patrolling monocyte A form of circulating monocyte that adheres to and surveys the vascular endothelium, distinguished from classical monocytes by its low expression of Ly6C.
pattern recognition receptors (PRRs) Receptors of the innate immune system that recognize common molecular patterns on pathogen surfaces.
PD-1   Programmed death-1, a receptor on T cells that when bound by its ligands, PD-L1 and PD-L2, inhibits signaling from the antigen receptor. PD-1 contains an ITIM motif in its cytoplasmic tail. Target of cancer therapies aimed at stimulating T-cell responses to tumors.
PD-L1 (programmed death ligand-1, B7-H1) Transmembrane receptor that binds to the inhibitory receptor PD-1. PD-L1 is expressed on many cell types and is upregulated by inﬂammatory cytokines.
PD-L2 (programmed death ligand-2, B7-DC) Transmembrane receptor that binds to the inhibitory receptor PD-1; mainly expressed on dendritic cells.
PECAM See CD31.
pembrolizumab Human anti-PD-1 antibody used for checkpoint blockade in treatment of metastatic melanoma.
pemphigus vulgaris An autoimmune disease characterized by severe blistering of the skin and mucosal membranes.
pentameric IgM Major form of the IgM antibodies produced by the action of J chain resulting in higher avidity for antigens.
pentraxin A family of acute-phase proteins formed of five identical subunits, to which C-reactive protein and serum amyloid protein belong.
pepsin A protease that cleaves several sites on the carboxy-terminal side of the disulfide linkages, producing the F(abʹ)2 fragment and several fragments of the Fc region.
peptide-binding cleft The longitudinal cleft in the top surface of an MHC molecule into which the antigenic peptide is bound. Sometimes called the peptide-binding groove.
peptide editing   In the context of antigen processing and presentation, the removal of unstably bound peptides from MHC class II molecules by HLA-DM.
peptide-loading complex (PLC) A protein complex in the endoplasmic reticulum that loads peptides onto MHC class I molecules.
peptide:MHC tetramers Four specific peptide:MHC complexes bound to a single molecule of ﬂuorescently labeled streptavidin, which are used to identify populations of antigen-specific T cells.
peptidoglycan A component of bacterial cell walls that is recognized by certain receptors of the innate immune system.
peptidoglycan-recognition proteins (PGRPs) A family of Drosophila proteins that bind peptidoglycans from bacterial cell walls that serve to initiate the proteolytic cascade of the TOLL pathway.
periarteriolar lymphoid sheath (PALS) Part of the inner region of the white pulp of the spleen; it contains mainly T cells.
peripheral lymphoid organs, peripheral lymphoid tissues The lymph nodes, spleen, and mucosa-associated lymphoid tissues, in which adaptive immune responses are induced, as opposed to the central lymphoid organs, in which lymphocytes develop. They are also called secondary lymphoid organs and tissues.
peripheral tolerance Tolerance acquired by mature lymphocytes in the peripheral tissues, as opposed to central tolerance, which is acquired by immature lymphocytes during their development.
Peyer’s patches Organized peripheral lymphoid organs under the epithelium in the small intestine, especially the ileum, and in which an adaptive immune response can be initiated. They contain lymphoid follicles and T-cell areas. They are part of the gut-associated lymphoid tissues (GALT).