Influenza NP (311-325) is a biologically active peptide composed of amino acids 311 to 325 derived from the influenza virus nucleoprotein (NP). As a bona fide MHC class II-restricted epitope, it is instrumental for studying host immune responses during viral infection. It notably induces robust gamma interferon (IFN-gamma) production in intracellular cytokine assays while not stimulating CD8 T-cells in mice. Additionally, the presence of Glutamine (Q) or Glutamic acid (E) at the N-terminus can lead to the spontaneous formation of Pyroglutamyl (pGlu) peptides, which may enhance peptide stability against gastrointestinal proteases through the hydrophobic γ-lactam ring of pGlu. Consequently, these peptides are considered a normal variant and factored into the peptide purity measurements in HPLC analysis.