Antigenic Shift:
Antigenic shift is the process by which two different strains of influenza combine to form a new subtype having a mixture of the surface antigens of the two original strains. The term antigenic shift is specific to the influenza literature; in other viral systems, the same process is called reassortment or viral shift.
Antigenic shift occurs only in influenza A because it infects more than just humans.
Affected species include other mammals and birds, giving influenza A the opportunity for a major reorganization of surface antigens.
Antigenic Drift:
Antigenic drift is the natural mutation over time of known strains of influenza to evade the immune system.
Antigenic drift occurs in all types of influenza including influenza A, B and C.