bacteria A vast kingdom of unicellular prokaryotic microorganisms, some species of which cause infectious diseases in humans and animals, while others make up most of the body’s commensal microbiota. Disease-causing bacteria may live in the extracellular spaces, or inside cells in vesicles or in the cytosol.
BAFF   B-cell activating factor belonging to the TNF family that binds the receptors BAFF-R and TACI to promote B cell survival.
BAFF-R Receptor for BAFF that can activate canonical and non-canonical NF-κB signaling and promote survival of B cells.
bare lymphocyte syndrome See MHC class I deficiency, MHC class II deficiency.
base-excision repair Type of DNA repair that can lead to mutation and that is involved in somatic hypermutation and class switching in B cells.
basiliximab Antibody to human CD25 used to block IL-2 receptor signaling in T cells for treatment of rejection in renal transplantation.
basophils Type of white blood cell containing granules that stain with basic dyes. It is thought to have a function similar to mast cells.
BATF3 A transcription factor expressed in dendritic cells belonging to the AP1 family, which includes many other factors such as c-Jun and Fos.
B cells, B lymphocytes One of the two types of antigen-specific lymphocytes responsible for adaptive immune responses, the other being the T cells. The function of B cells is to produce antibodies. B cells are divided into two classes. Conventional B cells have highly diverse antigen receptors and are generated in the bone marrow throughout life, emerging to populate the blood and lymphoid tissues. B-1 cells have much less diverse antigen receptors and form a population of self-renewing B cells in the peritoneal and pleural cavities.
B-cell antigen receptor, B-cell receptor (BCR) The cell-surface receptor on B cells for specific antigen. It is composed of a transmembrane immunoglobulin molecule (which recognizes antigen) associated with the invariant Igα and Igβ chains (which have a signaling function). On activation by antigen, B cells differentiate into plasma cells producing antibody molecules of the same antigen specificity as this receptor.
B-cell co-receptor A transmembrane signaling receptor on the B-cell surface composed of the proteins CD19, CD81, and CD21 (complement receptor 2), which binds complement fragments on bacterial antigens also bound by the B-cell receptor. Co-ligation of this complex with the B-cell receptor increases responsiveness to antigen about 100-fold.
B-cell co-receptor complex A transmembrane signaling receptor on the B-cell surface composed of the proteins CD19, CD81, and CD21 (complement receptor 2), which binds complement fragments on bacterial antigens also bound by the B-cell receptor. Co-ligation of this complex with the B-cell receptor increases responsiveness to antigen about 100-fold.
B-cell mitogens Any substance that nonspecifically causes B cells to proliferate.
Bcl-2 family Family of intracellular proteins that includes members that promote apoptosis (Bax, Bak, and Bok) and members that inhibit apoptosis (Bcl-2, Bcl-W, and Bcl-XL).
Bcl-6 A transcriptional repressor that opposes differentiation of B cells into plasma cells.
BCMA   Receptor of the TNFR superfamily that binds APRIL.
Bcr–Abl tyrosine kinase Constitutively active tyrosine kinase fusion protein caused by a chromosomal translocation—the Philadelphia chromosome— between Bcr with the Abl tyrosine kinase genes associated with chronic myeloid leukemia.
BDCA-2 (blood dendritic cell antigen 2) A C-type lectin expressed selectively as a receptor on the surface of human plasmacytoid dendritic cells.