaged tissue, inflammation and/or senescent cells
(3) The ability to kill and to remove abnormally mutated cells so as to monitor and inhibit the growth of malignancies in the body (immunological surveillance). Thus, immunogenicity reflects the strength of these three functions.
Read: Antigenecity and Immunogenicity
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Allergenicity refers to the ability of an antigen to induce an abnormal immune response, which is an overreaction and different from a normal immune response in that it does not result in a protective/prophylaxis effect but instead causes physiological function disorder or tissue damage.
To further simplify, each antigen carries immunogenic and allergenic properties: