Final answer:
A cell surface IgM receptor is generated by naive B cells as a monomer that binds antigens. It triggers B cell differentiation into plasma cells that secrete pentameric IgM, forming the primary response to pathogens. This process involves antigen presentation, T-helper cell interaction, and DNA transposition, and can evolve into producing class-switched, high-affinity IgG antibodies.
Step-by-step explanation:
The generation of a cell surface IgM receptor involves a complex process where naive B cells express monomeric IgM on their surface to function as receptors for antigens. Upon encountering an antigen, these receptors bind to the antigen and the B cell engulfs it through phagocytosis. The antigen is then degraded into peptides and presented on the B cell surface in conjunction with major histocompatibility complex II (MHC-II). When T-helper cells recognize this complex, they secrete cytokines, stimulating the B cell to differentiate into a plasma cell, a process that involves DNA rearrangement known as 'transposition'. As a result, the plasma cells produce large amounts of pentameric IgM antibodies, which are the first antibodies to respond to an infection. The pentameric IgM, containing five IgM monomers linked by a J chain, can bind to up to ten identical antigens, providing a strong initial immune response.
IgD, like IgM, is a membrane-bound receptor on B cells but is not secreted and is found in low serum levels. In contrast, plasma cells secrete pentameric IgM with identical antigen specificity to the original BCRs. The immune response can be further strengthened through class switching to IgG antibodies after subsequent antigen exposures, enhanced by affinity maturation due to mutations in the immunoglobulin gene variable regions.