MicroRNA (miRNA) control has emerged as a critical regu latory circuit of the immune system. Measuring changes in miRNA abundance in the blood of human subjects in health and disease is therefore a promising new fi eld of investigation. Th ese short non-coding singlestranded RNAs about 22 nucleotides in length have been found to play essential regulatory roles [130] [131] [132] . Th ese molecules exhibit highly specifi c, regulated patterns of expression and control protein expression by trans lational repression, mRNA cleavage, or promotion of mRNA decay. Interestingly, thanks to their small size, miRNA molecules are stable and can be measured not only in blood cells but also in circulation in the serum [133] . Th ey are thus not only potentially important contributors to immune function, but also potential sources of biomarkers.