Kidd antigens: A red cell has a ton of molecules on the cell surface that helps it do what a cell gotta do. One such molecule is an ion transporter (helps urea travel from one side of the cell membrane to the other. Almost all people have this protein, but at some point there was a mutation, so about 50% of people have “version a” while about 50% of people have “version b” and another 25% have both “Jka” and “Jkb”. These are the main Kidd antigens (Jka and Jkb are abbreviations). So, really there’s nothing special about having a Kidd antigen on your red cell. The problem happens if you’re missing one (ie type Jkb only) and get exposed to some blood from a Jka person. There’s a chance your immune system notices the difference and forms anti-Jka antibodies. Once the hospital knows you have formed this antibody, they’ll do their best to give you Jka-negative blood to prevent the transfusion from being destroyed by your immune system.

Just as an aside, there are tons of different blood group system (types of molecules on the red blood cell surface that can vary between one person and another). It’s impractical to “match” a blood unit up front to each persons molecules except for the ABO and Rh+/- everyone’s talked about. So we only special-select blood if there’s a known antibody, with the exception of sickle cell patients.