1.in which是介词加关系代词,引导定语从句
in可以放到从句中去
our immune system breaks down in this condition
2.把in放进从句中应该是这样的：
it is transmitted from one person to another in the way.
不是都能组成动词短语的,你得了解介词加关系代词引导定语从句时,介词如何去选择,不光是由从句动词去决定,还可能是由先行词决定,等等.
（1）以从句中动词；形容词；名词与介词的固定搭配而定；
[高考考例]2.In the dark street,there wasn’t a single person _____ she could turn for help.[MET 1992]
A.that B.who C.from whom D.to whom
[思路点拨]答案为D.turn to sb for help 为固定搭配表示“求助于某人”
（2）以先行词与从句中的动词关系及所表达的含义而定或以先行词在定语从句中的作用与含义而定.
[考例]1.The medicine of science,______ progress has been very rapid lately,is perhaps the most important of all the science.
A.to which B.in which C.which D.with which
[思路点拨]答案为B.从句主谓结构搭配为：progress has been rapid in,这里的in意为“在某方面”,相当于rapid progress has been made in the science of medicine.
[考例]2.The professor made his first speech _____ he talked about some subjects the students were interested.
A.that B.which C.in which D.of which
[小结]关键词：介词的选用与从句主谓结构的搭配有关.
我帮你找了in which的用法,如果还有其他的问题的话,可以去小马过河问问那里专业的英语老师,