RAG-1, RAG-2 Proteins encoded by the recombination-activating genes RAG-1 and RAG-2, which form a dimer that initiates V(D)J recombination.
rapamycin An immunosuppressant drug that blocks intracellular signaling pathways involving the serine/threonine kinase mammalian target of rapamycin (mTOR) required for the inhibition of apoptosis and T-cell expansion. Also called sirolimus.
Raptor See mTORC1.
Ras A small GTPase with important roles in intracellular signaling pathways, including those from lymphocyte antigen receptors.
reactive oxygen species (ROS) Superoxide anion (O2–) and hydrogen peroxide (H2O2), produced by phagocytic cells such as neutrophils and macrophages after ingestion of microbes, and which help kill the ingested microbes.
rearrangement by inversion   In V(D)J recombination, the rearrangement of gene segments having RSS elements in an opposing orientation, leading to retention.
receptor editing The replacement of a light or heavy chain of a self-reactive antigen receptor on immature B cells with a newly rearranged chain that does not confer autoreactivity.
receptor-mediated endocytosis The internalization into endosomes of molecules bound to cell-surface receptors.
receptor serine/threonine kinases Receptors that have an intrinsic serine/threonine kinase activity in their cytoplasmic tails.
receptor tyrosine kinases Receptors that have an intrinsic tyrosine kinase activity in their cytoplasmic tails.
recombination signal sequences (RSSs) DNA sequences at one or both ends of V, D, and J gene segments that are recognized by the RAG-1:RAG-2 recombinase. They consist of a conserved heptamer and nonamer element separated by 12 or 23 base pairs.
red pulp The nonlymphoid area of the spleen in which red blood cells are broken down.
RegIIIγ An antimicrobial protein of the C-type lectin family, produced by Paneth cells in the gut in mice.
regulatory T cells   Effector CD4 T cells that inhibit T-cell responses and are involved in controlling immune reactions and preventing autoimmunity. Several different subsets have been distinguished, notably the natural regulatory T-cell lineage that is produced in the thymus, and the induced regulatory T cells that differentiate from naive CD4 T cells in the periphery in certain cytokine environments.
regulatory tolerance Tolerance due to the actions of regulatory T cells.
Relish A distinct member of the Drosophila NFκB transcription factor family that induces the expression of several antimicrobial peptides in response to Gram-negative bacteria.
resistance A general immune strategy aimed at reducing or eliminating pathogens; compare with avoidance and tolerance.
respiratory burst An oxygen-requiring metabolic change in neutrophils and macrophages that have taken up opsonized particles, such as complementor antibody-coated bacteria, by phagocytosis. It leads to the production of toxic metabolites that are involved in killing the engulfed microorganisms.
restriction factors Host proteins that act in a cell-autonomous manner to inhibit the replication of retroviruses such as HIV.
retinoic acid   Signaling molecule derived from vitamin A with many roles in the body. It is thought to be involved in the induction of immunological tolerance in the gut.
retrotranslocation complex The return of endoplasmic reticulum proteins to the cytosol.
retrovirus A single-stranded RNA virus that uses the viral enzyme reverse transcriptase to transcribe its genome into a DNA intermediate that integrates into the host-cell genome to undergo viral replication.
reverse transcriptase Viral RNA-dependent DNA polymerase that is found in retroviruses and transcribes the viral genomic RNA into DNA during the life cycle of retroviruses (such as HIV).