Well, the way that your immune system recognizes it is that all the cells of our body express a molecule on their surface, a membrane protein called the MHC1 complex.

而免疫系统识别的方法是,身体内所有细胞在其表面表达一种分子,一种叫做MHC1复合体的膜蛋白

耶鲁公开课 - 生物医学工程探索课程节选

Essentially, what they are is it's a protein that spans the membrane of a cell.

本质上来说，它是一个横跨细胞膜的蛋白质。

麻省理工公开课 - 化学原理课程节选

For now, just picture it in this simple way as a molecule, usually a protein, that's embedded in the cell membrane and many receptors have a part of them that is extracellular.

现在,我只把这种分子图示出来,受体通常是一种蛋白质,它们镶嵌在细胞膜上,很多受体还有部分结构露在细胞外

耶鲁公开课 - 生物医学工程探索课程节选