The difference in blood types means basically one man's food is another man's poison.
According to Dr. D'Adamo's web site, AVOID: Flocculates serum or precipitates serum proteins. It is listed as neutral for Type B's and Type O secretors. Avoid for all others.
My doctor simply said I could have regular salad dressing if I wanted, but if I experienced join pain I should avoid it because of the vinegar. I have been eating regular salad dressing now for about a week and haven't noticed any ill-effects.
I would not use apple cider for any medicinal effects it may have on Type B's or Type O Secretors. It seems to be high in magnesium and carbohydrates. I think there are better Type A friendly sources for this.