I dun have first hand experience with my pets regarding to autoimmune.. But I do have first hand experience in human.. Yes, I , myself have autoimmune disorder myself.. But in human, which I believe, the same with the animals, has a vast variety of it.. Some may be deadly...some may not.. As for me, mine is not deadly as my autoimmune loves to attack my hair follicle which makes the hair fall and started to leave bald patches on my head.. I was on steroid before... that is the only solution to it... Orally and direct injection to the spots... I remembered I used to get more than 3 injections of steroid when I was treated last year..
As per using steroid (tho my doc did not specifically announced this tho I asked several times, running away from the type of pills he is giving me )..I know it was steroid as my face bloated tho my body not..THen after awhile.. The steroid began to show its bad part where it eats up 2 of my toe nails.. I end up without any toe nails before ( which I was so damn freak out that I wont be having any)..Now, I am okay.. Now, I stopped the treatment as I am okay now.. as usual as normal, having great hair like others.. But then, I hav eto be careful to remind doctors that treat me afterwards that I have Alopecia Areata..which their antibiotic may harm me if it is too strong..as now my immune system already attacking my parts of body.. It goes away.. No steroids and my face back to normal..
As my cousin, she lost her battle to autoimmune also..Hers was deadly as the autoimmune attacks the white blood cells of hers or more known as LUPUS...
Some of you may think it is inherit..but it is not as until now..my doctor or any other doctors cannot tell why it is happening... My doc just said 1:1000 people has it and sorry to say..you are the one of one thousand...
So..as for your cat.. try to verify which autoimmune and which part it is attacking.. Ask your vet over and over again... Maybe consult more than 1 vet.. Get those who are serious in saving pets life..
As for my advise...your cat is unlucky..so dun make it be unlucky the rest of his/her life..