Where are blood group antigens widely distributed?

Blood group antigens are primarily found on the surface of red blood cells, but their distribution extends beyond just blood cells to various tissues and body fluids. These antigens are also present in bodily secretions such as saliva, tears, and other fluids, which can contain glycoproteins and glycolipids that have similar structures to those found on red blood cells. Additionally, different tissues can express these antigens, making them vital in understanding compatibility for blood transfusions and organ transplants. This extensive distribution underscores the importance of blood group antigens in immunology and serology.

The other options may include various body systems or components where certain proteins or markers might be found, but they do not encompass the comprehensive distribution of blood group antigens as thoroughly as the correct answer does.