This article is from the Magic: The Gathering Rules FAQ, by Patrik Linell (pls@claymore.nu) with numerous contributions by others.
Exactly four things, namely the following:
Cannot be D amaged by Foo sources.
Cannot be E nchanted by Foo enchantments or E quipped by Foo Equipment.
Cannot be B locked by Foo creatures.
Cannot be T argeted by Foo spells and abilities.
Mnemonic DEBT.
 
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