As we know iron rust when it react with oxygen then why iron in our body does not rust when it reacts with oxygen molecules in blood?
To an extent it does. The iron in our body is used to carry oxygen from our lungs to the other tissues of the body. to do this , each iron atom is bound to a large, multiringed molecule to form heme. Each heme is, in turn , bound to a subunit of a protein called hemoglobin. There are four of these subunits per hemoglobin protein, so each whole hemoglobin contains four bound iron atom that allows it to carry four oxygen molecules. In order to bind the heme ring and still have electrons available to bind oxygen, the iron atoms must be oxidised to Fe(II), or ferrous , atoms. That is, the iron in our blood is not metallic iron, but is already oxidised ("rusted") before it even sees oxygen. As it binds oxygen in the lungs, the ferrous iron atom donates an electron and becomes Fe(III) , or ferric atom. Put another way oxygen binds hemoglobin , it furthur oxidises ("rusts") the already oxidised iron contained in it.
Thank you vishnu