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leukoagglutinin 447 leukocyte common antigen (LCA, CD45)
leukoagglutinin
An antibody or other substance that induces the aggregation
or agglutination of white blood cells into clumps.
leukocidin
A cytolytic bacterial toxin produced especially by staphylococci.
It is toxic principally for polymorphonuclear leukocytes
and to a lesser extent for monocytes. It contains F and
S components that combine with cell membranes, causing
altered permeability. Less than toxic doses interfere with
locomotion of polymorphonuclear neutrophils (PMNs).
leukocyte
White blood cell. The principal types of leukocytes in the
peripheral blood of humans include polymorphonuclear
neutrophils, eosinophils and basophils (granulocytes), and
lymphocytes and monocytes.
leukocyte activation
The first step in activation is adhesion through surface
receptors on cells. Stimulus recognition is also mediated
through membrane-bound receptors. An inducible
endothelial–leukocyte adhesion molecule that provides
a mechanism for leukocyte–vessel wall adhesion has
been described. Surface-adherent leukocytes undergo
a large prolonged respiratory burst. NADPH oxidase,
which utilizes hexose monophosphate shunt-generated
NADPH, catalyzes the respiratory burst. Both Ca 2+ and
protein kinase C play key roles in the activation pathway.
Complement receptor 3 (CR3) facilitates the ability
of phagocytes to bind and ingest opsonized particles.
Molecules found to be powerful stimulators of polymorphonuclear
neutrophil (PMN) activity include recombinant
interferon-γ (IFN-γ), granulocyte–macrophage
colony-stimulating factor (GM-CSF), tumor necrosis
factor (TNF), and lymphotoxin.
leukocyte adhesion deficiency (LAD)
Recurrent bacteremia with staphylococci or Pseudomonas
linked to defects in the leukocyte adhesion molecules
known as integrins, which mainly alters the ability of leukocytes
to reach extracellular pathogens at sites of infection,
thereby preventing their effective eradication. These
include the CD11/CD18 family of molecules. CD18 β
chain gene mutations lead to a lack of complement receptors
CR3 and CR4 to produce a congenital disease marked
by recurring pyogenic infections. Deficiency of p150,95,
leukocyte-function-associated antigen 1 (LFA-1), and
complement receptor 3 (CR3) membrane proteins leads to
diminished adhesion properties and mobility of phagocytes
and lymphocytes. All three of these molecules share
a flaw in the synthesis of the 95-kDa β chain subunit.
The defect in mobility is manifested as altered chemotaxis,
defective random migration, and faulty spreading.
Particles coated with C3 are not phagocytized and
therefore fail to activate a respiratory burst. The CR3 and
p150,95 deficiencies account for the defective phagocytic
activity. The T cells of patients with LAD fail to respond
normally to antigen or mitogen stimulation and are also
unable to provide helper function for B cells producing
immunoglobulin. They are ineffective in fatally injuring
target cells, and they do not produce the lymphokine
interferon-γ (IFN-γ). LFA-1 deficiency accounts for the
defective responses of these T lymphocytes and all natural
killer (NK) cells that also have impaired ability to fatally
injure target cells. Clinically, the principal manifestations
are consequences of defective phagocyte function rather
than of defective T lymphocyte function. Patients may
have recurrent severe infections, defective inflammatory
responses, abscesses, gingivitis, and periodontitis. LAD
has two forms; those with the severe deficiency do not
express the three α and four β chain complexes, whereas
those with moderate deficiency express 2.5 to 6% of these
complexes. The two human LAD syndromes thus far
recognized include LAD I, in which the integrin family is
defective, and LAD II, in which sialyl Lewis X , the ligand
for selectins, is absent. LAD has an autosomal-recessive
mode of inheritance; LAD I is attributable to mutations
in the gene that encodes the CD18 protein which is part
of the β 2 integrins, whereas mutations in the gene that
encodes an enzyme needed for the synthesis of leukocyte
ligands for endothelial selectins causes LAD II.
leukocyte adhesion molecule-1
A homing protein found on membranes that combines with
target-cell-specific glycoconjugates. It helps regulate migration
of leukocytes through lymphocytes binding to high
endothelial venules and neutrophil adherence to endothelium
at inflammatory sites.
leukocyte adhesion molecules (LAMs)
Facilitators of vascular endothelium aggregation, chemotaxis,
cytotoxicity, binding of iC3b-coated particles,
lymphocyte proliferation, and phagocytosis. The three
main families of LAMs include the selectins, integrins, and
immunoglobulin superfamily. Leukocyte adhesion deficiencies
are partial or complete inherited deficiencies of cell
surface expression of CD18 and CD11a–c. These deficiencies
prevent granulocytes from migrating to extravascular
sites of inflammation, leading to recurrent infections and
possibly death.
leukocyte adhesion protein
A membrane-associated dimeric glycoprotein composed of
a unique α subunit and a shared 95-kDa β subunit involved
in cell-to-cell interactions. This protein group includes
lymphocyte function-associated antigen 1 (LFA-1) found on
lymphocytes, neutrophils, and monocytes; membrane attack
complex 1 (MAC-1) found on neutrophils, eosinophils,
natural killer (NK) cells, and monocytes; and p150,95,
which is common to all leukocytes.
leukocyte chemotaxis inhibitors
Humoral factors that inhibit the chemotaxis of leukocytes.
They play a role in the regulation of inflammatory
responses of both immune and nonimmune origin.
leukocyte common antigen (LCA, CD45)
A family of high molecular weight glycoproteins (180 to
220 kDa) densely expressed on lymphoid and myeloid cells,
including lymphocytes, monocytes, and granulocytes.
Expression of LCA on leukocytes but not on other cells
makes LCA a valuable marker in immunophenotyping
of tumors to determine histogenetic origin. LCA antigen
function is unknown, but it has a high carbohydrate content
and is believed to be associated with the cytoskeleton. LCA
molecules are heterogeneous and appear on T and B lymphocytes
and selected other hematopoietic cells. Some LCA
epitopes are present in all LCA molecules, while others
are confined to B lymphocyte LCA; still other epitopes are
associated with B cell, CD8 + T cell, and most CD4 + T cell
LCA molecules. About 30 exons are present in the gene that
encodes LCA molecules; it is designated as CD45.
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