Which of the following is true about Lewis antigens?

Lewis antigens are unique carbohydrate structures that are found primarily in body fluids, including plasma. They are not produced by red blood cells themselves; instead, they are formed from precursor substances that are secreted into body fluids and can be absorbed by red blood cells.

The correct choice states that Lewis antigens are not expressed on cord blood. This is accurate because newborns typically do not express Lewis antigens at birth. Instead, Lewis antigen expression develops over time as the infant grows, often becoming detectable weeks after birth.

Understanding why this is correct helps emphasize the distinction between Lewis antigens and other blood group systems such as the ABO and Rh systems, which are fully expressed at birth. The other options illustrate characteristics surrounding Lewis antigens but do not accurately represent the nature of these antigens. For instance, they are not found on all red blood cells and specifically do not play a role in Rh blood typing, which pertains to a completely different blood group system. Additionally, while Lewis antigens can be found in plasma, they are variable in stability and can be influenced by various factors.