system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are the different bloodtypes: "A", "B", or "O". Why aren't they "A", "B", or "C"?
gpt
"A" and "B" are just arbitrary letters. "O" was chosen cause it looks like a zero and represents the absence of A or B antigens on the red blood cells. Members of the AB group, unsurprisingly, have both A and B antigens on their blood cells.