I've been told I carry the Anti-M (MNS1) antibody. I'm B+ blood type. What exactly is Anti-M (MNS1)?

I donate blood on a regular basis and was sent a letter which did not explain what exactly it is but they gave me a card to carry in my wallet that says I carry this. I've checked the Internet and am finding it difficult to find anything out about it. Help! Please! Thanks!!