Antigen-presenting cells (APC) are developed by phagocytosis in the carrier's body (or animals? when antigen was injected for the first time. Fragments of antigen made by APCs will appear on the surface of cells along with MHC class II protein, and the helper T cells, with their characteristic T cell receptors on the surface, will bind itself to APCs to be activated.

B cells conjugated with activated helper T cells will be differentiated into plasma cells or memory cells. Plasma cells will secrete antibodies and memory cells which don't generate antibodies will act as specific antigen receptors to trigger stronger and faster immune reactions against the repeatedly encountered antigen.