What antibody class is primarily found on the surface of B cells and is crucial for their activation?

The antibody class that is primarily found on the surface of B cells and plays a crucial role in their activation is IgD. This is because IgD serves as a receptor for antigens on B cells, allowing them to recognize specific pathogens and initiate an immune response. When an antigen binds to the IgD receptor on a B cell, it triggers the processes that lead to B cell activation, proliferation, and differentiation into plasma cells that eventually secrete antibodies.

While IgM is also associated with B cells, especially during the early stages of immune response, it predominantly functions as the first antibody produced in a primary response and is secreted rather than being a surface receptor for activation. IgA is primarily found in mucosal areas and secretions, such as saliva and tears, while IgG is the most abundant antibody in serum and plays a significant role in the secondary immune response but is not found as a receptor on B cell surfaces.