Abstract
The immune system's efficacy depends on the coordinated yet diverse responses of individual cells to varied stimuli. While single-cell RNA sequencing (scRNA-seq) has provided unprecedented resolution into cellular transcriptomes, it often fails to capture the functional state of a cell, which is primarily governed by the proteome. This study investigates the heterogeneity of immune cell activation pathways using emerging single-cell proteomic technologies, including Single-Cell Chemical Proteomics (SCCP) and Single-Cell Network Profiling (SCNP). By analyzing protein expression and post-translational modifications at the individual cell level, we identify significant stochasticity in signaling pathway activation that is not apparent in transcriptomic data. Our results demonstrate that even within morphologically identical subsets, CD4+ T cells and B cells exhibit a spectrum of 'effectorness' and metabolic commitment. We observe that activation of the NF-κB and MAPK pathways follows a non-linear trajectory, influenced by the cell's metabolic state and prior epigenetic priming. Furthermore, we explore the implications of this proteomic heterogeneity in the context of aging and chronic inflammation, revealing how specific sub-populations drive pathological responses in autoimmune and oncological settings. These findings underscore the necessity of proteomic-level analysis for a comprehensive understanding of immune regulation and the development of targeted therapies. By bridging the gap between transcriptomic signatures and functional protein outputs, this research provides a robust framework for dissecting the molecular mechanisms of immune cell activation in health and disease, offering new insights into the precision medicine of immunological disorders.