Some proteins in body are usually not seen by immune system e.g. lens of eye. If eye is injured and lens protein gets into blood, antibodies are made to it. Diseases sometimes stimulate antibodies that can also react with a normal body protein e.g. rheumatic fever, in which antibodies against streptococcus later react with heart muscle proteins. But, some have the allele but not the disease. One copy of the gene doesn't make enough product.
Precursors build up in blood and cause dementia.
Too much enzyme activity. Normally a minor side pathway from adenosine to inosine. Most adenosine normally goes to ATP, only a small amount to inosine. Mutant that makes 50 x too much ADA shunts most adenosine to inosine, leading to ATP deficiency, which gives fragile red blood cells and anemia.