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helminth 315 hematopoiesis
thereby diminishing the effectiveness of bound antibodies.
The immune response may also be downregulated during
infection as antigen-specific responsiveness of local and circulating
T cells is diminished. The complexity of the disease
makes development of effective vaccines difficult. An oral
subunit vaccine used with a mucosal adjuvant has protected
animal models. Thus immunotherapy may be a future option
for prevention.
helminth
A parasitic worm. Infections by helminths induce a T H2regulated
immune response associated with inflammatory
infiltrates rich in eosinophils and IgE synthesis.
helper CD4 + T cells
CD4 + T lymphocytes that facilitate antibody formation by
B cells following antigenic challenge. T H2 T cells that synthesize
cytokines IL-4 and IL-5 represent the most efficient
helper T cells in contrast to other helper CD4 + T cells.
helper/suppressor ratio
The ratio of CD4 + helper/inducer T lymphocytes to CD8 +
suppressor/cytotoxic T lymphocytes. This value normally
ranges between 1.5 and 2.0; however, in certain virus infections
(notably AIDS), the ratio becomes inverted as a consequence
of greatly diminished CD4 + lymphocytes and either
stationary or elevated levels of CD8 + lymphocytes. Inversion
of the ratio continues as the clinical situation deteriorates.
Eventually, the CD4 + cells may completely disappear in an
AIDS patient. Other conditions in which the helper/suppressor
(CD4/CD8) ratio is decreased include other viral
infections such as herpes, cytomegalovirus, Epstein–Barr
virus, and measles, graft-vs.-host disease, lupus erythematosus
with renal involvement, severe burns, exercise,
myelodysplasia, acute lymphocytic leukemia in remission,
severe sunburn, exercise, and loss of sleep. Elevated ratios
may occur in such conditions as atopic dermatitis, Sezary
syndrome, psoriasis, rheumatoid arthritis, primary biliary
cirrhosis, lupus erythematosus without renal involvement,
chronic autoimmune hepatitis, and type I insulin-dependent
diabetes mellitus.
helper T cells
CD4 + helper/inducer T lymphocytes that represent a subset
of T cells critical to induction of an immune response
to a foreign antigen. Antigen is presented by an antigenpresenting
cell, such as a macrophage, in the context of
self major histocompatibility complex (MHC) class II
antigen and interleukin-1 (IL1). Once activated, the CD4 +
T cells express IL2 receptors and produce IL2 molecules
that can act in an autocrine fashion by combining with the
IL2 receptors and mediating the CD4 + cells to proliferate.
Differentiated CD4 + lymphocytes synthesize and secrete
lymphokines that affect the functions of other cells of the
immune system such as CD8 + cells, B cells, and natural
killer (NK) cells. B cells differentiate into plasma cells that
synthesize antibody. Activated macrophages participate in
delayed-type hypersensitivity (type IV) reactions. Cytotoxic
T cells also develop. Murine monoclonal antibodies are
used to enumerate CD4 + T lymphocytes by flow cytometry.
hemadsorption inhibition test
A red blood cell suspension is added to a tissue culture
infected with a hemagglutinating virus. Viral hemagglutinin,
expressed at the tissue culture cell surfaces, facilitates
the hemadsorption of erythrocyte aggregates to the tissue
culture surfaces. Antiviral antibody added to the culture
prevents this hemadsorption, which serves as the basis for
testing for antiviral antibody.
RBC
RBC
IgM
Hemagglutination.
RBC
RBC
hemagglutination
The aggregation of red blood cells by antibodies, viruses,
lectin, or other substances.
hemagglutination inhibition reaction
A serological test based upon inhibiting the aggregation of
erythrocytes bearing antigen. It may be employed for diagnosis
of such viral infections as rubella, variola–vaccinia, rubeola,
herpes zoster, herpes simplex types I and II, cytomegalovirus,
and Epstein–Barr virus. It is also used in the diagnosis of
adenovirus, influenza, coronavirus, parainfluenza, mumps,
and such viral diseases as St. Louis, Eastern, Venezuelan, and
Western equine encephalitides. It has also been used in diagnosing
various bacterial and parasitic diseases.
hemagglutination inhibition test
An assay for antibody or antigen based on the ability to
interfere with red blood cell aggregation. Certain viruses
are able to agglutinate red blood cells. In the presence of
antiviral antibody, the ability to agglutinate erythrocytes is
inhibited, thus serving as a basis to assay the antibody.
hemagglutination test
An assay based upon the aggregation of red blood cells into
clusters either through the action of antibody specific for
their surface epitopes or through the action of a virus that
possesses a hemagglutinin as part of its structure and does
not involve antibody.
hemagglutinin
A red blood cell agglutinating substance. Antibodies, lectins,
and some viral glycoproteins may induce erythrocyte
agglutination. In immunology, hemagglutinin usually refers
to an antibody that causes red blood cell aggregation in
physiological salt solution either at 37°C (warm hemagglutinins)
or at 4°C (cold hemagglutinins).
hematocytes
Cells resembling leukocytes that appear in invertebrate
hemolymph.
hematogones
Early precursor B cells that express cell surface antigens
such HLA-DR, CD10, CD19, and CD20. Morphologically
they appear as small compact lymphocytes.
hematopoiesis
The development of the cellular elements of the blood
including erythrocytes, leukocytes, and platelets from
pluripotent stem cells in the bone marrow and fetal liver.
Various cytokine growth factors are synthesized by bone
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