Allergic Temmie has the ability to deal 1 damage to all enemy monsters whenever another kind of Temmie is summoned, making it act like Woshua.
This includes Temmie, Temmie 2, Trader Temmie and even other Allergic Temmies. Only Temmie Statue won't activate it. And unlike Temmie Statue, its ability will also activate whenever the enemy summons one.
This means that if you have an Allergic Temmie on the board, the enemy can't place a Temmie or Temmie 2, because they only have 1 HP, making them die instantly. (It can be canceled out with the help of Diamond Boy 1, though.)