Red blood cells come in many "blood types" including type A, type B, type AB, type O [lacking proteins A and B], Rh positive, and Rh negative [lacking Rh+] and many others. If blood is transfused, the recipient detects any new or "foreign" proteins. These blood type proteins are
in the plasma where they have been secreted by the red blood cells.
inside the red blood cell cytoplasm.
on the outer surface of the red blood cell membrane.
evenly distributed throughout the cell contents and plasma.
in the red blood cell nucleus.