What is an antibody, and is it bad to have that in a uncommon blood type?

Biologically, it is a protein produced by the body's immune system when it detects harmful substances, called antigens. Examples of antigens include microorganisms (bacteria, fungi, parasites, and viruses) and chemicals. Antibodies may be produced when the immune system mistakenly considers healthy tissue a harmful substance. This is called an autoimmune disorder. Each type of antibody is unique and defends the body against one specific type of antigen. In short, antibodies are the fighters inside our body.

Antibodies are just "germ fighters" kind of. Think of them like your body's own internal police or military force. If something nasty gets inside of you like a germ or virus your body makes and sends out these fighters to kill off the thing trying to make you sick. Antibodies can go wrong in a variety of ways too though! For example, people with certain immune deficiencies, like HIV, aren't able to create the right kind or amount of these antibodies and so are far more likely to be ill or an illness to become much more dangerous to them than a person with a healthy immune system. Or the body can confuse a natural, needed organism in your body as an "attacker" and fight to get rid of it which causes it's own kind of illnesses and problems. That's the REALLY short and simplified version! I'd suggest asking a qualified doctor or someone and do some research for a better answer to your question. Hopefully this gives you a beginning understanding though!