There's also some really compelling evidence that psoriasis is caused by bacteria. Psoriasis is often described as autoimmune, but is probably better described as immune-mediated disorder. Unlike autoimmune disorders like MS and lupus, psoriasis doesn't involve the body "attacking" itself.
Specifically, streptococcus bacteria such as Streptoccocus pyogenes, also called Group A Streptococcus, which is primarily responsible for rheumatic fever and scarlet fever in developing countries, and more commonly known in the West for causing strep throat (streptococcal pharyngitis).
We already know with almost certainty that S. pyogenes triggers a form of psoriasis called guttate, which in 60% of cases resolves on its own within 1-3 months. But in the remaining cases, it turns chronic. It can also come and go: spontaneously resolving and then coming back a few months after.
What's particular about strep is that it appears that it can lie dormant in the body. The tonsils of psoriasis sufferers have been found to contain strep bacteria, and a recent meta-study concluded that a tonsillectomy can significantly improve psoriasis in about 70% of patients who undergo the procedure; so it's likely that the tonsils act as a reservoir for continuous reactivation. (Apparently, people who have a tonsillectomy as a child are also less likely to develop psoriasis.) Strep bacteria are also thought to evade serological detection by hiding in biofilm, from which they periodically emerge to reactivate the immune response.
As to why only some people develop psoriasis, the explanation might be some kind of genetic predisposition. Psoriasis is often explained as maybe being caused by "cross talk" between the adaptive and innate immune systems.
Some papers:
Streptococcus pyogenes-induced cutaneous lymphocyte antigen-positive T cell-dependent epidermal cell activation triggers TH17 responses in patients with guttate psoriasis
https://pubmed.ncbi.nlm.nih.gov/27056267/
Specifically, streptococcus bacteria such as Streptoccocus pyogenes, also called Group A Streptococcus, which is primarily responsible for rheumatic fever and scarlet fever in developing countries, and more commonly known in the West for causing strep throat (streptococcal pharyngitis).
We already know with almost certainty that S. pyogenes triggers a form of psoriasis called guttate, which in 60% of cases resolves on its own within 1-3 months. But in the remaining cases, it turns chronic. It can also come and go: spontaneously resolving and then coming back a few months after.
What's particular about strep is that it appears that it can lie dormant in the body. The tonsils of psoriasis sufferers have been found to contain strep bacteria, and a recent meta-study concluded that a tonsillectomy can significantly improve psoriasis in about 70% of patients who undergo the procedure; so it's likely that the tonsils act as a reservoir for continuous reactivation. (Apparently, people who have a tonsillectomy as a child are also less likely to develop psoriasis.) Strep bacteria are also thought to evade serological detection by hiding in biofilm, from which they periodically emerge to reactivate the immune response.
As to why only some people develop psoriasis, the explanation might be some kind of genetic predisposition. Psoriasis is often explained as maybe being caused by "cross talk" between the adaptive and innate immune systems.
Some papers:
Streptococcus pyogenes-induced cutaneous lymphocyte antigen-positive T cell-dependent epidermal cell activation triggers TH17 responses in patients with guttate psoriasis https://pubmed.ncbi.nlm.nih.gov/27056267/
Group A streptococcal pharyngitis: Immune responses involved in bacterial clearance and GAS-associated immunopathologies https://pubmed.ncbi.nlm.nih.gov/28951419/
Psoriasis, chronic tonsillitis, and biofilms: Tonsillar pathologic findings supporting a microbial hypothesis https://pubmed.ncbi.nlm.nih.gov/29554401/
Psoriasis, chronic tonsillitis, and biofilms: Tonsillar pathologic findings supporting a microbial hypothesis https://pubmed.ncbi.nlm.nih.gov/29554401/
Tonsillectomy and the subsequent risk of psoriasis: A nationwide population-based cohort study https://pubmed.ncbi.nlm.nih.gov/33548305/
Mechanisms of microbial pathogenesis and the role of the skin microbiome in psoriasis: A review https://pubmed.ncbi.nlm.nih.gov/30981296/