P-selectin glycoprotein ligand-1 (PSGL-1) Protein expressed by activated effector T cells that is a ligand for P-selectin on endothelial cells, and may enable activated T cells to enter all tissues in small numbers.
pseudo-dimeric peptide:MHC complexes Hypothetical complexes containing one antigen peptide:MHC molecule and one self peptide:MHC molecule on the surface of the antigen-presenting cell, which have been proposed to initiate T-cell activation.
pseudogenes Gene elements that have lost the ability to encode a functional protein but that are retained in the genome and may continue to be transcribed normally.
psoriasis   Chronic autoimmune disease thought to be driven by T cells manifested in skin, but which can also involve nails and joints (psoriatic arthropathy).
psoriatic arthropathy See psoriasis.
pTα See pre-T-cell receptor.
purine nucleotide phosphorylase (PNP) deficiency An enzyme defect that results in severe combined immunodeficiency. The deficiency of PNP causes an intracellular accumulation of purine nucleosides, which are toxic to developing T cells.
purinergic receptor P2X7 An ATP-activated ion channel that allows potassium efﬂux from cells when activated, which can trigger inﬂammasome activation in response to excessive extracellular ATP.
pus Thick yellowish-white liquid typically found at sites of infection with some types of extracellular bacteria, which is composed of the remains of dead neutrophils and other cells.
pus-forming bacteria Capsulated bacteria that result in pus formation at the site of infection. Also called pyogenic (pus-forming) bacteria.
PYHIN A family of four intracellular sensor proteins containing an H inversion (HIN) domain in place of the LRR domain found in most other NLR proteins. The HIN domain functions in recognition of cytoplasmic dsDNA. Examples are AIM2 and IFI16.
pyogenic arthritis, pyoderma gangrenosum, and acne (PAPA)
Autoinﬂammatory syndrome that is caused by mutations in a protein that interacts with pyrin.
pyogenic bacteria See pus-forming bacteria.
pyrin   One of several protein interaction domains, structurally related to but distinct from CARD, TIR, DD, and DED domains.
pyroptosis A form of programmed cell death that is associated with abundant pro-inﬂammatory cytokines such as IL-1β and IL-18 produced through inﬂammasome activation.
Qa-1 determinant modifiers (Qdm) A class of peptides derived from the leader peptides of various HLA class I molecules that can be bound by the human HLA-E and murine Qa-1 proteins, and then are recognized by the inhibitory NKG2A:CD94 receptor.
quasi-species The different genetic forms of certain RNA viruses that are formed by mutation during the course of an infection.
Rac See Rho family small GTPase proteins.
radiation-sensitive SCID (RS-SCID) Severe combined immunodeficiency due to a defect in DNA repair pathways, which renders cells unable to perform V(D)J recombination and unable to repair radiation-induced double-strand breaks.
RAE1 (retinoic acid early inducible 1) protein family Several murine MHC class Ib proteins; these are orthologs of human RAET1 family proteins, including H60 and MULT1, and are ligands for murine NKG2D.
RAET1 A family of 10 MHC class Ib proteins that are ligands for NKG2D, and includes several UL16-binding proteins (ULBPs).
Raf A protein kinase in the Raf–MEK1–Erk signaling cascade that is the first protein kinase in the pathway, and is activated by the small GTPase Ras.