Johnson N C
Immunohematology Reference Laboratory, Greater Chesapeake and Potomac Region, 4700 Mt. Hope Drive, Baltimore, MD 21215, USA.
Immunohematology. 2011;27(2):68-71.
The XG blood group system is best known for its contributions to the fields of genetics and chromosome mapping. This system comprises two antigens, Xg(a) and CD99, that are not antithetical but that demonstrate a unique phenotypic relationship. XG is located on the tip of the short arm of the X chromosome with exons 1 to 3 present in the pseudoautosomal region of the X (and Y) chromosome(s) and exons 4 to 10 located only on the X chromosome. Xg(a) demonstrates a clear X-linked pattern of inheritance. MIC2, the gene encoding the CD99 antigen, is found in the pseudoautosomal region of both the X and Y chromosomes. Anti-Xg(a) is comparatively rare, and only two examples of anti-CD99 have ever been identified. Alloanti-Xg(a) is considered clinically insignificant; only one example of autoanti-Xg(a) has been reported, but it resulted in severe hemolytic anemia. Insufficient data exist to determine the clinical significance of anti-CD99. Linkage of XG to several X-borne genes encoding inherited disorders has been demonstrated. CD99 is an adhesion molecule, and high levels are associated with some types of cancer.
XG血型系统因其在遗传学和染色体图谱领域的贡献而最为人所知。该系统包含两种抗原，即Xg(a)和CD99，它们并非等位基因，但呈现出独特的表型关系。XG位于X染色体短臂的末端，其外显子1至3存在于X（和Y）染色体的假常染色体区域，而外显子4至10仅位于X染色体上。Xg(a)呈现出明显的X连锁遗传模式。编码CD99抗原的基因MIC2位于X和Y染色体的假常染色体区域。抗Xg(a)相对罕见，仅鉴定出两例抗CD99。同种抗Xg(a)被认为在临床上无显著意义；仅报道过一例自身抗Xg(a)，但它导致了严重的溶血性贫血。目前尚无足够数据确定抗CD99的临床意义。已证明XG与几个编码遗传性疾病的X连锁基因存在连锁关系。CD99是一种黏附分子，高水平与某些类型的癌症相关。
