CD9 and periodontitis: Exosomes in periodontitis act as immune modulators by transporting key molecules that influence inflammatory activity [35]. Salivary exosomes enriched with complement proteins (C6, C8A, C8B) and CCL28 enhance innate immune and complement responses, while decreased exosomal tetraspanins (CD9, CD81) may weaken intercellular communication and contribute to tissue breakdown [32,34]. Elevated exosomal PD-L1 mRNA in advanced disease suggests an adaptive role in suppressing excessive inflammation, highlighting exosomes as active regulators of immune balance in periodontitis [35].