The effectiveness of antigens
We introduced the concept of antigens in lecture this past week, and defined them as aspects of a pathogen that induce an immune response. We further carried over the concept of “patterns” that we learned about from innate immunity, describing antigens as patterns with significantly more specificity than molecules like peptidoglycan. This raises the fascinating question of what makes an effective antigen, and why should we care?
We are already familiar with the most significant aspects of the adaptive immune system:
- it is acquired, and not present at birth
- it demonstrates increased specificity relative to innate pattern recognition
- it demonstrates memory, so that subsequent exposures are faster and more effective than initial exposures to a given antigen.
A “good” antigen therefore would be one that allows a “better” immune response; that it, the host is either able to fight off the infection more rapidly, or even better, not demonstrate any disease symptoms at all. This is the basic principle behind vaccination, where we purposefully expose an individual to an innocuous antigen, and an immune response is generated against that antigen so that when the individual comes into contact with the pathogen that bears that antigen, an adaptive immune response is all ready to protect the individual. A “poor” antigen in this case would be undesirable, as it would not induce an appropriate immune response, and the individual would have no significant protection against the pathogen.
So let’s think about what types of molecules are effective antigens, and how can we increase this from a therapeutic point of view. We stated in lecture that larger molecules are better antigens than small molecules, and that chemically complex molecules are also better antigens. We can then evaluate biological molecules for their antigenic effectiveness:
|Carbohydrate||Sugars||Moderate to poor|
|Lipid||Fatty acids||Very poor|
|Nucleic acid||Nucleotides||Very poor|
We also introduced the concept of exogenous, endogenous, and autoantigens in lecture. Autoantigens are antigens which we find on our own cells, and which our immune systems will delete from the repertoire of potential immune responses during the process of fetal and newborn development. Effectively this means that if a pathogen were to have an antigen that closely resembles one of our own autoantigens, it should be poorly effective at initiating an immune response.
Vaccine development takes all of these things into consideration when trying to decide what will make a “good” antigen, and therefore be effective at providing immunologic protection. The big drawback with most vaccines though is that a vaccine typically is challenging the individual with only a portion of a pathogen, and are less effective than exposure to the whole pathogen. Vaccines will circumvent this difficulty with the administration of adjuvants, compounds that are given with an antigen to increase the effectiveness of the immune response. It is actually the adjuvants that people generally have adverse reactions to with vaccinations, not to the antigens in the preparation themselves.
BONUS: Using your favorite search engine, identify a vaccine currently in use, and indicate what the antigen (or antigens) are in the vaccine. Because this is a bit more in depth than some others, I declare that this bonus is worth 2 points. Please include your source when you comment!