How can we assess GVHD through blood omics? The number of high-throughput omics tools that can be used to profile the human immune system in the blood is increasing rapidly. Genomic approaches consist of measuring abundance of cellular RNA and also microRNAs (miRNAs) that are present in cells or in the serum/plasma. Other genomic approaches consist of determining gene sequence and function (eg, genome-wide association studies, RNA-interference screens, exome sequencing, next-generation whole-genome sequencing). Proteomic assays are used to determine antibody specificity or measure changes in serum/plasma levels of cytokines or chemokines using multiplex assays or global profiles through tandem-mass spectrometry. Cellular profiling assays or cytomics are used to phenotype immune cells based on intracellular or extracellular markers using polychromatic flow cytometry or mass cytometry. CircRNA, circular RNA; lnRNA, long noncoding RNA; mRNA, messenger RNA; RNAseq, RNA sequencing; ULS, Universal Linkage System.