Not only it won't work as you think, it might actually make them more vulnerable to death by (immune) overreaction.
You see, the inside of our body is a really good place to live in: it is nicely warm and there's plenty of resources such as cells and nutrients for bacteria, viruses and fungi to enjoy to their heart's content. To make sure they don't get the chance to trouble us, our immune systems are here to protect us from outside agents.
The problem? "outside agents" usually only stretches to other organisms that don't belong in our bodies and some types of organic toxins that don't kill us too quickly, and even then there is a large number of exceptions, as our bodies often get in contact with and house many substances and microorganisms which are neutral at worst and helpful at best, such as seen in our gut bacteria, which can offer help in functions that can range from digestion to aiding in our very immunity.
Now, to make sure we're on the same tracks: our immune response essentially works in 3 stages assuming we don't die midway:
Stage 0: an external agent enters our body, and it hasn't been detected yet.
Stage 1: our immune system detects the thing as a threat, usually by engulfing one of the invaders, and begins an immediate, less specialized reaction (usually causing inflamation) while it takes the information it got by eating some of the invaders to essentially research a more effective response.
Stage 2: the immune system cracks the code and crafts antibodies specifically designed to target the invading elements, and so these are produced and released In mass, neutralizing the threat, and the information on it is stored, grating us immunity against it. If the same invader comes back later and hasn't changed enough, the immune system will immediately recognize it and jump directly to the stage 2 response again with little to no side effects or symptoms. This is what we know as a secondary immune response, and it's because of it we'll create to vaccines against diseases, especially those that are usually too debilitating and/or too aggressive for the infected person to develop an immunity in time.
With that said, we come to the main problem: for our immune system to work in the first place, there must be something to identify. And that's not how everything works:
ionizing radiation, especially in cases like Gamma radiation, causes damage by essentially breaking you apart on a genetic level. As the thing is a wave and essentially energy, there's nothing to be picked up.
heavy metals are a problem and accumulate on our bodies precisely because, despite toxic, they behave very similarly to other non-toxic elements we do need, the difference/problem being that they also work in ways that are detrimental to us. A good example is Carbon monoxide, which forms much stronger links to the hemoglobin than oxygen does, meaning it never lets go, doesn't enter the bloodstream and accidentally leaves that red blood cell unusable for oxygen transportation. Again, there's nothing to detect, and even when there is, it works too similarly to other chemicals to be perceived as a threat, because inherently, it really isn't unless it starts to pile up in quantities considered as too large, at which point it starts interacting with our bodies in ways that mess up too many important chemical processes for our continued existence to be maintained (in some cases "too much" may equate to "virtually anything more than none of it", but still).
Essentially, a realistic super immune system, as far as I understand, would work extremely fast, and could probably make it so you never got anything worse than a minor cold and normally short lasted inflammations, as it'd fight off and develop an immunity faster than the problem could spread, and thus you wouldn't really need to worry about death by viral or bacterial diseases, poisons or venoms, since all of these are natural organic substances your body can and will develop an immunity against under the right conditions. However, it doesn't really help to make you immune to everything, far from it. In fact, it won't even help reducing aging, as the inflammatory reactions caused by our normal immune response is a contributor to that, and the degradation of telomeres, the biggest link to aging we seem to have, isn't really something out immune system can help with. Rather, not getting hurt or stressed is the best way to ensure they don't need to be degraded by "unnecessary" cell division.
There's also one problem you need to consider: an immune system like that can be more trouble than it's worth. Our immune system is pretty crafty, but not the brightest, and like everything in our body, it must act in a balanced way. A weak immune system leaves us vulnerable, but an overagressive and oversensitive immune system can be worse.
He's incapable of getting sick, or dying of bacteria, viruses, venoms, poisons, drugs, chemicals, alcohol, radiation, basically any outside substance that can be consumed or injected.
Is how you described it. But do you know what has a similar description? Allergies. Allergies are basically what happens when your immune system detects and reacts to an outside substance, but in an overagressive and damaging way. Essentially, the immune system is so sensitive that it detects things as threats when they're not, and reacts to them in a violent way, which can even cause someone to die by eating a harmless peanut butter sandwich, or petting a cat. An immune system that targets EVERY outside substance isn't going to make you healthier. In fact, it might kill you depending on how it actually works.
Another big complication is that, as no immune system can be perfect, it can detect harmless things as dangerous invaders, and sometimes, that something is yourself. Auto immune diseases are essentially that: you mistook yourself for an invader and now your body started breaking part of itself apart. problem: you still need that to survive, and when you immune system is violent and efficient enough to grant you immunity to venom in seconds, such a mistake could easily be a fatal one.
So summing it up:
while you don't get to age well if your immune system does not work, it's not your immune system that controls your aging. A super immune system doesn't make you age slower, and might actually make you age faster if it's super sensitive as well.
if it is or becomes super sensitive though, you might not need to worry about aging, because auto immune responses and anaphylatic shock (an extreme and dangerous allergic reaction to a normally harmless outside substance) might do you in long before your first wrinkles form without proper (and perhaps constant) medical assistance.
despite all this, you're still very much in risk of dying from radiation poisoning, mercury poisoning, suffocation by carbon dioxide and other types of poisoning related to heavy metals, as there's really nothing to be detected as a threat, and if your immune system somehow does detect it, chances are that it's already too sensitive for your own good and is already targeting the substances the heavy metals are usually mistaken for.
things like fire, deep enough cuts and bullets will also still do you in without medical assistance (and sometimes even despite that). Your immune system is ultimately still organic and carbon based, and it doesn't really work well at over 80 degrees, or when most of your blood is leaving your body.
To get something closer to the kind of semi immortality you want you'd basically need the following:
- an immune system more like a bat's, being effective but not too keen at causing inflammation.
- different genetics, including the ability to regrow telomeres in all cells and rapidly divide them in cancer fashion. The ability to edit your RNA like cephalopods do could also help you rapidly adapt to certain things such as temperature changes.
- extensive genetic repair mechanisms and special protection substances and processes, allowing you to naturally resist certain things, like how targigrades protect themselves against radiation through special proteins shielding their DNA.