A lot of research has been done that shows benefits from probiotics.
I read a book The Probiotics Revolution by Gary Huffnagle. He's a researcher on interactions between gut microbes and the immune system.
Apparently one's gut microbes can have a huge effect on health. Gary Huffnagle says that he personally had severe allergies and asthma, and after about a month of doing things to improve his gut ecosystem, his previously severe mold allergy was much better! Gradually in the years after that, his allergic rhinitis became so mild it no longer bothers him.
I've been quite sick for years, and it may be the result of being on antibiotics for months at a time. My immune system is be very sensitized, I have (probably two) autoimmune diseases - and it may have been very bad for me, to further harm my gut bacteria. Also since sugar picks me up when I'm sick with allergies, I've tended to eat a lot of it, and that may encourage Candida, which probably makes allergies worse. Gary Huffnagle did research on animals where a Candida overgrowth made their allergies worse.
I've been culturing vegetables. It's easy to grow lactic acid bacteria by shredding vegetables and letting them sit in a salty brine for about 5 days. The "good" bacteria are anaerobic and there are special pickling jars to create an anaerobic environment and encourage more of them. But I get lots of acid, just leaving the vegetables in saltwater in plastic jars - so I think even doing it that simply, works fairly well. The "good" lacto-bacteria make lactic acid as they grow. It's similar to making alcohol with yeast fermentation.
The cultured vegetables are tasty. I've been culturing veggies I'm not allergic to. So far, daikon and spaghetti squash are good this way.
I'm getting some pickling jars though, to do it more anaerobically and hopefully make a better probiotic.
That looks like a good book.
It is, and I'm HOPING that his optimism about probiotics is true. My allergies are better - they've gone from atrocious to horrible.
Probiotics are becoming mainstream, as more is found out about just how important the gut microbiome is.
There's a Ubiome project that tells people the composition of the microbial communities on their bodies, while they contribute data for research.