Question

You have three layers of skin which act as a barrier against

Answer 1

You have three layers of skin which act as a barrier against diseases.

这个句子本来就是定语从句.我不知道楼主哪里看到说这个是宾语从句的.

这个句子主谓宾都有：You have three layers of skin.这就是个完整的句子,后面which引导就是个定语从句,来修饰three layers of skin,并且which做从句的主语.

给你举2个宾语从句的例子,你就明白这两种句子的不同了.

I don't know （where he is from）.我不知道（他从哪儿来）

I think （that you are right）.我认为（你是对的）.

括号内为宾语.这就是宾语从句.