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

• red-green colour blindness
• red-green dichromacy
• red-grey
• red-handed
• red-hot
• red-hot poker
• red-ink
• red-lavender
• red-lead putty
• red-legged partridge
• red-letter
• red-letter day
• red-light district
• red-necked grebe
• red-orange
• red-pencil
• red-purple
• red-rimmed
• red-shafted flicker
• red-shouldered hawk
• red-skinned onion
• red-spotted purple
• red-streaked
• red-striped
• red-tailed hawk