Final answer:
Small changes in the viral coat over time are called antigenic drift, caused by point mutations affecting surface proteins. Major changes through gene reassortment are known as antigenic shift. These mechanisms are part of viruses’ adaptive immune evasion tactics.
Step-by-step explanation:
Small changes in the viral coat that happen over time are referred to as an antigenic drift. This type of antigenic variation is due to point mutations in the genes that code for surface proteins like hemagglutinin (H) and neuraminidase (N). In contrast, antigenic shift is a major change that occurs from gene reassortment, typically when two different influenza viruses infect the same host cell.
Adaptive immune evasion strategies, like antigenic drift and antigenic shift, allow viruses to survive against immune defenses by rapidly changing non-essential antigens. The rate of antigenic variation in viruses like influenza is very high, undermining the ability of the immune system to recognize different strains and necessitating new vaccine formulations each flu season.
Pathogens such as the human immunodeficiency virus (HIV) and bacteria like Borrelia burgdorferi and Neisseria gonorrhoeae also display antigenic variation, which contributes significantly to their ability to cause chronic diseases and evade the immune system.