Why should all types avoid pork?

Hog is very "A-like" immunologically, which makes it an avoid if you happen to have antibodies to the blood type A antigen, like type B's and O's do.

Paradoxically enough, hog also has an antibody (iso-hemmaglutinin) in its tissues which reacts to the A antigen, so it should be avoided for this reason by A's and AB's as well.