B
43•
B B cells or B lymphocytesCELLS ORB LYMPHOCYTES
B lymphocytes, also known as B cells, are one of the five
types of white blood cells, or leukocytes, that circulate
throughout the blood. They and T-lymphocytesare the most
abundant types of white blood cells. B lymphocytes are a vital
part of the body’s immune system. They function to specifi-
cally recognize a foreign protein, designated as an antigen,
and to aid in destroying the invader.
B lymphocytes are produced and mature in the bone
marrow. The mature form of the cell is extremely diverse, with
a particular B cell being tailored to recognize just a single anti-
gen. This recognition is via a molecule on the surface of the B
cell, called a B cell receptor. There are thousands of copies of
the identical receptor scattered over the entire surface of a B
cell. Moreover, there are many thousands of B cells, each dif-
fering in the structure of this receptor. This diversity is possi-
ble because of rearrangement of genetic material to generate
genes that encode the receptors. The myriad of receptors are
generated even before the body has been exposed to the pro-
tein antigen that an individual receptor will recognize. B cells
thus are one means by which our immune system has
“primed” itself for a rapid response to invasion.
The surface receptor is the first step in a series of reac-
tions in the body’s response to a foreign antigen. The receptor
provides a “lock and key” fit for the target antigen. The anti-
gen is soluble; that is, floating free in the fluid around the B
cell. A toxin that has been released from a bacterium is an
example of a soluble antigen. The binding of the antigen to the
B cell receptor triggers the intake of the bound antigen into the
B cell, a process called receptor-mediated endocytosis. Inside
the cell the antigen is broken up and the fragments are dis-
played one the surface of the B cell. These are in turn recog-
nized by a receptor on the surface of a T lymphocyte, which
binds to the particular antigen fragment. There follows a series
of reactions that causes the B cell to differentiate into a plasma
cell, which produces and secretes large amounts of an anti-
bodyto the original protein antigen.Plasma cells live in the bone marrow. They have a lim-
ited lifetime of from two to twelve weeks. Thus, they are the
immune system’s way of directly addressing an antigen threat.
When the threat is gone, the need for plasma cells is also gone.
But, B lymphocytes remain, ready to differentiate into the
antibody–producing plasma cells when required.
Within the past several years, research has indicated that
the deliberate depletion of B cells might aid in thwarting the
progression of autoimmune disease—where the body’s
immune system reacts against the body’s own components—
and so bring relief from, for example, rheumatoid arthritis.
However, as yet the data is inconclusive, and so this promis-
ing therapy remains to be proven.See alsoAntibody and antigen; Antibody formation and kinet-
ics; Immunity, active, passive and delayed; Immunity, cell
mediated; ImmunizationBBacillus thuringiensis, insecticideACILLUS THURINGIENSIS, INSECTICIDE
Bacillus thuringiensisis a Gram-positive rod-shaped bac-
terium. This bacterium is most noteworthy because of its use
to kill butterfly and moth caterpillars (Lepidoptera), the larvae
of mosquitoes, and some species of black fly, that are a dam-
age to economically important plants or a health threat.
The basis of the bacterium’s insecticidal power is a pro-
tein endotoxin (an endotoxin is a toxin that remains inside the
bacterium). More correctly in terms of the lethal activity, the
toxin is actually a so-called protoxin. That is, the molecule
must be processed to some other form before the toxic activ-
ity is present.
Inside the bacterium the protoxin molecules collect
together to form a crystal. These crystals are visible as two
pyramids associated with each other when the bacteriaare
examined in light microscopy. Often the bacteria contain a
bright spot under light microscopic illumination. This spot is
an endospore (a spore that is contained within the bacterium).womi_B 5/6/03 1:09 PM Page 43