This lecture covers the activation of B cells in response to T-dependent antigens, including affinity maturation, class switching, and germinal center reactions. It explains the signal transduction pathways downstream of the B cell receptor and the role of co-receptors in informing B cells of the antigen environment. The process of antigen recognition, interaction between B and T cells, and antigen presentation to T cells are also discussed, along with the consequences of B cell activation by signal 1 and 2.