FAQ for
AVP: Alien vs. Predator (2004) More at IMDbPro »

The following FAQ entries may contain spoilers. Only the biggest ones (if any) will be covered with spoiler tags. Spoiler tags have been used sparingly in order to make the page more readable.

For detailed information about the amounts and types of (a) sex and nudity, (b) violence and gore, (c) profanity, (d) alcohol, drugs, and smoking, and (e) frightening and intense scenes in this movie, consult the IMDb Parents Guide for this movie. The Parents Guide for AVP: Alien vs Predator can be found here.

Neither and both. Alien vs Predator sets up a whole new premise whereby the creatures from the Alien series and the predators from the Predator series meet and do battle in Antarctica. A sequel, AVPR: Aliens vs Predator - Requiem, was released in 2007. It is however a prequel to the Alien films and a sequel to the predator films in most respects.

It's a direct nod to the original AvP comics. Which was the inspiration for both this film and the video games. In the comic book, the lead Predator protects the humans from both Alien and Predators because they found him unconscious and wounded and nursed him back to health. That Predator however, never killed any humans.

In this film, the Predator Scar kills humans without hesitation (like he does with Charles Weyland). When he confronts Lex, he is ready to kill her (he actually yells at her, probably hoping she will run, as it is no sport to kill a static prey). But at this point, she gives him his shoulder cannon. So at this point he probably respected her enough to spare her life and decided that her help was better than being on his own.

The Predators are not mindless killers, they definitely have a strong sense of honor and ethics. Remember that Dutch from Predator could have been easily killed, but he was ultimately granted the opportunity to a 'fair' fight (without laser weapons) because that Predator probably judged him as a worthy prey, respecting him for putting up such a good fight. Also, Lt. Harrigan (Danny Glover) from Predator 2 managed to kill a Predator, and was left alive by his brethren out of respect. He even got a trophy for it. In AvP, Scar the Predator was surprise-attacked by a Xenomorph and knocked unconscious, while Lex was quick enough to grab a spear and impale the Alien as it lunged toward her. So in a sense, Lex has saved Scar's life, and his honor may dictate that he help and save her in return.

He may also see her as something of an equal, as she was quick enough to dispatch an Alien. As he walks away, she confronts him. Scar growls and advances at her, trying to intimidate her, but he notices that she is not easily scared. So he may see that she has the right skills for a warrior, and grants her the opportunity to make it out there alive. He even respected her enough to give her a mark making her a warrior of his people.

Since the Aliens are not released until the Predators take their plasma cannons from the sarcophagus, they would have had to wait for a day before they could hunt the creatures, had the gestation time been the same as in the previous Alien movies. Therefore, it would be logical for the Predators to have sped up the Alien life cycle. The manner in which they accomplished this is uncertain.

(Theory 1) One possibility is that the Aliens' life cycle becomes faster in the atmosphere they're exposed to. In other words, on planets with lesser atmosphere, their life cycle takes longer and on planets with stronger atmopshere, their life cycle becomes faster.

*Lets recap*

Alien: On LV-426, there was probably little oxygen (it was a 'primordial atmosphere' according to Ash). It took about 24 hours for an Alien embryo to grow and almost 24 hours for an Alien to mature. And the rest of the story continued aboard the Nostromo, which has oxygen tanks but is still different from Earth's.

Aliens: Also on LV-426, this time with a breathable atmosphere. However, it is probably not ready yet (lower oxygen content than on Earth). We see one person being attacked by a Facehugger (in the Special Edition), but we don't see the Chestburster hatch. We also have one woman being killed by an Alien bursting out of her, but no info as to when she was implanted. The fact that one subject was killed when the Facehugger was removed before embryo implanting suggests this process takes considerably longer than a few minutes.

Alien3: Took place on Fiornia 161 which had a tad different air than Earth, which would explain why the Alien grew faster than in Alien but slower than in AvP. The embryo in Ripley took about a week to grow, probably because the Alien was a Queen, which probably takes longer then the other Aliens.

Alien Resurrection: As in Alien, the movie took place on a ship, the Auriga, which has oxygen tanks but the air is different than on Earth, which would explain why it took a few hours for the Alien at the end to break out.

Alien vs. Predator: Took place on Earth with stronger atmosphere; on Earth, it may take 10 1/2 minutes for the Aliens to grow, much faster then on LV-426, Fiornia 161 and space ships, which would explain why the Predators picked Earth, because it also had humans they could use. And the Hybrid (PredAlien) inside Scar Predator took longer because it was a Queen.

Aliens vs. Predator Requiem: The Facehuggers on the Predator shuttle were in space ship oxygen for who knows how long, and as in Alien and Resurrection, it takes longer when exposed to air tanks, which is why it took a few hours for the Facehuggers' embryos to mature and hatch. And the rest of the Aliens were directly implanted into women's bellies by the Hybrid, which was a ProtoQueen, which is a stage between Chestburster and full Queen. In any case, they were implanted on Earth, which is why it took about 10 1/2 minutes for them to grow up.

(Theory 2) It's possible that the Aliens mature faster when they've recently been born. In other words, the younger the eggs and Facehuggers, the faster the Aliens grow and mature; the older the eggs and Facehuggers, the longer it takes for them to mature.

*Lets recap*

Alien: The eggs on the ship were perhaps thousands of years old, which could explain why it took 24 hours for the embryo to be implanted and mature, and almost 24 hours for the Alien itself to grow to full height.

Aliens: Same eggs as in ]Alien, which goes a long way to explain why these Aliens take some time to grow and mature. Also, the Queen was born from this hive, so she would also lay 'old' eggs with a slowed-down life cycle.

Alien3: Also eggs originating from a Queen that was born from thousand year old eggs. The Facehugger in this film was some kind of Queen Facehugger which means it could lay more then one embryo and a Queen. It took a few hours for the Runner (Dog Alien) to be born. As for the Queen embyro, it may take a longer time for the embryo to grow.

Alien Resurrection: It seemed the military might have had the eggs for a long while and they seemed to have bred 14 Aliens (including the Alien that blew up in the escape pod and the first Alien Ripley killed) in a few hours or days. So the eggs may have been laid a few days before they used the hosts. In the scene that shows the Alien eggs, if you count, there were seven people implanted and there seemed to be about 15 hosts, including Purvis, which means they may have bred seven Aliens at a time. Purvis was among the first batch of people to get implanted (he can be seen among them in the implantation scene), so it is unexplained why it takes his Chestburster until the end of the film to burst from his chest. Perhaps he is in ill health, which slows down the embryo's growth.

Alien vs. Predator: The team in the sacrificial chamber was implanted a few minutes after the eggs had been laid, so the eggs were relatively fresh. The Queen was probably older, but the Predators' methods to keep her in cryostasis may preserve her better than the ship on LV-426 did. So that may explain why they matured so fast. The PredAlien at the end is probably a Predator/Queen hybrid, which may explain its (relatively) longer maturation time (a few hours, compared to several days for the Queen in Alien 3).

Aliens vs. Predator Requiem: The Facehuggers in this film were kept in tubes for who knows how long, and after they found a host, it took about 24 hours for the Aliens to grow. The embryos the Hybrid laid in the women's bellies birthed a few minutes or hours later because they were just laid.

(Theory 3) My opinion is that all along, the Aliens have been hinted at as being engineered. If it was a weapon of war, you want some time from implantation to expulsion, so that the host can get back to his own kind and spread the infection, and to make sure the Chestburster doesn't pop out before the host is back where you want him or her to be. I think the Predators may have re-engineered the Alien Queen in the chamber to produce quicker maturing eggs that produce quicker growing Aliens, so that the Predators could get there and have a lot of fast prey to hunt in an hour or so, instead of just sitting around all day twiddling their thumbs, reading, watching their equivalent of TV and movies, and/or playing their equivalent of cards, darts, chess, etc. while their game grows.

Then again, one simple answer could be a combination of the first two theories; varying factors can affect an outcome, which is why no two people are the same.

A 'Predalien', also known as a 'Hybrid', an Alien that resembles a Predator. It has been established that Aliens take on certain characteristics of the host (such as bipedalism, as seen in Aliens that came from humans). This is demonstrated in "Alien 3" when a chestburster that comes out of a dog (or in the Special Edition, an Ox) has dog/quadruped-like characteristics.

They didn't bother to check.

They knew he was dead, so they had no reason to believe he had a living Alien embryo inside.

(Also, the Predators weren't wearing masks; remember that Scar had to alter his vision mode to see the embryo inside Sebastian)

The explosion of the Predator bomb could have raised the ambient temperature to a bearable level.

This actually provides a clever and reasonable explanation why humans in the future know nothing about the Alien species until the first Alien film. The cold did affect her, but while trying to run from the queen and just trying to survive in general, she would likely have had raised body heat and heart rate, so the cold would not have affected her right away. However, after the Predator ship took off, there is a high probability she froze to death - hence why she is not in the sequel. We see her walking towards what appears to be the Snow tractor, so she might have made it, but whether she survives is ambiguous at best.

Even if Lex survived, it seems reasonable for her not to be in the sequel, since AvP:R takes place immediately after AvP. Lex would still be making her journey back to civilization while the events in AvP:R are happening. This means it's possible she could return in AvP3.

This is unlikely however, as the Strauss Brothers have stated that AvP:R tied up all the loose ends for the present setting and the next film will take place in the future.

Predators do not see infrared without their helmets. The Predator's ability to see in infrared without his helmet, as seen in Predator 2, is a goof.

However, a Predator can still see without his helmet...even though the environment would be mostly red. For example, the first Predator saw Dutch just fine. Even though the Predator in AVP couldn't see that well without his helmet, it would be hard to miss a gigantic Queen.

Predators can see fine without their helmets - it may not look like much to us, but we're human and are only used to the way WE process the visible spectrum. When the Predator takes off his mask, he presumably sees at least some of the visible spectrum, but in a simplified view. Aliens, humans, other Predators would all be the same colour so to speak. The use of infrared vision is just a visual aid to assist during a hunt.

The hunting ritual did not go as planned; the humans were not supposed to take the plasma cannons, the Predators were. The Predators were taking part in a 'coming of age' ritual and were quite young (technically teenagers). However, there were still humans running around in the pyramid who were keeping the Predators too busy to be minding the Aliens. Normally, there would only be enough people for breeding Aliens, and they would be dead already when the hunt began.

The first Predator was distracted by Lex so that the Alien was able to stab him in the back. The second Predator got careless towards the end of his fight with the main Alien. He likely assumed the Alien was trapped in the net and helpless, but the Alien broke free as the Predator was about to strike the final blow.

According to AvP backstory, no Predator has survived a fight with an Alien with only its wristblades. Basically, these Predators didn't have a chance, especially considering they weren't using acid resistant equipment.

Either;

The pyramid in the flashback was a different one. (The flashbacks depicted multiple pyramids, it's possible that there may still be some scattered across the Earth)

Or another group of Predators could have rebuilt it for their hunting purposes. (In AVPR, a Predator scout-ship departs from the leaving mother-ship, possibly so that they can start the process again)

-In this film Lance Henriksen plays Charles Bishop Weyland. Founder of Weyland industries.

-In Aliens, Bishop is an Android modeled after the founder of Weyland-Yutani (either CB Weyland, Michael Bishop, or both)

-In Alien 3 (as a subject of much debate) Bishop II (sometimes known as Michael Bishop) is either a special android with red blood which is built specifically to deceive Ripley, or he is a direct descendant of Charles Bishop Weyland and is the heir and CEO of Weyland-Yutani in the 22nd century and designed the Bishop android modeled on himself and his descendants (ie, Charles Bishop Weyland).

No she isn't. There are several reasons for this:

1) The two ships look nothing alike; they have completely different shapes.

2) The Predators look nothing like the dead alien creature ('Space Jockey') found inside the Derelict. The dead creature is several times larger, has a large proboscis and lacks the typical Predator mandibles.

3) The dead alien inside the Derelict is fossilized. Fossilization takes tens of thousands of years at least; there are just 118 years between Alien Vs. Predator and Alien.

4) It is unlikely that the ship would just happen to crash on LV-426 (the planet from Alien where the Derelict is found), which is located far away from Earth.

It has been stated that in the sequel to this movie,AVPR: Aliens vs Predator - Requiem, the Predator ship crashes on Earth, reducing chances of her being the Derelict from Alien to zero. However, that Predator ship is not the one that crashed in Colorado in AvPR, because the ship that crashed was a shuttle, not the mother ship. There is a scene in the unrated edition of AVPR which shows the mother ship leave Earth after AVP1, and a shuttle jetison from the mother ship, which goes back to Earth where she crashes, releasing the Aliens in the town in Colorado. Why the scene with the shuttle dispatching from the mother ship was not used is unknown.

With no mention about the Queen from this movie returning in the sequel AVPR: Aliens vs Predator - Requiem, and the fact that she would be subject to tremendous water pressure (with open wounds in her armored plating), it seems likely that she sank so deep that she crushed to death. There is the likelihood that should she survive the pressure but not manage to escape, she would eventually starve. It is, however, not impossible for her to rip free from the chains tying her to the sinking container. It would probably have taken some days to get out and get back to the mainland, but she would not be able to mass-reproduce (as her egg positor was ripped off her).

Her absence in AvP:R does not prove that she is dead; AvP:R takes place only shortly after the end of AvP and it is situated in an entirely different location, so even if the Queen managed to escape, she would hardly show up a day later in a place thousands of miles away. This was the most likely reason for the Queen not being mentioned in AvP:R.

Some say the Queen would not starve. She either gets crushed by the depths, freezes, or both. Which leaves it open to bringing her back. However, she would not starve for the simple fact that Aliens don't NEED food. Don't forget that the Alien eggs in the Derelict spacecraft remained dormant for thousands of years. The Queen's goal in life is simple: lay eggs, gather hosts, make more aliens. Not mate, feed, kill repeat. The reason the Alien in Alien 3 kept killing everyone was that they were all a threat to it. In Alien, there were only 6 people versus one Alien. It only killed 2 of them technically; it cocooned Brett and Dallas in the extended cut, Ash was a robot, it birthed from Kane, so it only killed Lambert and Parker. In Alien 3 it was 1 alien vs roughly 30 people, plus Ripley had a queen inside of her, the Alien could afford to kill off most of the convicts to ensure the queen's safety.

It has been suggested that the Queen can't die because Aliens can survive in outer space, but this is not sustained in any of the movies. We see and hear the Alien from Alien and the Queen from Aliens getting expelled into space, shrieking and thrashing about, but this merely indicates that they can survive in deep space for a few seconds. Even humans could theoretically survive a few seconds in space, but the extreme cold and vacuum would eventually kill them. Aliens have somewhat tougher body structures, so they might last a little longer, but even if they could survive the vacuum, they would eventually freeze solid.

What we know little about is the Aliens' dependance on oxygen. We see them breath in the movies, but we don't know if their blood takes something from the air. If it is either oxygen or nitrogen, the Queen would eventually suffocate under water, unless she can hold her breath very long, or can go without air for a longer time.

The water looked pretty deep, it's possible that sinking that deep in that cold the Queen would freeze solid and the deep gravitation pull would crush her into a million pieces. (Note: gravity has nothing to do directly with crushing a sinking object; it's the water pressure that does it. Gravity only supplies two things; the impetus for nonbouyant objects to sink and the weight of the column of water that crushes the object.)

But as seen in the beginning of the film, the Queen can't die from being frozen. As she was frozen for 100 years and thawed at the start of the ritual.

In 1904 the Predators used the inhabitants of the whaling station, which Lex refers to. As for every century since the pyramid's creation this *could* be a major plothole, as the only previous bodies found inside belong to the ancient civilization. So after the civilization was wiped out, they could have abducted people and brought them to the island. Used animals, or perhaps the pyramid layed dormant until the Whaling station was built there. Then 100 years later when our technology was advanced enough, they could lure people there with the heat bloom.

The most likely answer (given that there were no bodies except those in the sacrificial chamber), is that the pyramid laid dormant until 1904. It is likely the 1904 Predators were successful in their trial and cleaned up whatever mess was left.

The movie offers an explanation for this, albeit an incorrect one, as mentioned in the Goofs section.

It is said that Antarctica (including Bouvet Island, although this is not part of Antarctica as suggested in the movie) used to be free of ice, making it habitable.

However, this was several million years ago, before humans ever evolved. There is another possible explanation for this, however. As seen in the movie, the pyramid is located in a cave under the ice. Bouvet Island is prone to volcanic activity, and geothermal warmth is strong enough to melt the ice and snow in some surface areas. Geothermal warmth could have also sustained the inhabitants of the pyramid inside the cave, along with warmth produced by the Predator technology embedded in the pyramid.

Though we don't actually know how old the human race is and since the civilization was in Antarctica before the humans acrossed the world after the explosion, we found no evidence there because we did not think to look and even so the evidence might have been wiped out by the Predator explosion, so it's possible the human race started a lot longer ago then we think and was wiped out so it still would be possible for the humans to build the pyramids on the Antarctica way before it iced over. But that's just me.

These could have been different clans, each with their own different hunting rituals. Furthermore, since Aliens form the 'ultimate prey' and are not easily obtainable, perhaps only the most promising hunters are eligible for this ritual. This excludes the majority of Predators from participating in this particular ritual, who may have to resort to other ways of obtaining a respectable status.

Another theory is that first they hunt humans as their initiation test. If they are successful in hunting worthy humans, they then team up with two other hunters of equal standing and hunt the Aliens to become official clan members. (Though this contradicts part of the story-telling in some of the Alien vs. Predator comics/novels where humans are the ULTIMATE prey. So it would be vice-versa and the Aliens would be the initation and the humans would be the ultimate. Though the other way around makes more sense as Aliens can see predators when they're cloaked, they don't panic and even when wounded or maimed they still pose a great threat. Wheras humans are the opposite.)

Another theory could be this, the mark means that you are one of the skilled hunters, which means if you slay an Alien then you are an ultimate hunter. So it could mean they did not have marking because they did not take part in the ritual and didn't mark themselves.

In AvP backstories (especially AvP: Prey), the hunting of the aliens preceeds the hunting of humans. The novice hunters go with a group to hunt aliens and become 'blooded', after which they are allowed to hunt on their own. Chances are this part of the backstory was not invented before the earlier movies, and therefore would not have existed.

Or he never actually killed an Alien.

They most likely just didn't have time, they strung them up to skin them but Quinn spotted them so they had to chase him down and take care of him first, and they followed him down the ice chute so they didn't bother wasting time going back up to skin their first victims.

Predators are on Antarctica for hunting Aliens for a coming of age ritual, the humans are merely collateral damage and were used for breeding the Aliens.

The information is contradictory. In Predator it is said that the Predators only appear in 'the hottest years'. In Predator 2 it is said that, in the past, Predators have appeared in Iwo Jima, Beirut and Cambodia, as they are drawn to heat and conflict. While the latter two areas have a hot climate, Iwo Jima was very cold and rainy during the U.S. offensive of World War II. On the other hand, Dark Horse Comics's Predator: COLD WAR (early 90s) features predator's hunting humans in Siberia. Also in the comic (and book of the same name) the predators crash landed in Siberia, and the crash gave off a lot of radiation warming up the surrounding area. They were not affected by the cold. In the Alien versus Predator game, it is made evident that between the humans, the aliens, and the predators, the latter is the toughest and most resistant of all three species, being able to withstand fire, falls, and wounds better than the other two species. In short, it has never been stated that Predators can not stand cold. Even though they prefer heat, they can apparently also function when it is cold.

Also, the Predators were on a ritual. The extreme cold could be an additional challenge for them.

At best, the only human survivor from this movie is Lex. Even if she did survive, most likely no one believed her story. She was there on behalf of Weyland, so anything she tells the company will be kept under wraps. Soon after, the Yutani Corp gains video footage of one of the Aliens from Gunnison and a piece of Predator technology (through Colonel Stevens in AVPR, who worked for the same unit as Peter Keys from 'Predator 2').

In Alien, the Weyland-Yutani Corporation knew of the existence of the Aliens for some time, but they chose to keep it under wraps so they could use them for their own means; the crew of the Nostromo were not supposed to know about it, except Ash (Special Order 376 was for 'Science Officer Eyes Only').

Remember, this is the early days of the corporation; Weyland and Yutani are separate companies. They do not merge until the time leading up to 2030 (from the tie-in game 'Predator: Concrete Jungle').

It is true that Aliens can sense an Alien embryo inside a person such as in Alien 3 when the Dog Alien wouldn't kill Ripley because she had an Alien Queen inside her.

However, that was because the Dog Alien was a drone, and had no reproductive capabilities. It needed to protect the queen. In this film, it was the Queen who was the last one standing, so it risked killing one of it's children by killing scar in order to save itself. She is the queen and can reproduce, if there's the choice between killing one of her children that's growing inside of a dangerous enemy, so be it.

Also, remember that the Queen pierced Scar just below the chest...

When Lex and Scar reach the foot of the ice-tunnel, Lex looks up the tunnel and expresses puzzlement. The camera looks from her point of view and we see a faint silhouette moving and flickering in the light at the top of the tunnel; the Queen has already made her way to the surface.

Scar and Lex are thrown a fair distance away from the mouth of the tunnel and always keep running on top of the ice as it collapses from beneath their feet. Obviously, the Queen was buried in the collapsing snow and ice, which is why she still emerges from the ice moments later. (Remember, you can be buried in an avalanche!)

It was confirmed by writer/director Paul W.S. Anderson that Schwarzenegger was going to reprise his role as Dutch and appear at the end of the film if he lost the California election, but since he won he turned down the role and was written out of the script. It is unknown how he was going to make his appearance.

One possible approach would have been to appear at the whaling station to investigate the loss of communication with the Weyland team and the explosion at the whaling station. Since he was the leader of a rescue team in Predator, he would have probably arrived at the whaling station and found Lex after her meeting with the Predator tribe, and probably would've taken her back to the boat.

An alternate theory: originally, in Predator 2 the role of Peter Keyes was actually supposed to be Dutch Schaeffer who is now part of an elite unit whose specific duty was to track and try to capture the Predator. This was changed because Arnold turned down the role because he didn't like the idea of taking the Predator into the city.

So likely his cameo would have re-envisioned this role. Either Lex being rescued by him and his team while they were trying to track the Predators or something along those lines.

Scar got his name because of the mark he burned into his forehead. Grid was named for the grid-like battle scars on his head. Chopper was named for his long arm blades (he is also referred to as Gill, due to the gill like designs on his helmet). Celtic was probably named due to the designs of his mask and armor that give him a slightly Gaelic look.

Before Time: It dates back to Antarctica during the dawn of the human race where a Predator tribe comes to Earth and teach the humans how to built a civilization of pyramids and the Predators used them as hosts to breed the Aliens for the Predator's coming of age ritual. But something went wrong, the Aliens grew too large, the Aliens overwhelmed the city and the Predators were forced to blow it sky high. Either one Pyramid survived or the Predators re-built one.

1715: A Predator aquired old pistol. The gun the Elder Predator gave Harigan.

1904: A whailing station is attacked by three more Young Blood Predators who used the workers as hosts for the Aliens, as shown in an alternate opening, the disappearance of the workers mentioned by Lex, and the diles of coffin last set 1904.

1987: Predator

1997: Predator 2

2004, October 3: Begining of AVP.

2004, October 10: Weyland Industries heads for the Pyramid awakening the Alien Queen, stealing the guns, and the Young Bloods arrive to become men.

AVPR takes up right after AVP left off, but the Predator mother-ship reached Saturn when the Scout seperated and headed back to Earth; this distance was most likely a month at least, considering that one person commented "Isn't Halloween in October?" (You'd say something like that after Halloween)

After AVPR: Ms. Yutani covered up the events of Gunnison as she stated "The world isn't ready for this" and she must have signed an agreement with Weyland's heir forming Weyland-Yutani. Then, in 2030, Weyland and Yutani finalize their merger ('Predator: Concrete Jungle').

2122: Alien

2179: Aliens

Maybe a month later (most say a year later): Alien3

2379/2380: Alien Resurrection

No. If you look closely once the Predator ship takes off the camera pans out and Lex is clearly seen walking back to the vehicle. Even in the commentary of the DVD, Paul W.S Anderson, and Sanaa Lathan both point out that the character survives and the original plans to include her in the sequel.

Think about this for a second...If the movie was strictly predators vs aliens it would get extremely tedious, to have 90-120 minutes of Predators fighting Aliens. Also, all the Alien Vs Predator stories are told from both the predator perspective and the human perspective. There have always been humans caught in the middle, in order for the audience to have something to relate to. Not to mention that in a film with only Predators and Aliens, neither of whom speak English, there would be no dialogue, and subtitles would be hokey.

In order to achieve the PG 13-rating for its theatrical release, the film had to be cut in terms of violence. The also available unrated edition runs almost 8 minutes longer than the theatrical cut and adds more story and gore to the film. A detailed comparison between both versions can be found here.

It's interesting to note, that in the AvP canon, the predators all have acid-proof armor and weapons. However in this film, when Celtic Predator was fighting the Alien, his wrist blade melts away after using it on an alien, then he gets some acid on his armor and it starts to melt. However, Scar Predator used his wrist-blades and many other weapons on the Aliens and they didn't melt at all. (We never see Gill/Chopper Predator use his stuff because he got killed too quick). This could have partly meant why the ritual went awry so fast is that the team leader (Celtic) was not properly equipped for the hunt, he may have grabbed the gear for hunting humans instead of the gear for hunting aliens, or there was a supply mix up. (this would explain why he paused to look at his wrist-blades and seemed surprised it had melted) It is not uncommon for a military to accidentally send the wrong equipment into a battlefield, so what's to say the Predators don't make mistakes.

Also, it's possible that Scar was the only one who knew about the acidic blood; he might have been the only one who actually did his homework and researched the Aliens, either forgetting or choosing not to pass the relavent information on to the other two. (Equivalent to a group of College/University room-mates where only one actually studies!)