system
Given the following post, respond as if you're commenting on Reddit:
human
Are there: Evolutionary benefits to blood types?

Hello,

I understand blood types are pretty incredible and varied, but are there any evolutionary benefit for these blood types?  

Furthermore, as a follow-up question, are there animals who only have a single blood type (excluding Rh +/- factors?)
gpt
Pathogens like bacteria or viruses can carry antigens (chemical signatures, like blood type) from other hosts. Current idea is that, if those pathogens are equipped with their former host's type B, say, then since you're type A, your immune system has a much easier time recognizing the pathogen as an invader--they're very different, after all--and can respond to it more quickly and effectively. 

It's just a means by which different immune systems can respond better to invaders.