Final answer:
Lan is the high-incidence series antigen that is usually IgM and occasionally causes hemolysis.
Step-by-step explanation:
In the high-incidence blood group antigen series, the antigen that is usually IgM and occasionally causes hemolysis is Lan (Langereis blood group system). IgM is a type of antibody that is often the first to be made in response to an infection and has the ability to cause hemolysis, which is the destruction of red blood cells. While most of the high-incidence antigens are not typically associated with severe transfusion reactions, Lan is an exception because when someone has an IgM antibody to Lan, it can activate complement, which can lead to the hemolysis of red blood cells.