Final answer:
In the context of MNS blood group system antigens, Anti-U is directed at a high-incidence antigen, Anti-M's clinical significance varies, and Anti-S is reactive with enzyme-treated cells. Anti-N is not commonly found.
Step-by-step explanation:
Regarding antibodies to MNS blood group system antigens, it is true that Anti-U is directed at a high-incidence antigen. This is because Anti-U targets an antigen that is present on the red blood cell surfaces of most individuals within the population. The antibodies are typically not naturally formed but can be clinically significant when they appear.
The option stating that 'Anti-N is commonly found' is not correct, as anti-N antibodies are not typically found in the general population. Also, the assertion that 'Anti-M is always clinically significant' is not accurate, as the clinical significance of anti-M antibodies varies with each case. Lastly, the statement 'Anti-S is reactive with enzyme-treated cells' is correct; treating red blood cells with enzymes enhances the reaction with Anti-S antibodies.