For reasons not fully understood, organic chemistry demonstrates a significant statistical preference for the use of "left-handed" amino acids in cellular life: of the twelve intelligent biological species currently known to inhabit the galaxy, only quarians and turians use dextro-amino acids in their biological processes; asari, batarians, drell, elcor, hanar, humans, krogan, salarians, volus, and vorcha all metabolise levo-amino acids. This trend appears to be universal: only about 15% of "garden worlds" surveyed within Citadel Space are dextro-amino, the remainder are levo-amino.
Although commonly referred to as an "allergic reaction" in non-medical circles, acute chirality poisoning is a distinct condition that occurs on a cellular level when cross-chiral materials are absorbed and become metabolically toxic. Although incidental exposure via ingestion, inhalation, or prolonged skin contact is universally harmless, the contamination of significant portions of a patient's nutritional intake can lead to a wide variety of symptoms ranging from malnutrition to stomach and intestinal cramps, bloating, diarrhea, internal bleeding, complete intestinal organ failure, and gum disease.
Prompt treatment with genetically-engineered probiotics are capable of alleviating the symptoms of chirality poisoning in most cases, although these alterations to gut flora typically have adverse side effects on the digestive system or its equivalents. Low doses of drugs capable of binding to cross-chiral molecules to form inert compounds are routinely taken in order to eliminate stress on the liver and kidneys when exposed to trace amounts of cross-chiral material in public environments where individuals of different species tend to congregate.
While the krogan are commonly considered "bi-chiral," their metabolic processes are in fact strictly levo-amino- this misconception stems from the presence of several varieties of symbiotic bacteria within the krogan stomach that enable the full digestion and conversion of dextro-amino materials. Periodic attempts to market products derived from these bacteria as a means of enabling other species to consume cross-chiral foodstuffs or as an improved treatment for chirality poisoning have run afoul of Council restrictions on genetic engineering and the "ownership" of sapient tissues. Additionally, levo- and dextro-amino compounds often differ significantly in taste.
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Author's Note:
I had just a whale of a time trying to track down any actual scholarly information on the impact or toxicity of dextro-amino acids- apparently nobody in the entire history of medical science thought to feed rats food laced with dextro nutrients and write down whether or not they died. Eventually I was able to locate a systematic review from way back in 1953 (Physiology of the D-Amino Acids: Clarence P. Berg, Physiological Reviews, Volume 33: 145) which did just that. By modern standards it's rather vague, many of the cited studies demonstrate severe methodological flaws, and not having any real knowledge of the field a lot of it just didn't make sense; but it describes rats being fed diets of up to 12% by mass dextro-amino materials and surviving long enough for their growth to be measured, so while switching rations with a turian probably wouldn't be very good for you in the long run (and would probably taste extremely odd) I highly doubt any reasonable amount of contact is going to have physiological effects. I suppose it's possible that someone could develop an allergic reaction to alien proteins, but that would be rare, no different from any other sort of allergy, and no more likely to occur with quarians than with asari.
I had considered making a Thing of this, having fears of 'allergic reaction' being semi-deliberately promulgated as a way to keep turians isolated from the other races and tying into other in-universe animalization or 'dangering-up' of their species, but then I remembered I myself live in a world where people still think cellular telephones can cause cancer and there really doesn't need to be any deeper reason why uninformed medical scare-stories would develop and circulate.
Observant readers will note the complete absence of the Yahg in the list up above. Once again, I really did not see the purpose of adding them in LOTSB when the Broker could've just been an AI or something; I think the idea of a non-FTL species so aggressive they had to be quarantined by the Council was interesting (in fact, one of my complaints about LOTSB is that no other development of the Yahg was attempted beyond it) but, like Liara, PD did not have "room" to develop them and so they became something of a sacrifice of expedience.