See also B-CELL LYMPHOCYTE; CELL STRUCTURE AND
FUNCTION; GRANULOCYTE; HEMATOPOIESIS; LYMPHOMA;
MONOCYTE; MULTIPLE MYELOMA; NATURAL KILLER(NK)
CELL; THYMECTOMY.
lymphocytopenia A decline in the number of
lymphocytes in the BLOODto fewer than 1,000
lymphocytes per microliter of whole blood. Lym-
phocytes circulate in the blood and the LY M P H,
their primary role being to identify and attack
invading pathogens to prevent and fight INFECTION.
Lymphocytopenia often accompanies IMMUNODEFI-
CIENCYdisorders, notably HIV/AIDS(in which it may
be one of the earliest indications of infection),
infections such as TUBERCULOSISand HEPATIS, and
AUTOIMMUNE DISORDERSsuch as SYSTEMIC LUPUS ERY-
THEMATOSUS (SLE) and MYASTHENIA GRAVIS. Other
causes include RADIATION THERAPYas cancer treat-
ment, long-term PUVA (psoralen plus ultraviolet
light of A wavelength) PHOTOTHERAPYfor treatment
of PSORIASIS, severe stress, and medications such
as CORTICOSTEROID MEDICATIONS. Lymphocytopenia
may be transitory, with the LYMPHOCYTE level
returning to normal when the underlying cause
improves. Depending on the cause, people who
have lymphocytopenia may show few symp-
toms. Treatment targets the underlying condition.
The health consequences of lymphocytopenia
vary with the overall status of the IMMUNE SYSTEM.
See also LEUKOPENIA; NEUTROPENIA; THROMBOCY-
TOPENIA.
lymphoma A type of CANCER that affects the
hematopoietic functions of the LY M P Hsystem that
results in the uncontrolled proliferation of lym-
phocytes, the type of LEUKOCYTE(white BLOODcell)
that the lymph tissues primarily produce. The
lymphocytes congregate in the lymph tissues to
form tumors.
LYMPHOMA VS. LEUKEMIA
LEUKEMIAand LYMPHOMAare both cancers that can
affect the lymphocytes. However, leukemia is a
CANCERof the BONE MARROWthat alters the devel-
opment and proliferation of lymphocytes that
enter the BLOODcirculation. Lymphoma is a can-
cer of the lymphatic tissues that produce lym-
phocytes.Though there are nearly three dozen identified
types of lymphoma doctors assign them to one of
two major categories, Hodgkin’s lymphoma and
non-Hodgkin’s lymphoma. Doctors diagnose about
60,000 people with lymphoma in the United States
each year. Lymphoma is the fifth most common
kind of cancer among American adults and the
third most common kind of cancer among children.How Lymphoma Develops
Lymphomas originate in the reticuloendothelial or
clone cells in the lymph structures that produce
lymphocytes, notably the lymph nodes and the
SPLEEN. Most lymphomas affect B-cell lymphocytes
(B-cells) though some affect T-cell lymphocytes
(T-cells). Hodgkin’s lymphoma involves a specific
kind of B-cell called a Reed-Sternberg cell. In all
lymphomas, the affected lymphocytes proliferate
and migrate to lymph tissues, such as lymph
nodes and the spleen. The lymphocytes cluster
into tumorous formations that drain the NUTRIENTS
and other resources healthy cells require, causing
the healthy cells to die and allowing the cancerous
lymphocytes to continue proliferating.
A key marker for the extent and severity of
lymphoma is whether tumors are present on only
one side or on both sides of the DIAPHRAGM. Lym-
phomas present only on one side of the
diaphragm (either above or below) tend to be less
aggressive than those that are present in LY M P H
NODEregions on both sides of the diaphragm, as
well as more responsive to treatment (particularly
those above the diaphragm). Cancerous lympho-
cytes can also metastasize to other kinds of tissues
throughout the body, primarily traveling through
the lymphatic system. The most common sites for
lymphoma METASTASISoutside the lymphatic sys-
tem are the BRAIN, SKIN, BONE, and BONE MARROW.
However, because the lymphatic network extends
throughout the interstitial tissues, metastases in
advanced disease can appear anywhere.Hodgkin’s Lymphoma
Hodgkin’s lymphoma, also called Hodgkin’s dis-
ease, accounts for about 15 percent of diagnosed
lymphomas. It most commonly affects people
between ages 16 to 34 and over age 55. The pres-
ence of specifically abnormal B-cells, Reed-Stern-
berg cells, is the hallmark of Hodgkin’s lymphoma.152 The Blood and Lymph