Can opponents respond to summoning units, and does it matter if the unit has an on-summon ability?

Ruling: Summoning a unit by tapping runes does not start a trigger that opponents can respond to. However, if a unit has an ability that triggers when it enters the game (like Riptide Rex), both players can respond to that ability, though the unit still lands on the board before responses occur.

Sequence:

• Unit is summoned and lands on the board
• If the unit has an on-summon ability, that ability triggers
• Both players can then respond to the triggered ability

Nuances:

• Units without on-summon abilities create no trigger and cannot be responded to
• Gears resolve directly with no opportunity to respond (same as units without abilities)