It's important

Ken immediately listened carefully, there was a feeling he couldn't explain spread in his heart and it became a big question mark for him but instead of him worrying about his feelings now he prefers to focus on listening to all the stories the nurse told him.

 "So... your mother has a different blood type from you, it doesn't matter if it's just a different blood type, but... What's different from your blood group is also in her rhesus. Your mother has a rhesus negative blood group , whereas you have a blood type with a positive rhesus."

 Ken does not understand about the biological system in the human body, but he has never heard of problems or material about rhesus. That gives it a little idea that there are actually other blood groups besides the ABO blood group in the human blood group system.