: Can the immune system break down anything? Every time one takes a pill, drinks a glass of wine, or uses an illicit drug, it is understood that the effects are temporary. Caffeine only keeps one awake for a few hours, alcohol

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Actually, in the case of most drugs, it has nothing to do with the immune system per se. Generally, drugs are either chemically modified (often in the liver) to make them inert, or excreted (either in the urine or the stool) thus removing them from the body. When the liver or the kidneys are not working properly, drugs fail to be excreted or digested, resulting in accumulation, which may be toxic.

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Any drug taken by mouth are being absorbed by the digestive system where a large amount may be destroyed by metabolic enzymes (first-pass metabolism) and enters the hepatic portal system. Then it's carried through the portal vein into the liver to be absorbed into the bloodstream and excrete the waste through bile. Some other bypass the liver, entering the blood directly. Only around 2-5% of the dose is excreted in an unchanged form in the urine.

The biological half-life of a substance refers to the body's cleansing through the function of kidneys and liver in addition to excretion functions to eliminate a substance from the body (time it takes for the blood plasma concentration of a substance to halve its steady-state).

Here are the common prescription medications which has its high half-life:

4–16 days: Fluoxetine,
5 weeks: Dutasteride,
22-110 days: Amiodarone
5.5 months: Bedaquiline

And here are the metals:

Mercury (as methylmercury) in the body has a half-life of about 65 days.
Lead in the blood has a half life of 28–36 days. In bone about 10 years.
Cadmium in bone about 30 years.
Plutonium in the liver about 40 years. Plutonium in bone about 100 years.

I hope that helps.

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First of all, it's typically liver and the kidneys responsible for removing unwanted chemicals from the body, not the immune system. It should be noted that liver and kidneys have no "knowledge" which chemical is "bad" and which is "good". They have evolved to let some chemicals pass, react some chemicals etc. But there is plethora of compounds that had so little influence on evolutionary process, that we're not ready to handle them, since not being able to handle them was not selected against in the process of evolution.

So can the body break down anything?

tl;dr: No, there are many chemical compounds that are not handled by human body ideally and that, even if there's enough time for the body to handle them, still cause major harm.
Not sure what breaking down exactly means here, but certainly there are some compounds that can get stuck in the body, notably heavy metals. Those and some others accumulate in the body, possibly but not necessarily causing harm in the future. Let's have some examples:
Bioaccumulation
Bioaccumulation is a process where organism (eg. you) absorbs certain chemical compound at faster rate than it excretes or processes it, which is probably what you meant by "breaking down". See bellow some examples of such accumulation. Remember that the compound to be accumulated doesn't need to be exactly toxic, but as the poison is in the dose, as long as you accumulate something, it eventually will became toxic, should it even be water.
Silver
Silver is a heavy metal and was used as disinfection in the first half of 20th century[1]. Silver accumulates in the human body, causing argyria. This often happens to people who adhere to so-called "alternative medicine", which includes promoting silver suspension as a cure to all kinds of diseases.[2]
Lead
Lead is more dangerous than silver and is recognised as important health problem by WHO.
Other examples
I will defer listing of other examples on wikipedia.
Toxification
As I said in the introduction, evolution didn't prepare liver and kidneys for many chemicals. Sometimes the attempt to react them starts the toxic reaction itself and it would be better if the chemical stayed in blood as-is and was excreted in urine. Such event is a type of toxification, toxification can generally occur anywhere, not just liver.
Methanol
Methanol is known as possible toxic component that can accidentally occur in alcoholic beverages. Methanol shares toxicity properties with ethanol, however on top of that, it is metabolized into formic acid in the liver. This is why, if there are no better alternatives, ethanol can be administered to prevent further methanol poisoning since it occupies the liver and therefore stops the toxification process. Bear in mind that this only works before the methanol was already turned into formic acid.[3]
Arsenic
Apart from accumulating, arsenic also is oxidated in body which produces much more toxic arsenic (III) oxide which is the famous deadly poison.
Refferences:

[1] "Chapter VIII: Germicides and Disinfectants". The Use of Colloids in Health and Disease
[2] Over-the-counter drug products containing colloidal silver ingredients or silver salts
[3] Methanol poisoning overview

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Certainly not. By definition, it cannot break down poisons (at least not quickly enough) to stop the body being harmed or killed. In addition, it can not break down (or "neutralize") many radioactive substances. A famous example of this was the murder of Alexander Litvinenko:

On 1 November 2006, Litvinenko suddenly fell ill and was hospitalised. His illness was later attributed to poisoning with radionuclide polonium-210 after the Health Protection Agency found significant amounts of the rare and highly toxic element in his body.