How are B cells activated?

A phagocyte will engulf the pathogen by endocytosis. The vesicle containing the pathogen will fuse with the lysosome where it is degraded by hydrolytic enzymes. Short peptides of the pathogen will be displayed on the surface of the phagocyte. This is called an antigen presenting cell. A T helper cell will bind to the antigen expressed on the surface of the antigen presenting cell. This will activate the T helper cell. This T helper cell will then go on to bind a specific B cell.
The B cell expresses the same antigen on its surface. This occurs as the B cell binds to the antigen and engulfs it by phagocytosis, and the vesicle fuses with the lysosome. Peptides of the antigen will be expressed on the surface of the B cell. An activated T helper cell will bind to the antigen peptide expressed on the plasma membrane of the B cell, and activates it. The B cell replicates by clonal expansion and forms plasma cells which release antibodies for the specific antigen.