Depending on your interpretation, Grendel's blood might be acidic or merely hot. Either way, it's quite a protective adaptation -- and one the xenomorph utilizes with brutal efficiency as well.* Cut it and you risk burning a hole in your ship's hull. Blast it with a shotgun and endure a flesh-eating arterial spray. In its facehugger stage, the creature is able to secrete these corrosive fluids in order to penetrate helmets and armor.
Xenomorph blood burns rapidly through inorganic material and liquefies human flesh on contact. The acidity neutralizes shortly after exposure, but not before dishing out extensive damage. Naturally, the creature is impervious to harm from its own corrosive fluids in the same way that our stomach linings protect us against our own stomach acid.
How can we possibly explain this adaptation? Well, we have two distinct possibilities to examine.
A Modest, Explosive Proposition
What if the acidic "blood" isn't blood at all? We see the thing excrete the acid when harmed. We see it weaponized the stuff. What if it's actually an acid stored throughout the body to react to physical harm, in turn harming the attacker?
We see something similar in terrestrial termites and ants, who also serve a queen. Many species practice autothysis, the process by which a natural-world organism destroys itself via the internal rupturing of an organ or gland that in turn ruptures the skin. It’s a purely muscular exercise, caused by deliberate contractions around the engorged tissue.
Just consider the termite Neocapritermes taracua -- the body buster par excellence. This species’ workers grow abdominal sacks of toxic blue crystals throughout their lives, but these “explosive backpacks” are most pronounced in elderly workers. Their mandibles are dull and useless, but their swollen poison pouches five them one last purpose in the defense of the colony. When enemy invaders bite into their bodies, the blue crystals combine with salivary secretions to produce a deadly chemical weapon.
Perhaps the xenomorph simply boasts a system of pressurized acid tubes and we merely witness (and suffer) the rupture of those tubes -- an act of highly-evolved alien autothysis.
Truly Alien Blood
If we're forced to consider a literal interpretation, however, we have to come to terms with actual acidic blood. What sort of organism might have blood that is either incidentally or adaptively corrosive to our sort of flesh life?
One possibility is that the xenomprophs are silicon-based rather than carbon-based lifeforms. Even here on carbon-biased Earth, silicic acid (a weak acid) occurs in hair, nails and the epidermis -- and this It seems conceivable that this or Hexafluorosilicic acid might factor into a silicone xenomorph’s anatomy. Furthermore, this is the exact sort of biochemistry that could enable a lifeform to thrive in a sulfuric-acid-rich environment.
Other possibilities present themselves. Some experts, including Carl Sagan, have speculated that alternate astrobiologies could still entail carbon, but depend on something other than water -- such as ammonia or hydrofluoric acid. Here's what the late astrobiologist had to say on the matter in 1980's "Cosmos."
Biochemists have also presented sulfuric acid as a possibility -- and it’s certainly more cosmically common. In the 2007 book "The Limits of Organic Life in Planetary Systems" by the National Research Council, we need look no further than Venus for inspiration:
Whether a matter of acidic autothysis or just exotic blood, the xenomorph's internal fluids are enough to make any creature think twice about fighting back -- though there's no stopping a determined hero's quest to wipe out your hive queen.
* Perhaps the xenomoprh and Grendel are one and the same?
Monster of the Week is a - you guessed it - regular look at the denizens is of our monster-haunted world. Sometimes we'll focus on the cultural aspects, but mostly we'll look at the possible science behind a creature of myth, movie or legend. Be sure to explore the Monster Gallery as well as the Monster Science video series.