system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:. Why do auto immune diseases like psoriasis only target certain parts of the body or skin and not everything?
gpt
Psoriasis does affect the entire body, it simply is worse in some places for any of dozens if not hundreds of reasons. Some factors are sun damage, friction damage, chemical agitation, etc. 

There are also forms of psoriasis that affect more than the skin, such as psoriatic arthritis which affects joints.  With this form you will have more skin reactions at the joints damaged by the arthritis.