Final answer:
Antigenic drift is the process by which new influenza virus strains emerge due to point mutations that cause minor changes in HA and NA proteins, whereas antigenic shift is a major change from gene reassortment that can lead to pandemics.
Step-by-step explanation:
Point mutations in the hemagglutination (HA) or neuraminidase (NA) genes during viral ribonucleoprotein (RNP) replication can lead to the emergence of new influenza virus strains. This mechanism, known as antigenic drift, involves slight changes in the spike proteins hemaglutinin (H) and neuraminidase (N). In contrast, a major change called antigenic shift occurs when gene reassortment happens due to simultaneous infection of a cell with two different influenza viruses, producing a virus with a new combination of these proteins. Antigenic drift allows the influenza virus to evade a host's immune response by introducing new types that the immune system does not recognize, often leading to epidemics. While antigenic drift causes small antigenic changes over time, antigenic shift can result in new influenza strains that are significantly different from previous ones, potentially leading to pandemics.