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xenotype 754 X-linked lymphoproliferative syndrome
Xenopus.
to potentially increase the supply of organs for human
transplantation.
xenotype
Molecular variations based on differences in structure and
antigenic specificity. Examples include membrane antigens
of cells or immunoglobulins from separate species.
xenozoonosis
Transmission of an infection that may be a consequence of
xenotransplantation. Such infections may involve infection
of recipient cells with endogenous retroviral sequences
from donor cells, giving rise alone or after recombination
with human endogenous retroviral sequences to previously
unknown pathogenic viruses. Such new viruses may be
pathogenic for humans other than the xenograft recipient.
xeroderma pigmentosum (XP)
A rare disorder attributable to XPA-XPG gene mutations
that participate in the nucleotide excision pathway of DNA
repair. It is associated with profound sensitivity to sunlight
and variable CID.
Xg a
The sex-linked blood antigen is an antibody more common
in women than in men. It is specific for the Xg a antigen, in
recognition of its X-borne pattern of inheritance. The table
below shows phenotype frequencies in Caucasian males and
females. The antibody is relatively rare and has not been
implicated in hemolytic disease of the newborn or hemolytic
transfusion reaction although it can bind complement
and may occasionally serve as an autoantibody. Anti-Xg a
antibodies may be of value in identifying genetic traits
transmitted in association with the X chromosome.
Xg a : Sex-linked Blood Antigen
Reaction with
Anti-Xg a
Phenotype
Frequency
Phenotype
Male Female
Xg (a+) + 65.6 88.7
Xg (a–) 0 34.4 11.3
xid gene
An X-chromosome mutation. When homozygous, it leads
to diminished responsiveness to some thymus-independent
antigens, a limited decrease in responsiveness to thymicdependent
antigens, and defective terminal differentiation
of B cells. Many autoimmune features are diminished when
the xid gene is bred into autoimmune mouse strains. These
include reduced anti-DNA antibody levels, diminished renal
disease, and increased survival.
X-linked agammaglobulinemia (XLA)
A disease of Btk tyrosine kinase characterized by increased
susceptibility to infections beginning most often during the
first year of life when transferred maternal immunoglobulin
is catabolized. Arrested B cell development at the pre-B
cell stage blocks formation of mature B cells and antibody
synthesis. Serum analysis reveals a pronounced decrease in
immunoglobulin levels of all isotypes. IgA is usually undetectable
and a humoral response to recall antigens is virtually
absent. B lymphocyte and plasma cell numbers are markedly
decreased, whereas T lymphocyte subsets show a relative
increase. The defect is caused by a differentiation arrest
confined to the B cell lineage, distinguishing XLA from
most other immunoglobulin deficiencies. B lineage cells in
all organs are affected, resulting in reduced sizes of lymph
nodes and tonsils. The defective gene encodes a cytoplasmic
tyrosine kinase, designated Btk. Mutation analysis confirms
the diagnosis and can be used to identify healthy carriers.
Treatment consists of antibiotics to combat ongoing infections
and γ globulin substitution as a prophylaxis.
X-linked hyper-IgM syndrome
Refer to hyperimmunoglobulin M syndrome.
X-linked lymphoproliferative disease (XLP)
A combined immunodeficiency involving T and B lymphocytes.
It is exacerbated following exposure to Epstein–Barr
virus (EBV), one of the eight known human herpesviruses.
In older children and adults, EBV is the causative agent of
infectious mononucleosis, frequently a self-limiting polyclonal
lymphoproliferative disease with an excellent prognosis.
In immunosuppressed individuals, EBV infection
may lead to life-threatening lymphoproliferative disorders
and lymphoma. EBV has been primarily associated with
certain malignancies such as endemic Burkitt’s lymphoma
(BL), nasopharyngeal carcinoma (NPC), certain B and
T cell lymphomas, and approximately 50% of Hodgkin
disease. The XLP (LYP) gene locus has been mapped to
Xq25. In general, XLP has an unfavorable prognosis, but
transplantation of hematopoietic stem cells from a suitable
donor may cure this immunodeficiency. The SAP/SH2D1A
gene is altered. The two sets of target molecules for this
small SH2 domain-containing protein include (1) a family
of hematopoietic cell surface receptors (SLAM family), and
(2) a second phosphorylated adapter molecule. EAT-2, a
SAP-like protein, also interacts with these surface receptors.
SAP/SH2D1A is a natural inhibitor of SH2-domaindependent
interactions with SLAM family members.
Dysgammaglobulinemia and B cell lymphoma can occur
without prior EBV infection. The SAP/SH2D1A gene
controls signaling via SLAM surface receptors, thereby
playing a critical role in T cell and antigen-presenting cell
(APC) interactions during viral infections. XLP leads to an
inability to control EBV infections. It is characterized by
cytotoxic T lymphocyte-mediated immunopathologic injury
in response to EBV infections.
X-linked lymphoproliferative syndrome
A type of infectious mononucleosis in X-linked immunodeficiency
patients who manifest Epstein–Barr virus nuclear
antigen (EBNA)-positive lymphoid cells, polyclonal B lymphocyte
proliferation, and plasma blasts. The disease may