Antigens in eukaryotic cell membranes are specific surface molecules

Steps:

• Recall that cell membrane antigens are typically glycoproteins and glycolipids that trigger immune responses.
• Identify components in the diagram: assume 1 is glycoprotein, 2 is glycolipid, 3 is phospholipid, 4 is cholesterol based on standard membrane models.
• Match antigens to surface-exposed parts that can be recognized by antibodies.
• Select pairs where both are antigenic: 1 and 2 fit as they are carbohydrate-bearing molecules.

Why A is correct:

• Antigens are defined as molecules like glycoproteins (1) and glycolipids (2) that protrude from the membrane and elicit immune recognition.

Why the others are wrong:

• B: 3 is phospholipid, which forms the membrane backbone but does not act as an antigen.
• C: 2 is antigenic, but 3 (phospholipid) is not.
• D: 4 is cholesterol, a structural lipid without antigenic properties.

Final answer: A