autoinflammatory diseases   Diseases due to unregulated inﬂammation in the absence of infection; they can have a variety of causes, including inherited genetic defects.
autophagosome A double bilayer membrane structure that functions in macroautophagy by engulfing cytoplasmic contents and fusing with lysosomes.
autophagy The digestion and breakdown by a cell of its own organelles and proteins in lysosomes. It may be one route by which cytosolic proteins can be processed for presentation on MHC class II molecules.
avidity The sum total of the strength of binding of two molecules or cells to one another at multiple sites. It is distinct from affinity, which is the strength of binding of one site on a molecule to its ligand.
avoidance   Mechanisms that prevent a host's exposure to microbes, such as anatomic barriers or particular behaviors.
azathioprine A powerful cytotoxic drug that is converted to its active form in vivo, which then kills rapidly proliferating cells, including proliferating lymphocytes; it is used as an immunosuppressant to treat autoimmune disease and in transplantation.
B-1 B cells A class of atypical, self-renewing B cells (also known as CD5 B cells) found mainly in the peritoneal and pleural cavities in adults and considered part of the innate rather than the adaptive immune system. They have a much less diverse antigen-receptor repertoire than conventional B cells and are the major source of natural antibody.
B7 molecules, B7.1 and B7.2 Cell-surface proteins on specialized antigen-presenting cells such as dendritic cells, which are the major co-stimulatory molecules for T cells. B7.1 (CD80) and B7.2 (CD86) are closely related members of the immunoglobulin superfamily and both bind to the CD28 and CTLA-4 proteins on T cells.
β1i (LMP2), β2i (MECL-1), β5i (LMP7) Alternative proteasome subunits that replace the constitutive catalytic subunits β1, β2, and β5 that are induced by interferons and produce the immunoproteasome.
β5t Alternative proteasome subunit expressed by thymic epithelial cells that substitutes for β5 to produce the thymoproteasome involved in generating peptides encountered by thymocytes during development.
β-defensins Antimicrobial peptides made by virtually all multicellular organisms. In mammals they are produced by the epithelia of the respiratory and urogenital tracts, skin, and tongue.
β sandwich A secondary protein structure composed of two β sheets that fold such that one lies over the other, as in an immunoglobulin fold.
β sheets A secondary protein structure composed of β strands stabilized by noncovalent interactions between backbone amide and carbonyl groups. In ‘parallel’ β sheets, the adjacent strands run in the same direction; in ‘antiparallel’ β sheets, adjacent strands run in opposite directions. Immunoglobulin domains are made up of two antiparallel β sheets arranged in the form of a β barrel.
β strands A secondary protein structure in which the polypeptide backbone of several consecutive amino acids is arranged in a ﬂat, or planar, conformation, and often illustrated as an arrow.
β2-microglobulin The light chain of the MHC class I proteins, encoded outside the MHC. It binds noncovalently to the heavy or α chain.
B and T lymphocyte attenuator (BTLA) An inhibitory CD28-related receptor expressed by B and T lymphocytes that interacts with the herpes virus entry molecule (HVEM), a member of the TNF receptor family.