This diagram illustrates the structure of the membrane-bound IgM (B-cell receptor). The antigen-recognition Fab regions, including the variable regions, extend outward to interact with antigens. The constant regions provide structural stability and aid in signal transduction. The receptor is anchored to the B-cell membrane via membrane-spanning domains, ensuring its proper localization. Glycosylation sites are also present, which play a role in stability and function. Unlike soluble pentameric IgM, which exists as a secreted antibody forming a five-unit structure with a J-chain, membrane-bound IgM remains monomeric and functions as part of the B-cell signaling complex.