Lymphocytes
are small white blood cells that bear the major responsibility
for carrying out the activities of the immune system; they number
about one trillion. The two major classes of lymphocytes are:
B cells, which grow to maturity independent of the thymus, and
T cells, which are processed in the thymus. Both B cells and
T cells recognize specific antigen targets.
B cells work chiefly by secreting soluble substances called
antibodies into the body's fluids, or humors. (This is known
as humoral immunity.)
Antibodies typically interact with circulating antigens such
as bacteria and toxic molecules, but are unable to penetrate
living cells. T cells, in contrast, interact directly with their
targets, attacking body cells that have been commandeered by
viruses or warped malignancy. (This is cellular
immunity.)
Although small lymphocytes look identical, even under the microscope,
they can be told apart by means of distinctive molecules they
carry on their cell surface. Not only do such markers distinguish
between B cells and T cells, they distinguish among various
subsets of cells that behave differently. Every mature T cell,
for instance, carries a marker known as T3 (or CD3); in addition,
most helper T cells carry a T4 (CD4) marker, a molecule that
recognizes class II MHC antigens. A molecule known as T8 (CD8),
which recognizes class I MHC antigens, is found on many suppressor/cytotoxic
T cells. In addition, different T cells have different kinds
of antigen receptors-either alpha/beta or gamma/delta.
