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B7.2 costimulatory molecule 92 B cell activation
B7.2 costimulatory molecule
A costimulatory molecule whose sequence resembles that
of B7. Dendritic cells, monocytes, activated T cells, and
activated B lymphocytes may express B7.2, which is an
80-kDa protein that serves as a costimulatory ligand for
CD28 but as an inhibitory ligand upon interacting with
CTLA-4 molecules. Also called CD86.
B220
A form of CD45, a protein tyrosine phosphatase.
b allotype
A rabbit immunoglobulin κ light chain allotype encoded by
alleles at the K1 locus.
B antigen, acquired
The alteration of A1 erythrocyte membrane through the
action of such bacteria as Escherichia coli, Clostridium
tertium, and Bacteroides fragilis to make it react as if it
were a group B antigen. The named microorganisms may
be associated with gastrointestinal infection or carcinoma.
B blood group
Refer to ABO blood group system.
B cell activation
B cell response to antigen, whether T-independent or
T-dependent, results in the conversion of small resting
B cells to large lymphoblasts and then into either plasma
cells that form specific antibody or long-lasting memory
B cells. Thymic-independent antigens such as bacterial
polysaccharides can activate B cells independently of
T cells by crosslinking of the B cell receptor. By contrast,
protein antigens usually require the intimate interaction of
B cells with helper T cells. Antigen stimulation of the B
cell receptor leads to endocytosis and degradation of the
antigen captured by the B cell receptor. Peptides that result
from degraded antigen are bound to major histocompatibility
complex (MHC) class II molecules and transported
to the cell surface for presentation to T lymphocytes. T
Naive CD4 +
T Cell
CTLA-4 CD40L
TCR
B7-2
CD28
( = Antigen)
ICAM-1
MHC41 DENDRITIC CELL
IL-2
(proliferation)
IL-2Rα
NK CELL
CTLA-4
IL-2
IFN-γ
IL-4
CD40L
CD40
IFN-γ
B7-2
cells bearing a specific T cell receptor that recognizes the
peptide–MHC complex presented on the B cell surface are
activated. Activated T cells help B cells either via soluble
mediators such as cytokines (i.e., IL-4, IL-5, and IL-6) or
membrane-bound stimulatory molecules such as the CD40
ligand. In germinal centers, B cells are converted to large
replicating centroblasts and then to nonreplicating centroblasts.
In germinal centers, frequent immunoglobulin (Ig)
region mutations and the switch from IgM to IgG, IgA, or
IgE production occur. Mutation increases the diversity of
antigen-binding sites. Mutations that lead to loss of antigen
binding cause the cells to die by apoptosis. The few cells
for which mutation gives an immunoglobulin product that
has high affinity for antigen are selected for survival. These
antigen-selected cells differentiate into plasma cells that
produce antibody or into small long-lived memory B cells
that enter the blood and lymphoid tissues.
CTLA-4 *
*Delayed appearance of
CTLA-4 followed by B7-ligation
downregulates T cell activity
Antigen
Plasma
Cell
T CELL
(Cytokine activation)
LFA-1
ICAM-1
NAIVE B
CELL
slg
?
Hypothetical B7/CD40 pathway for B cell activation.
B´´ Cell
MCH41
B Cell
B´ Cell
ACTIVATED
B CELL
CD40L
IgG
ACTIVATION
CD40
IgA
IgE
DIFFERENTIATION
Plasma
Cell
IFN-γ
IL-4
IL-4
IL-4
IFN-γ
TH3 CELL
IL-5
PROLIFERATION
B´´ Cell
B cell activation.
(If Antigen is very
abundant, very little
IL-4 is produced)
ANTIBODY
PRODUCING
B Cell
TGF-β
IL-6