Q.B cells are activated by :

B cells are activated when a specific antigen binds their surface B-cell receptor, triggering proliferation into plasma cells and memory cells.

Each B lymphocyte displays a unique antibody molecule on its surface that acts as its antigen receptor. When an antigen with a matching epitope binds this receptor -- typically together with signals delivered by helper T cells that recognise the same antigen presented on MHC-II -- the B cell is activated. It then undergoes clonal proliferation, differentiating into plasma cells that secrete large quantities of soluble antibody specific to that antigen, and into long-lived memory B cells. Interferons are antiviral signalling proteins, co …