What would be a valid reason for human blood/sweat/body fluids being toxic/harmful/caustic to an alien species' skin?
What would the aliens' organic biochemistry be like and what would their DNA be based on for this to occur in a plausible way?
Any variable degree of reaction to the aliens' skin?
The first thing that came to my mind was iron. Human, and mammalian blood in general, 'captures' more oxygen than the normal oxidation process of iron. This process is complex, and requires many stages and components (enzymes, for instance). The blood system, and red blood cells (RBCs) in particular, have developed unique defensive mechanisms to preserve this functionality.
Studies from our laboratory have shown that oxidative stress plays a significant role in damaging the RBC membrane and impairing its deformability. RBCs are continuously exposed to both endogenous and exogenous sources of reactive oxygen species (ROS) like superoxide and hydrogen peroxide (H2O2). The bulk of the ROS are neutralized by the RBC antioxidant system consisting of both non-enzymatic and enzymatic antioxidants including catalase, glutathione peroxidase and peroxiredoxin-2. However, the autoxidation of hemoglobin (Hb) bound to the membrane is relatively inaccessible to the predominantly cytosolic RBC antioxidant system. This inaccessibility becomes more pronounced under hypoxic conditions when Hb is partially oxygenated, resulting in an increased rate of autoxidation and increased affinity for the RBC membrane.
From Red blood cell oxidative stress impairs oxygen delivery and induces red blood cell aging for instance.
So the alien species, having evolved an entirely different blood system, would have a completely different method to preserve tissue integrity. If, for instance, the alien method required ROS (from the quote) systems to function properly, our blood antioxidant defensive system is designed to naturally neutralize such systems. For instance, if their planet were not rich in iron, they may have used hydrogen peroxide as a transport method. Hydrogen peroxide is very reactive, and eagerly gives off the extra oxygen molecule to form water. Their 'hemoglobin' could conceivably have evolved into a method that binds an extra oxygen molecule to water in the 'lungs', and then releases it in the tissue.
The very systems that our blood has evolved to protect the functionality of our blood hemoglobin system could be precisely the system that destroys their naturally evolved systems to protect their equivalent to our hemoglobin.
Co-incidentally, this would make their blood just as 'toxic' to humans. It would be like pouring hydrogen peroxide on it. Note, hydrogen peroxide was once used as an antiseptic, until it was found that its oxidizing properties also destroys healthy cell tissue, and delays healing.
Why Does Hydrogen Peroxide Fizz On Cuts?
Despite its negative effect on healthy cells, our bodies' cells naturally produce hydrogen peroxide when we metabolize food and turn it into energy. So how can a cell produce something that can destroy its own walls? That's where catalase steps in: when a cell creates hydrogen peroxide, it stores it inside the cell's specialized organelles, called peroxisomes, which contain hydrogen peroxide-busting catalase. Inside of a peroxisome, hydrogen peroxide decomposes and is turned into harmless water and oxygen gas .
This quote, incidentally, also posits a credible system for a hydrogen peroxide based blood oxygen delivery system, since the mechanism is already present in some form in earth-based mammals.
Your question did not mention how fast you want the reaction to be.
The most unexotic answer would be that we are the peanuts to their allergy. Some little molecule from the plethora of substances in our blood triggers their immune response which then goes into a tailspin, sowing destruction on their tissues and organs. The molecule would need to be small to guarantee fast diffusion through the aliens' skin, and ubiquitous in humans - nucleotides come to mind.
We would not even need to carry the actual allergen, we might just carry a protein that works as an enzyme on some molecule prevalent on the aliens' skin, breaking that down into an allergen.
There are many ways to kill an organism - I propose you look at a suitable killer substance or mix (VX to humans, Roundup to green plants, Oxygen to anaerobic bacteria) and just decree that something in the humans' blood works on the aliens in an equivalent way. - Note that sometimes, as in the case of Roundup, a mixture of substances is needed, as the actual killer can not enter into the organism alone.
