Why there is no reaction against Rh factor on first transfusion?

There is no reaction against the Rh factor during the first blood transfusion because the recipient's immune system has not yet been sensitized to the Rh antigens. When Rh-positive blood is transfused into an Rh-negative individual for the first time, their body does not have pre-existing antibodies against the Rh factor, allowing the transfusion to occur without an immediate immune response. However, if an Rh-negative person is exposed to Rh-positive blood again, their immune system may recognize the Rh antigens and mount a reaction.