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This passes coordination and modification syntactic tests that a truly idiomatic noun phrase should fail. "The ABO and Rh blood-antibody systems remain a mainstay." DCDuring TALK 20:07, 8 January 2010 (UTC)
- It is not a set phrase (per above).
- The meaning is instantly deducible from the components: most important blood type classification system for human blood transfusions + collection of organized things, whole composed of relationships among the members
- AFAICT any mystery is due to the use of an abbreviation out of its normal usage context.
- -- DCDuring TALK 22:04, 9 January 2010 (UTC)