The problem is that the cancerous cells are themselves descended from healthy cells; other than the small differences that make them cancerous, they are in fact the same thing.

Many of the cutting edge immunotherapies for cancer essentially teach the immune system to target the cancerous cells.

However, in combination with an autoimmune disease like Chrons where the immune system has already learned to react to healthy cells, there is a much higher chance that an immunotherapy intended to target only cancerous cells also causes the immune system to target more healthy cells.

Yes, exactly. The specific risk for Crohn's is that the amped up immune system will rapidly kill ALL the gut cells that they've been pissed about for decades.

For the unfamiliar: Crohn's is to guts what eczema is to skin. They are both autoimmune diseases where the immune system attacks a specific kind of healthy cell. Unhelpful.