system
Given the following post, respond as if you're commenting on Reddit:
human
If cancer and psoriasis are both overactive cell growth, what is it that makes them different?

If you were to compare skin cancer vs. psoriasis they are both described as overactive cell growth, though with different results and degrees of danger. A) how are they different, and B) what is it that makes cancer more dangerous than psoriasis?
gpt
oncology doc - Interesting question!

The short answer is that you're right, psoriasis and cancer are both due to overgrowth of cells - but different kinds of cells, and for very different reasons. 

Try to visualize this a pyramid. At the apex is a single cell type. On the next level, there are two cells. Then 4 cells on the level below that. That's sort of how cellular differentiation works in the body; a non-specific cell gives rise to slightly more differentiated cells, which gives rise to slightly more differentiated cells, etc, until you have mature cells of X,Y,or Z function. 

There are a multitude of different kinds of cancer, all of which are due to an uncontrolled proliferation of a certain cell type. Leukemia, for instance, is due to clonal overgrowth/expansion of an immature blood cell precursor. Something like, say, breast cancer, is due to an uncontrolled overgrowth of the cells that comprise certain types of breast tissue. Cancer cells have typically undergone/acquired some mutation that allows them to either proliferate without the need for external stimulus, or ignore external stimuli telling them to stop dividing, or shuts off a cellular 'self-destruct' mechanism that should make them stop once DNA damage occurs.

In psoriasis, there is an overgrowth of a well differentiated type of cell - the keratinocyte. HOWEVER, and this is the key, the overgrowth is not due to a driver mutation as in cancer, but rather, the result of an inflammatory stimulus. In psoriasis, there is hyper activation of the pro-inflammatory/immune cells within the skin. That activation causes them to release cellular signals (cytokines) that encourage the keratinocytes to proliferate and divide far faster than they usually would, because those signals are meant to be released when local destruction occurs and extra cells are needed to fill in the damage. That proliferation isn't due to some mutation in the keratinocyte, but rather, overactivation of immune cells, which are telling them to do so. To take it a step further, the keratinocytes are then attacked by the immune system, release signals that cause the recruitment of immune cells (because there is localized inflammation), which continues the process. 

TL;DR: Cancer is an uncontrolled proliferation of cells. Psoriasis is an immune deregulation where inflammatory cytokines are causing local inflammation, which causes cells to proliferate secondarily. 

I hope that helps!