As someone with MM, I've read a lot and talked to various experts. They often say there's no single cause for MM but a combination of genetic and environmental factors. A weakened immune system might not directly cause MM, but it can certainly play a role in how the disease manifests and progresses.

Hi there. I'm sorry to hear about your brother's diagnosis. I'm not a doctor but i dont think a weakened immune system is a direct cause of Multiple Myeloma

Another way to look at this is that Multiple Myeloma, as well as some treatments, challenge immunity.