Anti Poison

I based an ability off 'Slow Poison' and I was wondering how I could make another ability that could block that. For example, A unit without 'Anti Poison' gets attacked by a unit with 'Slow Poison'. The unit without antipoison takes damage from the poison as well as regular damage. A unit with a possible counter for 'Slow Poison' only takes damage from regular attack and poison does nothing. Would the best way to do it be: When the unit obtains the buff of the poison ability, it will remove it and set the health of unit to: current health of unit + damage of poison??

Help would be appreciated

You should have the poison be a buff, and then for anyone who has the buff remove 10 life every second or whatever. But if the unit has resistance, just remove the buff every now and then so they dont get posioned.