A macrophage engulfs a virus particle by phagocytosis and partially digests it inside a phagolysosome. Which statement correctly describes what the macrophage does next to help trigger the specific immune response?

AS & A Level Biology 9700: Macrophages as Antigen-Presenting Cells

Step 1: Recall what happens after phagocytosis

After a macrophage engulfs and partially digests a pathogen inside a phagolysosome, it does not simply discard the fragments. Instead, it moves fragments of the pathogen’s antigens to its own cell-surface membrane and displays them there. A macrophage doing this is called an antigen-presenting cell.

Step 2: Recall why this matters for the specific immune response

Displaying the antigen on its surface allows the macrophage to present it to T-helper cells. A T-helper cell with a receptor complementary in shape to this presented antigen can bind to it, and this binding is an important step in activating the T-helper cell, which then goes on to help activate B-lymphocytes and other phagocytes. This links the non-specific phagocytic response to the specific response involving lymphocytes.

Step 3: Evaluate each option

A: correct. This matches antigen presentation exactly: antigen fragments are displayed on the macrophage’s surface, and T-helper cells with a complementary receptor recognise them.
B: incorrect. Antigen fragments are displayed on the macrophage’s own surface membrane, not released loosely into the plasma for a B-lymphocyte to find by chance.
C: incorrect. Macrophages are not lymphocytes and do not divide to form clones of memory cells; only B- and T-lymphocytes form memory cells during the specific immune response.
D: incorrect. Secreting antibodies is the role of plasma cells (derived from B-lymphocytes), not macrophages.

Final answer

The macrophage presents antigen fragments on its own surface membrane, becoming an antigen-presenting cell recognised by T-helper cells, option A.