Encyclopedia of Health and Medicine

lymphocyte 151
debris from the lymph. Follicles within the lymph
node contain B-cells and T-cells, which proliferate
and mature in the follicles. The B-cells produce
antibodies specific to the antigens the lymph carries
into the lymph node. The lymph node adds
these antibodies to the lymph as the lymph exits
the node. The lymph node’s follicles release additional
T-cells as necessary to fight INFECTION,
responding to chemicals PHAGOCYTOSIS releases.
Extensive networks of lymphatic capillaries carry
lymph among the lymph nodes as well as to and
from the larger LYMPH VESSELS.
Lymph nodes commonly swell when they are
actively responding to infection because they fill
with the pathogenic cells they filter from the
lymph, a circumstance doctors call LYMPHADENOPA-
THY. LYMPHADENITIS occurs when the infection
involves the lymph node itself. The lymph nodes
also can become seeding sites for cancer cells that
are metastasizing (spreading) to other parts of the
body. Most operations to remove cancerous tumors
also include removal of adjacent lymph nodes to
examine them for the presence of cancer cells,
which is key to the STAGING AND GRADING OF CANCER.
See also ANTIBODY; ANTIGEN; LYMPHEDEMA; METAS-
TASIS; PATHOGEN; SENTINEL LYMPH NODE DISSECTION.
lymphocyte A type of LEUKOCYTE (white BLOOD
cell) that primarily resides in the LYMPH and lymph
tissues. Lymphocytes are the body’s primary
immune defense and move through the lymph in
response to antigens and pathogens. When more
rapid deployment is necessary, lymphocytes enter
the bloodstream. About 1 percent of the body’s
lymphocyte population circulates in the blood,
making up about 25 percent of the circulating
leukocytes. There are two major types of lymphocytes—T-cells
and B-cells—and natural killer cells.
Each type has different immune responsibilities.
T-Cells
T-cells, which make up about 75 percent of lymphocytes,
originate in the BONE MARROW and
migrate to the THYMUS to come to maturity. In the
thymus T-cells acquire the ability to distinguish
between “self” and “nonself,” an essential function
of determining whether the particles the T-cells
encounter are invaders. Mature T-cells carry kinds
of antibodies, identified as CLUSTERS OF DIFFERENTIA-
TION, that denote the T-cell’s immune function.
There are numerous subtypes of T-cells, the most
common being
• helper T-cells, which secrete a cytokine called
CD4 (for cluster of differentiation 4) that directs
the response of other T-cells
• cytotoxic T-cells, which attack invading cells by
releasing chemicals that penetrate their cell
membranes, which causes them to rupture and
die
• suppressor T-cells, which reign in the IMMUNE
RESPONSE after the immune attack has squelched
the threat
• memory T-cells, which retain the ability to produce
antibodies against the same ANTIGEN
should it reappear in the body
B-Cells
B-cells, which make up about 10 percent of lymphocytes,
originate in the bone marrow and
migrate to the lymph tissues to come to maturity
and await activation via contact with an antigen.
When such contact occurs, the individual B-cell
develops antibodies specific to the antigen, differentiates
into either a memory B-cell or a PLASMA
cell and then proliferates within the lymph tissues,
lymph, and bloodstream. Memory cells “remember”
the specific antigen and produce antibodies
whenever the antigen again enters the body. This
process provides long-term protection against
pathogens. Plasma cells generate copious antibodies
as they replicate, providing an immediate
immune response to the PATHOGEN.
Natural Killer Cells
Natural killer (NK) cells are specialized lymphocytes
that attack and destroy self cells that have
become defective in some way. Researchers
believe one function of NK cells is to attack
tumors as they are beginning to develop, preventing
them from taking root. NK cells also appear to
attack cells that viruses hijack, preventing the
VIRUS from replicating and causing infection.
For further discussion of lymphocytes within
the context of blood and lymph structure and
function please see the overview section “The
Blood and Lymph.”