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epithelial membrane antigen (EMA) 253 Epstein–Barr virus (EBV)
origins such as breast, ovary, colon, and lung. Its expression
in normal tissue is very limited.
Epithelial membrane antigen⎯squamous cell carcinoma.
epithelial membrane antigen (EMA)
A marker that identifies, by immunoperoxidase staining,
most epithelial cells and tumors derived from them, such
as breast carcinomas. However, various nonepithelial neoplasms
such as selected lymphomas and sarcomas may also
express EMA. Thus, it must be used in conjunction with
other markers in tumor identification and/or classification.
epithelial thymic-activating factor (ETAF)
An epithelial cell-culture product capable of facilitating
thymocyte growth. The activity is apparently attributable to
interleukin-1 (IL1).
epithelioid cell
A particular type of cell characteristic of some types of
granulomas such as in tuberculosis, sarcoidosis, leprosy,
etc. The cell has poorly defined cellular outlines; cloudy,
abundant eosinophilic cytoplasm; and an elongated and
pale nucleus. By electron microscopy, the cell shows a few
short and slender pseudopodia and well developed cellular
organelles. Mitochondria are generally elongated, the Golgi
complex is prominent, and lysosomal dense bodies are
scattered throughout the cytoplasm. Strands of endoplasmic
reticulum, free ribosomes, and fibrils are present in
the ground substance. The epithelioid cell derives from the
monocyte–macrophage system. Peripheral blood monocytes
adhered to cellophane strips and implanted into the
subcutaneous tissue of an experimental animal develop
into epithelioid cells. Conversion of the macrophage to an
epithelioid cell is not preceded by a mitotic division of the
macrophage. On the contrary, epithelioid cells are able to
divide, resulting in round, small daughter cells that mature
in 2 to 4 days, gaining structural and functional characteristics
of young macrophages. Material that is taken up by
macrophages but cannot be further processed prevents the
conversion of epithelioid cells. The lifespan of the epithelioid
cell is 1 to 4 weeks.
epitope
An antigenic determinant. It is the simplest form or smallest
structural area on a complex antigen molecule that can
combine with an antibody or form the major histocompatibility
complex (MHC)-binding peptide recognized by T
lymphocyte receptors. It must be at least 1 kDa to elicit an
antibody response. A smaller molecule such as a hapten
may induce an immune response if combined with a carrier
protein molecule. Multiple epitopes may be found on
large nonpolymeric molecules. Based on x-ray crystallography,
epitopes consist of prominently exposed “hill and
ridge” regions that manifest surface rigidity. Antigenicity
is diminished in more flexible sites. Native and denatured
proteins, chemical groups, lipids, carbohydrates, nucleic
acids, or other constituents may comprise B cell epitopes.
By contrast, T cell epitopes are comprised of a complex of
antigenic peptide linked to either MHC class I or class II
molecules.
epitope spreading
Increased diversity of the response to autoantigens with time.
Immune response against a single epitope induces tissue injury
that exposes epitopes previously hidden, thereby activating
further lymphocyte clones. Participates in tumor regression,
chronic graft rejection and autoimmune responsiveness. Also
termed determinant spreading and antigen spreading.
epitype
A family or group of related epitopes.
Epivir ®
A synthetic nucleoside analog approved by the U.S. Food
and Drug Administration for the treatment of human
immunodeficiency virus (HIV). The mechanism of action
includes phosphorylation of the drug to its active 5′ triphosphate
metabolite that inhibits reverse transcriptase via DNA
chain termination after incorporation of the nucleoside
analog. Lamivudine is the active component of the drug.
EPO
Refer to erythropoietin.
Epstein–Barr immunodeficiency syndrome
Duncan’s X-linked immunodeficiency. This is an X-linked
or autosomal-recessive condition associated with congenital
cardiovascular and central nervous system defects. Patients
may develop fatal infectious mononucleosis. Aplasia of the
bone marrow, agammaglobulinemia, and agranulocytosis
occur, and the response to mitogens and antigens by B
cells is greatly diminished. Natural killer cell activity is
decreased, and T cells are abnormal. Patients may develop
hepatitis, B cell lymphomas, and immune suppression.
Epstein–Barr nuclear antigen
A molecule that occurs in B cells before virus-directed
protein can be found in nuclei of infected cells. Thus, it is
the earliest evidence of Epstein–Barr virus infection and
can be found in patients with conditions such as infectious
mononucleosis and Burkitt’s lymphoma.
Epstein–Barr virus (EBV)
A DNA herpes virus linked to aplastic anemia, chronic
fatigue syndrome, Burkitt’s lymphoma, histiocytic sarcoma,
hairy cell leukemia, and immunocompromised patients.
EBV may promote the appearance of such lymphoid
proliferative disorders as Hodgkin and non-Hodgkin
lymphoma, infectious mononucleosis, nasopharyngeal
carcinoma, and thymic carcinoma. It readily transforms B
lymphocytes and is used in the laboratory for this purpose
to develop long-term B lymphocyte cultures. Antibodies
produced in patients with EBV infections include those that
appear early and are referred to as EA, antibodies against
viral capsid antigen (VCA), and antibodies against nuclear
antigens (EBNA). EBV selectively infects human B cells by
E