PD-L1 is a type I transmembrane protein of 290 amino acids, and it is a member of the B7 family. Human PD-L1 has 70% amino acid identity to its mouse orthologue. Binding of PD-L1 to its receptor PD-1 leads to the inhibition of T cell receptor–mediated lymphocyte proliferation and cytokine secretion. PD-L1 induces IL-10 production in T cells stimulated with low levels of anti-CD3. PD-L1/PD-1 interaction suppresses immune responses against autoantigens and tumors and plays an important role in the maintenance of peripheral immune tolerance. Disruption of the PD-L1 gene leads to up-regulated T cell responses and the generation of self-reactive T cells. Antibodies against PD-1 or PD-L1 leads to increased antitumor immunity. PD-L1 has an important role in conferring fetomaternal tolerance in an allogeneic pregnancy model; antibodies against PD-L1 lead to a breakdown in maternal tolerance to the fetus. PD-L1 shares its receptor with PD-L2 (CD273, B7-DC). PD-L2 has a more limited expression than PD-L1, being expressed on activated macrophages and dendritic cells. PD-L1 is expressed in many tumors, and the interaction with its receptor activates signaling pathways that inhibit T-cell activity and therefore the antitumor immune response. Antibodies targeting PD-1 or PD-L1 block the PD-1 pathway and reactivate T cell activity.
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本试验阳性结果主要见于下列几种情况：
1.自身免疫性贫血，（IgG）型引起的溶血性贫血，本试验直接反应常呈强阳性，间接反应大多阴性，但亦可阳性。
2.药物诱发的免疫性溶血性贫血①α-甲基多巴型：直接及间接反应均阳性。②青霉素型：直接反应阳性，间接反应阴性，以上二型如以正常红细胞先与有关药物于37℃培育后再加病人血清、间接反应均为阳性。③福阿亭型：（奎宁等药物）抗体通常为IgM，偶有IgG型者，直接反应为阳性，间接反应阴性，但如用IgG抗血清做试剂则结果大部分均为阴性，但如培育时加入新鲜正常人血清（供应补体）则结果为阳性。
3.冷凝集素综合征直接反应阳性，间接反应阴性（试验需在37℃下进行）由于本病红细胞膜附着的是补体C4和C3而不是IgG或IgM，如果用抗IgG或抗IgM抗血清做试验时，则结果阴性，如以抗补体的抗血清做试验则直接反应阳性。
4.新生儿同种免疫溶血病，因Rh血型不合所致溶血病，直接及间接反应均强阳性，持续数周、换血输血后数天内可变为阴性，由于“ABO”血型不合引起的溶血病，结果常为阴性或弱阳性。
5.红细胞血型不合引起的输血反应，ABO或Rh血型不合输血，供者的红细胞被受者的血型抗体致敏，在供者被致敏的红细胞完全破坏以前，直接反应阳性，Rh阴性者如过去不曾接受过Rh阳性者的血或曾妊娠胎儿为Rh阳性者，间接反应阳性，如无上述接触，第一次输血后（Rh阳性的血），数天之内间接反应也会变为阳性。
6.其它在传染性单核细胞增多症、SLE、恶性淋巴瘤、慢性淋巴细胞白血病、癌肿、铅中毒、结节性动脉周围炎、EVan氏综合征等，病人直接反应亦可阳性，阵发性寒冷性血红蛋白尿症患者中，急性发作后用抗补体血清做试验直接反应常为阳性。展开
