My daughter is constantly sick. And has asthma problems breathing what can I do to strengthen her immune system?

Consider Probiotics. There have been a couple well-executed (prospective, randomized, controlled) clinical trials of probiotics in children, published in the journal, pediatrics. Both found a significant reduction in the frequency, severity, and duration of uris, in children taking a daily probiotic preparation. Further research suggests that the effect is immune-mediated. Please check my blog for more on probiotics.
Adequate sleep. One of the most important interventions anyone can do to help maintain or improve their immune system is to obtain adequate sleep. There are many studies, which show we typically do not get the sleep we need.
IG deficiency? Would consider asking her physicians if she has been checked for immunoglobulin deficiencies, which can manifest as recurrent respiratory infections. (especially iga).