Webster's New World Medical Dictionary

autosomal dominant trait
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B cell
A type of white blood cell that has an
important role in producing antibodies for the
immune system. B cells are lymphocytes that mature
in the bone marrow (as opposed to T cells, lym-
phocytes that mature in the thymus). Many B cells
go on to become plasma cells and produce anti-
bodies (immunoglobulins); some B cells mature
into memory B cells. See also 
memory B cell;
plasma cell.
B variant GM2-gangliosidosis
See 
Tay-Sachs
disease.
B. quintana
See 
Bartonella quintana.
Babinski reflex
A reflex used to determine ade-
quacy of the higher (central) nervous system. The
Babinski reflex is obtained by stimulating the out-
side of the sole of the foot, causing extension of the
big toe while fanning the other toes. The examiner
begins the stimulation at the heel and goes forward
to the base of the toes. Most newborn babies and
young infants are not neurologically mature, and
they therefore show a Babinski reflex. A Babinski
reflex in an older child or an adult is abnormal and
is a sign of a problem in the brain or spinal cord. A
Babinski reflex that is present on one side but not
the other is also abnormal, and it can indicate
which side of the brain is involved. Also known as
plantar response, big toe sign, and Babinski phe-
nomenon, response, or sign.
baby teeth
See 
primary teeth.
bacillus
A large family of bacteria that are rod-
like in shape. They include the bacteria that cause
food to spoil, as well as those that are responsible
for some diseases. Helpful members of the bacillus
family are used to make antibiotics or colonize the
human intestinal tract and aid with digestion.
back pain
Pain felt in the low or upper back.
Causes of pain in the low and upper back include
conditions affecting the bony spine; discs between
the vertebrae; ligaments around the spine and discs;
spinal inflammation; spinal cord and nerves; mus-
cles; internal organs of the pelvis, chest, and
abdomen; tumors; and the skin.
back pain, low
Pain in the lower back area that
can be caused by problems with the lumbar spine,
the discs between the vertebrae, the ligaments
around the spine and discs, the spinal cord and
nerves, muscles of the low back, internal organs of
the pelvis and abdomen, or the skin covering the
lumbar area. See also 
sciatica.
backbone
The spine, a flexible row of bones
stretching from the base of the skull to the tailbone.
See also 
vertebral column.