Antibodies have 2 identical binding sites for antigens, causing them to clump up and precipitate out of blood. Helper T cells activate the B cells. Each B or T cell has a single specific antibody it can make. B cells have IgM on surface. If an antigen binds to the IgM, it is taken into the cell by endocytosis, digested to peptides, then the peptides are displayed on the cell surface on an MHC class 2 molecule.
Macrophages eat everything non-specifically.
They also take potential antigens in by endocytosis, digest them to peptides, and display them on the cell surface on an MHC class 2 molecule. A helper T cell whose specific antibody binds to the peptides displayed on the macrophage becomes activated. The activated helper T cell then binds to the same antigen displayed on the surface of a B cell. The helper T then secretes interleukin, which actives the B cell. The B cell then divides and differentiates into plasma cells, which secrete antibodies, and memory cells, which are held in reserve until next appearance of antigen.