Can you react with a reaction card to triggered abilities from permanents (like units) that start a chain, or does rule 538 prevent players from receiving priority before they resolve?

Ruling: You can react to triggered abilities from permanents. Rule 538 only applies when the actual card itself is played onto the chain, not when triggered abilities from permanents are added to the chain.

Sequence:

• When triggered abilities are added to the chain, players receive priority and can respond with reactions
• After each card/ability resolves in a chain, players get another opportunity to add reactions (starting with the owner of the next item on the chain)
• This continues until both players pass, then the next item in the chain resolves
• The process repeats until the entire chain is resolved

Nuances:

• Triggered abilities are treated differently from cards played directly onto the chain
• Players can let parts of the chain resolve and then react again while the chain still has unresolved items
• Once all players pass on an empty chain (after everything resolves), nothing more can be added to that chain