64 有關第一型與第二型主要組織相容性複合體（MHC class I and class II）的分子之描述，下列何者正確？

MHC class I有較多的多樣性（polymorphism）

共顯表現來自父母的對偶基因

MHC class I可以表現在所有的細胞上

與抗原蛋白片段結合區均由2條胜肽鏈組合而成

answer B

相容性複合體: major histocompatibilty complex

MHC class I molecules are one of two primary classes of major histocompatibility complex (MHC) molecules (the other one being simply MHC class II) and are found on every nucleated cell of the body (and thus not on red blood cells though paradoxically are found on platelets). Their function is to display fragments of proteins from within the cell to T cells; healthy cells will be ignored while cells containing foreign proteins will be attacked by the immune system. Because MHC class I molecules present peptides derived from cytosolic proteins, the pathway of MHC class I presentation is often called the cytosolic or endogenous pathway.

MHC (major histocompatibility complex) Class II molecules are found only on a few specialized cell types, including macrophages, dendritic cells and B cells, all of which are professional antigen-presenting cells (APCs).

The peptides presented by class II molecules are derived from extracellular proteins (not cytosolic as in class I); hence, the MHC class II-dependent pathway of antigen presentation is called the endocytic or exogenous pathway.

Loading of class II molecules must still occur inside the cell; extracellular proteins are endocytosed, digested inlysosomes, and bound by the class II MHC molecule prior to the molecule's migration to the plasma membrane.