it 句子成分剖析

Cholesterol builds up in our body and it's this build-up that's thought to be harmful.
请问老师，后面的it指代的是什么呢？that's thought to be harmful怎么样剖析？

Cholesterol builds up in our body and it's this build-up that's thought to be harmful.

【翻译】胆固醇在大家体内积聚，正是这种积聚被觉得是有害的。

黑体部分为强调句。it is 引出被强调的成分，本句被强调的成分是主语  this build-up。

还原剖析：This build-up （主语）is thought （谓语动词）to be harmful（主语补足语）. 

不定式 to be harmful， 在主动句中是宾语补足语；在被动句中是主语补足语。