What does the "S" suffix in a molecular designation indicate?

The "S" suffix in a molecular designation indicates that the protein is secreted. In immunology, particularly when discussing major histocompatibility complex (MHC) molecules and other proteins associated with the immune response, this suffix helps to differentiate proteins that are found in bodily fluids or secretions from those that are membrane-bound or present in cell structures.

Understanding this distinction is crucial for the functionality of proteins involved in immune responses, as secreted proteins often play roles in signaling and communication within the immune system. For example, many cytokines and antibodies are secreted proteins that carry out their functions in the extracellular environment, whereas other proteins that do not carry this suffix may be more structural or involved in direct cellular interactions. This clarifies the specific characteristics and potential roles of the protein in various biological contexts.