Describe how phagocytosis of a virus leads to presentation of its antigens and how presentation of this antigen leads to the secretion of an antibody against this virus antigen.

The macrophage engulfs the virus and the phagosome fuses with a lysosome, the virus is then broken down by hydrolytic enzymes inside the lysosome, and virus' antigens are displayed on the cell surface membrane. A helper T cell then binds to the antigen on the antigen-presenting cell, which stimulates a highly specific B cell which is complementary to this antigen on the antigen-presenting cell and this B cell then clones, and these B cells secrete the antibody.