This mechanic is off by default. It can be toggled in the mod’s config, found in the ESC menu while the mod is enabled.
Organ rejection may occur when a patient receives an organ transplant while their Immunity is higher than 10%. If organ rejection occurs, the organ that was transplanted will immediately gain 100% damage, making it useless.
Reducing Immunity is primarily done using Azathioprine.
At 100% Immunity, the chance for organ rejection is 60%. The formula, where 1 is 100%, is as follows: