How do you give first aid In left 4 dead 2?

In Left 4 Dead 2, to give first aid, you need to have a medkit in your inventory. Approach the injured player and press the use key (usually "E" on PC) to start healing them. The healing process takes a few seconds, during which the player cannot move or shoot, so ensure the area is safe. Once healed, the player will regain health, and you can continue fighting the infected together.