What are antigens?

• A. Proteins found on the surface of red blood cells
• B. Antibodies found in plasma
• C. Lipids in the cell membrane
• D. Nucleic acids in the nucleus

Answer: (A)

Antigens are substances that trigger the immune system to respond by binding to specific antibodies. On red blood cells, markers on the cell surface act as these antigens and help the immune system distinguish self from non-self, which is why they’re central to blood typing and transfusion compatibility.

The answer states that these antigens are proteins on the surface of red blood cells, which aligns with the idea that surface molecules seen by antibodies mark the cells for recognition. When foreign antigens are present, antibodies can bind to them and may cause reactions like agglutination.

The other options describe components that are not what antigens are for this purpose: antibodies are the immune proteins that recognize antigens, not the antigens themselves; lipids in the membrane aren’t the typical target described for defining red blood cell antigens in this context; and nucleic acids in the nucleus aren’t surface molecules that antibodies normally recognize on cells.