Which class of immunoglobulin is thought to function as an antigenic receptor site on the surface of B lymphocytes?

The class of immunoglobulin that functions as an antigenic receptor site on the surface of B lymphocytes is IgD. While IgM is found on the surface of B cells and plays a role in the initial stages of activation, IgD is specifically recognized as a key component of the B cell receptor complex. This means that IgD binds antigens directly, triggering the B cell's activation and subsequent immune response.

The presence of IgD on B cells allows them to recognize specific antigens. Once a B cell encounters its specific antigen, it can initiate a cascade of activation that leads to cell proliferation and differentiation into plasma cells, which produce antibodies. This process is crucial for the adaptive immune response, enabling the body to target pathogens effectively.

IgA and IgG serve important roles in immune function but are primarily associated with secreted antibodies and are not involved as antigen receptors on B cells. Understanding the specific functions of each immunoglobulin class in the immune response is vital for grasping the complexities of immunology.