But the concept of original sin says that everyone is tainted with the sin that got Adam and Eve evicted from the Garden of Eden, and is born with the burden of that inherited guilt. Not, of course, God's only-begotten son whose destiny it was to save mankind from their punishment for sin. Then, since he was born of a mortal woman and could be thought to have himself inherited the sin from her, his mother was deemed to be without original sin, too. I am not sure how her immunity came about.
It gets very complicated. It was all explained to me once, but the details have faded.