Antigenic drift is the minor change that occurs in the antigenic structure of a disease-causing agent that allows it to evade attack by the immune system of a host in the course of an infection. It is a mutational alteration in the surface proteins (which are target sites of antibodies) of pathogenic strains of microorganisms in such a way that antibodies that recognizes these antigenic sites do so less effectively or no longer recognizes them as it used to previously. Such drift or subtle changes in the antigenic surfaces of pathogens are a setback for most vaccination exercises because some vaccines have a short lifespan and, thus new forms of such vaccines must be produced and administered in order to confer a better protection against the antigen it is directed to. Both antigenic drift and antigenic shift are significant in epidemiological surveys because they help epidemiologists to recognize a pandemic or an epidemic in a population with regards to the number of susceptible individuals to those that are actually exposed to the disease being investigated. This is because whenever there is an antigenic drift or shift in a community, there is the likelihood of an increase in the number of susceptible individuals for the reason that the immune system of these persons has not yet been exposed to the mutant strains of the pathogens which resulted from a mutational change. In such populations, the morbidity rate will increase when the proportion of susceptible individuals is well above the allowed threshold density of susceptibles in the population.
Antigenic shift is a major change in the antigenic structure of a pathogen that gives it the exceptional ability to be unrecognized by the immune system of a host. It is a mutational change in the antigenic structure of a microbe that allows it to change from one antigenic type to another and thus continue to remain unnoticed by the immune system mechanisms of a host. In antigenic shift, there is a reassortment or rearrangement of the genetic makeup (otherwise known as gene reassortment) of pathogenic strains of microbes, and this allows them to produce a distinct type of surface proteins that are quite different from the original surface proteins of the organism. When antigenic drift or antigenic shift occurs in a population, it will be difficult or futile for a previous immunization exercise or immunity against a given infection to be beneficial and active against genetically new strains of the pathogen for which the immunity was initially developed. After an immunization in a given community, antibodies against an anticipated antigen will be developed and mobilized in the immune system of the immunized population. The mobilized antibodies will specifically bind to the surface proteins of the antigens (pathogens) whenever they appear in the host and deactivate their untoward effects in vivo. But due to antigenic shift, these antibodies will be ineffective as they will have no binding surfaces. Therefore, the target of most immunization programmes has been to increase the number of individuals immunized against a given disease in a particular population so that these vaccinated persons will help reduce the spread of the infection by protecting those who are not reached by the immunization exercise. This is called herd immunity.
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