Red blood cell
Red blood cells contain antigens in their plasma membranes that distinguish them as part of a specific category of blood cell. These antigens can be polysaccharides, glycoproteins, or GPI (a glycolipid) -linked proteins. Antigens range in complexity, from small molecules bound to the extracellular side of the phospholipid bilayer, to large membrane proteins that loop many times between both sides of the membrane. The smaller polysaccharide antigens classify blood cells into types A, B, AB, and O, while the larger protein antigens classify blood cells into types Rh D-positive and Rh D-negative. While the biological role of the correct blood type is unclear and may be vestigial, the consequences of incorrect blood types are known to be severe. The same cells that recognize PAMPs on microbial pathogens may bind to the antigen of a foreign blood cell and recognize it as a pathogen because the antigen is unfamiliar. It is not easy to classify red blood cell recognition as intrinsic or extrinsic, as a foreign cell may be recognized as part of the organism if it has the right antigens.