“Does open heart surgery affect your immune system?”

The procedure is done using cardiopulmonary bypass or a beating heart.
To my understanding the immune system is not affected except in cases in which the patient has a already present a non-diagnosed pre-existing immune condition.

No. It doesn’t.

The simple answer is NO. Good luck.

Open heart surgery is a major stress for the body. So, like any other major surgical operation, definitely impacts on the immune system. Furthermore, the use of the heart lung machine (pump) caused a systemic inflammatory response that may cause further changes to the immune system response, so immunocompromised patients can be negatively affected by the consequences of a cardiac surgery operation having a potential higher risk of infection and delayed healing. On the other hand, cardiac surgeons routinely safely perform cardiac surgery operations in immunocompromised patients with astonishing outcomes.