Which antigen is found in plasma and absorbed onto the red blood cells?

Which antigen is found in plasma and absorbed onto the red blood cells?

• A. Lea
• B. Lea^b
• C. M
• D. N

Answer: (A)

The key idea is that some antigens exist dissolved in plasma and can attach to red blood cell surfaces, rather than being built into the RBC membrane themselves. In the Lewis system, Lea and Leb are soluble antigens found in plasma and secretions. Because they’re not intrinsic to the RBC membrane, they can adsorb onto the RBC surface, making the cells carry that antigen even if the RBC itself doesn’t synthesize it. Among the options, Lea is the classic soluble Lewis antigen commonly found in plasma that can adsorb onto red blood cells, which is why it’s the best answer. The M and N antigens are inherent to the RBC membrane (on glycophorin A) and aren’t soluble plasma components that adsorb onto cells. Leb can also be present in plasma, but Lea is the typical example highlighted for adsorption onto RBCs.

The key idea is that some antigens exist dissolved in plasma and can attach to red blood cell surfaces, rather than being built into the RBC membrane themselves. In the Lewis system, Lea and Leb are soluble antigens found in plasma and secretions. Because they’re not intrinsic to the RBC membrane, they can adsorb onto the RBC surface, making the cells carry that antigen even if the RBC itself doesn’t synthesize it. Among the options, Lea is the classic soluble Lewis antigen commonly found in plasma that can adsorb onto red blood cells, which is why it’s the best answer. The M and N antigens are inherent to the RBC membrane (on glycophorin A) and aren’t soluble plasma components that adsorb onto cells. Leb can also be present in plasma, but Lea is the typical example highlighted for adsorption onto RBCs.