ANTIGENICITY V/S IMMUNOGENICITY || REQUIREMENTS FOR IMMUNOGENICITY

Adaptive immune responses arises as a result of exposure to foreign compounds. The compound of foreign origin that evokes the response is referred to as antigen. Example: Virus. It is a term initially coined due to the ability of these compounds to cause antibody responses to be generated. An antigen is any agent capable of binding specifically to a TCR or an antigen molecule. The ability of a compound to bind with an antibody or a TCR is referred to as antigenicity.

There is a functional distinction between the term antigen and immunogen. An immunogen is any agent capable of  inducing humoral or cell mediated immune responses. This distinction between the terms is necessary because there are many compounds that are incapable of inducing an immune response, yet they are capable of binding specifically with antibody or TCRs. Thus, all immunogens are antigens, but not all antigens are immunogens.

Requirements for immunogenicity : 

A substance must be posses the following characteristics -

Foreignness 

The most important feature of an immunogen that is effective immunogen must be foreign with respect to the host. The adaptive immune system recognizes and eliminates only foreign antigens. Self antigens are not recognized and thus individuals are tolerant to their own self molecules, even through these same molecules have the capacity to an immunogens in other individuals of the same species. 

SIZE

The second requirement for being immunogenic is that the compound must have a certain minimal molecular mass. There is a relationship between the size of immunogen and its immunogenicity. 

In general, small compounds with a molecular mass<1000Da are not immunogenic; those of molecular mass between 1000 and 6000 Da are may or may not be immunogenic; and those of molecular mass >6000Da are generally immunogenic. The most active immunogens tend to have a molecular mass of 100,000 Da or more. Relatively small substance have decreased immunogenicity, whereas large substance have increased immunogenicity.

 Chemical Complexity

 The third characteristic necessary for a compound to be immunogenic is a certain degree of chemical complexity. For example, homopolymers of amino acids or sugar are seldom good immunogens regardless of their size.

Similarly, a homopolymer of poly-gamma-D glutamic acid with a molecular weight of 50,000 Da is not immunogenic. The absence of immunogenicity is because these compounds, although of high molecular weight, are not sufficiently chemically complex. 

Virtually all proteins are immunogenic. Further more, the greater the degree of complexity of the protein, the more vigorous will be immune response to that protein.

Carbohydrates are immunogenic only if they have a complex polysaccharide structure or part of complex molecules such as glycoprotein. Nucleic acid and lipids are poor immunogens by themselves, but they become immunogenic when they are conjugated to protein carriers.