but their specificity is very different这句话是什么意思？

Antibodies and T cell antigen receptors (see Chapter 7) are the two classes of molecules used by the adaptive immune system to specifically recognize and respond to antigens (Table 5.1). Major histocompatibility complex (MHC) molecules also bind peptide antigens, but their specificity is very different and their function is to passively display peptides to T cell antigen receptors, not to respond to antigens in the way that antibodies on B lymphocytes and T cell receptors (TCRs) on T lymphocytes do

抗体和T细胞抗原受体(见第7章)是适应性免疫系统用来特异性识别和应答抗原的两类分子(表5.1)。主要组织相容性复合体(MHC)分子也结合肽抗原，但它们的特异性非常不同，它们的功能是被动地将肽提呈给T细胞抗原受体，而不是像B淋巴细胞上的抗体和T淋巴细胞上的T细胞受体(TCR)那样对抗原作出反应。

问题：but their specificity is very different这句话是什么意思？是说MHC之间is very different，还是MHC与 Antibodies is very different，在什么地方不同？还是有别的的意思