antigen-presenting cells (APCs) Highly specialized cells that can process antigens and display their peptide fragments on the cell surface together with other, co-stimulatory, proteins required for activating naive T cells. The main antigen-presenting cells for naive T cells are dendritic cells, macrophages, and B cells.
antigen processing The intracellular degradation of foreign proteins into peptides that can bind to MHC molecules for presentation to T cells. All protein antigens must be processed into peptides before they can be presented by MHC molecules.
antigen receptor The cell-surface receptor by which lymphocytes recognize antigen. Each individual lymphocyte bears receptors of a single antigen specificity.
anti-lymphocyte globulin Antiserum raised in another species against human T cells. It is used in the temporary suppression of immune responses in transplantation.
antimicrobial enzymes Enzymes that kill microorganisms by their actions. An example is lysozyme, which digests bacterial cell walls.
antimicrobial peptides, antimicrobial proteins Amphipathic peptides or proteins secreted by epithelial cells and phagocytes that kill a variety of microbes nonspecifically, mainly by disrupting cell membranes. Antimicrobial peptides in humans include the defensins, the cathelicidins, the histatins, and RegIIIγ.
antiserum The ﬂuid component of clotted blood from an immune individual that contains antibodies against the antigen used for immunization. An antiserum contains a mixture of different antibodies that all bind the antigen, but which each have a different structure, their own epitope on the antigen, and their own set of cross-reactions. This heterogeneity makes each antiserum unique.
antivenin Antibody raised against the venom of a poisonous snake or other organism and which can be used as an immediate treatment for the bite to neutralize the venom.
aorta-gonad-mesonephros (AGM) An embryonic region in which hematopoietic cells arise during development.
AP-1 A heterodimeric transcription factor formed as one of the outcomes of intracellular signaling via the antigen receptors of lymphocytes and the TLRs of cells of innate immunity. Most often, contains one Fos-family member and one Jun-family member. AP-1 mainly activates the expression of genes for cytokines and chemokines.
APECED See autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy.
An RNA editing enzyme that deaminates cytidine to uracil in certain mRNAs, such as apolipoprotein B, and which is related to the enzyme AID involved in somatic hypermutation and isotype switching.
apoptosis A form of cell death common in the immune system, in which the cell activates an internal death program. It is characterized by nuclear DNA degradation, nuclear degeneration and condensation, and the rapid phagocytosis of cell remains. Proliferating lymphocytes experience high rates of apoptosis during their development and during immune responses.
apoptosome A large, multimeric protein structure that forms in the process of apoptosis when cytochrome c is released from mitochondria and binds Apaf-1. A heptamer of cytochrome c-Apaf-1 heterodimers assembles into wheel-like structure that binds and activates procaspase-9, an initiator caspase, to initiate the caspase cascade.
appendix A gut-associated lymphoid tissue located at the beginning of the colon.
APRIL A TNF family cytokine related to BAFF that binds the receptors TACI and BCMA on B cells to promote survival and regulate differentiation.
apurinic/apyrimidinic endonuclease 1 (APE1) A DNA repair endonuclease involved in class switch recombination.