1. A classification system for the antigens of human blood; used in blood transfusion therapy; four groups are A and B and AB and O
   - ABO blood group system, ABO system

Type of: classification system

• open-air marketplace
• open-and-shut
• open-angle glaucoma
• open-class word
• open-collared
• open-door policy
• open-end credit
• open-end fund
• open-end investment company
• workflow
• workforce
• workhorse
• workhouse
• working
• working agreement
• working capital
• working day
• working dog
• working girl
• working group
• working memory
• working out
• working papers
• working party
• working person