PREX1 A guanine exchange factor (GEF) activated downstream of small G proteins in response to activation of GPCRs such as the fMLP or C5a receptor.
PrgJ A protein component of the Salmonella typhimurium type III secretion system inner rod used by the bacterium to infect eukaryotic cells. This protein is detected by NLR proteins NAIP2 and NLRC4.
primary focus Site of early antibody production by plasmablasts in medullary cords of lymph nodes that precedes the germinal center reaction and differentiation of plasma cells.
primary granules Granules in neutrophils that correspond to lysosomes and contain antimicrobial peptides such as defensins and other antimicrobial agents.
primary immune response The adaptive immune response that follows the first exposure to a particular antigen.
primary immunization See priming.
primary immunodeficiencies A lack of immune function that is caused by a genetic defect.
primary lymphoid follicles Aggregates of resting B lymphocytes in peripheral lymphoid organs. Cf. secondary lymphoid follicle.
primary lymphoid organs See central lymphoid organs.
priming The first encounter with a given antigen, which generates the primary adaptive immune response.
pro-B cells A stage in B-lymphocyte development in which cells have displayed B-cell surface marker proteins but have not yet completed heavy-chain gene rearrangement.
pro-caspase 1 The inactive pro-form of caspase 1 that is part of the NLRP3 inﬂammasome.
pro-inflammatory Tending to induce inﬂammation.
profilin An actin-binding protein that sequesters monomeric actin. Protozoan profilins contain sequences recognized by TLR-11 and TLR-12.
programmed cell death See apoptosis.
progressive multifocal leukoencephalopathy (PML) Disease in immunocompromised patients caused by opportunisitic infection by JC virus, for example as a consequence of immunotherapy.
propeptides   Inactive precursor form of a polypeptide or peptide, which requires proteolytic processing to produce the active peptide.
properdin See factor P.
prostaglandins Lipid products of the metabolism of arachidonic acid that have a variety of effects on tissues, including activities as inﬂammatory mediators.
prostatic acid phosphatase (PAP) Enzyme expressed by prostate cancer cells used as tumor rejection antigen in the vaccine Sipuleucel-T (Provenge).
proteasome A large intracellular multisubunit protease that degrades proteins, producing peptides.
protein inhibitors of activated STAT (PIAS) A small family of proteins that inhibit STAT family transcription factors.
protein-interaction domains, protein-interaction modules Protein domains, usually with no enzymatic activity themselves, that have binding specificity for particular sites (such as phosphorylated tyrosines, proline-rich regions, or membrane phospholipids) on other proteins or cellular structures.
protein kinase C-θ (PKC-θ) A serine/threonine kinase that is activated by diacylglycerol as part of the signaling pathways from the antigen receptor in lymphocytes.
protein kinases Enzymes that add phosphate groups to proteins at particular amino acid residues: tyrosine, threonine, or serine.
protein phosphatases Enzymes that remove phosphate groups from proteins phosphorylated on tyrosine, threonine, or serine residues by protein kinases.
proteolytic subunits β1, β2, β5 Constitutive components of the proteasome's catalytic chamber.
provirus The DNA form of a retrovirus when it is integrated into the host-cell genome, where it can remain transcriptionally inactive for long periods.
P-selectin See selectins.