It is well established that autism spectrum disorders have a genetic origin and that this genetic inheritance results in abnormal brain growth regulation. There is much less information about potential immune system involvement in autism. There have been many reports of persons with autism having verified immune system "problems." This has led many people to suggest that there is immune system dysregulation involved. Most of this speculation occurs in fringe journals with relatively few large scale studies conducted by researchers clearly qualified to conduct such research. There is certainly a need for more study.
One recent study by Rosen and colleagues (2007; Pediatrics) examined whether more infections occurred in the first two years of life for children with ASDs relative to typically developing peers. Interestingly, there were no differences between the populations though children with ASDs actually had slightly fewer ear infections than their typically developing peers though children w/ASDs were more likely to have an infection in the first month of life and more likely to have a genito-urinary infection. These data have some clear limitations as the authors suggest, they are not definitive but suggest the need for further study of autism and the immune system. Early higher rate of infection, such as at one month, is interesting when taken together with the known increased rates of autism detected in mothers who contract certain diseases while pregnant (like rubella). These early environmental exposures are contrary to what many who would prefer to blame vaccines for autism suggest may be related to the cause of autism. If more time and research money were placed in such potentially useful research into pre-natal or very early peri-natal exposure to environmental conditions, then this would be a welcome change.
This, of course, does not directly address your question. If you feel your child has problems with their immune system, discuss this with your pediatrician to determine whether it would be wise to consult with an immunologist. Though allergies are common in all populations, when a child has one or more, it is also a good idea to have allergies more precisely examined. One excellent source of information on allergies is the MedlinePlus:Allergy. I'm pasting in the address to this website.
http://www.nlm.nih.gov/medlineplus/allergy.html
Rosen NJ, Yoshida CK, Croen LA. (2007). Infection in the first 2 years of life and autism spectrum disorders. Pediatrics, Jan;119(1):e61-9.
This is just an idea. But you could try seeing a nutricianist so that they can firstly look at what she is eating and drinking to make sure it is balanced. They would then be able to advise you on some vitamins and minerals that might aid her immune systems and also help to keep detoxing. Nothing drastic, just good common sense stuff ie. remove all caffine and stimulants and sugar from the diet, lots of water, herbal teas with nettle extract (I find blackberry and nettle good), vitamin B complex and vitamin C complex etc. But take advice, because she is only 7 and you don't want to overdose her on any vitamins as too many can be as bad as too little. Goats milk soap is very good for the skin, I have heard that epsom salts baths can also be good.
Organic food if you can afford it - or grow your own. Exercise is also good.
Do you think she might have an allergy to any pets? If you suspect dust mites you can replace carpets with wooden floors and only keep a few stuffed toys which you wash at above 60 degrees or which you freeze overnight to kill mites.
You can also buy air filters that can take allergens out of the air in the house.
Make sure all washing materials in the house are all organic, or make your own - it is easy to do and more effective than the stuff you buy.