Blood types form a topological space (and a complete distributive lattice). There are three generators: A, B, and Rh+.
Above the “zero element” is the universal donor O− and the “unit element” is the universal receiver AB+.
A topological space contains a zero object, maybe other objects, and all unions ∪ & intersections ∩ of anything in the space. So taking the power set ℘ of {A, B, +} yields the “power set topology” which I drew above. AB+ is the 1 object and “nullset" O− is the 0 object.
A lattice has joins ∨ & meets ∧ which function like ∪ and ∩ in a topological space. Like 1 or True in a Heyting algebra, blood type as a power-set topology has one "master” object AB+.
