plasmacytoid dendritic cells (pDCs) A distinct lineage of dendritic cells that secrete large amounts of interferon on activation by pathogens and their products via receptors such as Toll-like receptors. Cf. conventional dendritic cells.
platelet-activating factor (PAF) A lipid mediator that activates the blood clotting cascade and several other components of the innate immune system.
pluripotent Typically referring to the capacity of a progenitor cell to generate all possible lineages of an organ system.
P-nucleotides Short palindromic nucleotide sequences formed between gene segments of the rearranged V-region gene generated by the asymmetric opening of the hairpin intermediate during RAG-mediated rearrangement.
Polη An error-prone, 'translesion', DNA polymerase involved in repairing DNA damage caused by UV radiation and in somatic hypermutation.
polyclonal activation The activation of lymphocytes by a mitogen regardless of antigen specificity, leading to the activation of clones of lymphocytes of multiple antigen specificities.
polygenic Containing several separate loci encoding proteins of identical function; applied to the MHC. Cf. polymorphic.
polymerase stalling The halting of RNA polymerase during the transcription of a gene at locations within the gene locus, known to be a regulated process, and involved in mechanisms of isotype switching.
polymeric immunoglobulin receptor (pIgR) The receptor for polymeric immunoglobulins IgA and IgM on basolateral surfaces of mucosal and glandular epithelial cells that transports IgA (or IgM) into secretions.
polymorphic Existing in a variety of different forms; applied to a gene, occurring in a variety of different alleles.
polymorphism Applied to genes, variability at a gene locus in which all variants occur at a frequency greater than 1%.
polymorphonuclear leukocytes See granulocytes.
polysaccharide capsules A distinct structure in some bacteria—both Gram-negative and Gram-positive—that lies outside cell membrane and cell wall that can prevent direct phagocytosis by macrophages without the aid of antibody or complement.
polyubiquitin chains Polymers of ubiquitin covalently linked from lysine residues within one ubiquitin monomer to the carboxy terminus of a second ubiquitin.
PorA Outer membrane protein of Neisseria meningitidis that binds C4BP, thereby inactivating C3b deposited on its surface.
positive selection A process occurring in the thymus in which only those developing T cells whose receptors can recognize antigens presented by self MHC molecules can mature.
post-transplant lymphoproliferative disorder B-cell expansion driven by Epstein–Barr virus (EBV) in which the B cells can undergo mutations and become malignant. This can occur when patients are immunosuppressed after, for example, solid organ transplantation.
pre-B-cell receptor Receptor produced by pre-B cells that includes an immunoglobulin heavy chain, as well as surrogate light-chain proteins, Igα and Igβ signaling subnits. Signaling through this receptor induces the pre-B cell to enter the cell cycle, to turn off the RAG genes, to degrade the RAG proteins, and to expand by several cell divisions.
pre-T-cell receptor Receptor protein produced by developing T lymphocytes at the pre-T-cell stage. It is composed of TCRβ chains that pair with a surrogate α chain called pTα (pre-T-cell α), and is associated with the CD3 signaling chains. Signaling through this receptor induces pre-T-cell proliferation, expression of CD4 and CD8, and cessation of TCR β chain rearrangement.
prednisone A synthetic steroid with potent anti-inﬂammatory and immunosuppressive activity used in treating acute graft rejection, autoimmune disease, and lymphoid tumors.