system
Given the following post, respond as if you're commenting on Reddit:
human
Why can't your immune get rid of bacterial STDs like syphilis etc?
gpt
It varies depending on the pathogen. When it comes to syphilis in particular, the outer cell membrane has unusually few of its own antigens (proteins) in it, which makes it harder for the immune system to detect it. It can also vary its antigens by gene conversion.

That said, the immune system can combat syphilis. It's just usually not enough to clear the infection. When the bacteria lyse (split open), their internal components are easily recognized by the immune system - their camouflage is only skin-deep. This can actually be a problem when treating syphilis, as a bunch of bacteria dying at the same time can make the immune system overreact to the suddenly abundant foreign antigens etc.

STDs aren't necessarily anything special in terms of ability to evade the immune system. Lots of pathogens have found ways to do that. Mycobacterium tuberculosis, for example, has a coating that allows it to survive inside the phagolysosome of macrophages; the substances the macrophage pumps into the phagolysosome would kill almost any other bacterium.