Explain why the Cell A (nerve cell) and Cell B (red blood cell) cannot switch jobs

Red blood cells carry a specific type of protein, called hemoglobin, which is able to bind oxygen and release it in other tissues. A nerve cell doesn't have this protein because it doesn't need to bind oxygen. Instead, the nerve cell uses other proteins that help it transmit signals to other cells.

That is copied from “gene expression” article

But all in all, the cells have different structures and different jobs. This means that they are produced for that one job and cannot be made to do another job that they aren’t specifically made for. As well the dna make up will produce these cells specifically and in different cells than the other.

Hopefully this helps some. Have a nice day!