Medicinal chemist here.

Two things: A full immune response is usually the body's response to biologics, not just random chemicals. The vast majority of marketed drugs are not biologics. There are certainly drugs that are biologics, though, so it's still a fair point of quandary.

The body does try to eliminate all foreign bodies, whether they're biologics or not. There's multiple areas of drug design that are dedicated to getting around this via either small changes to the chemical structure to combat digestion and enzymatic breakdown, clever formulation to assist with rapid transfer into the body... And most likely both of these things.

The drugs you see marketed usually don't have problems with immune response because they'd never see the light of day on the market of they did. Many drugs get pulled in clinical trials for adverse effects of various types, which is why you don't see them.

Most drugs very closely mimic the molecular shape of neurotransmitters/hormones/aminos/etc which are naturally occuring and serving important roles in the body. That's kinda the reason drugs do stuff to us!

However, some do incur significant immune response in some people. Others can weaken immune response. Lots of side effects are immune related. Lastly, Plenty of people are just straight up allergic to certain drugs.

Many, if not most, drugs may elicit an immune response (hypersensitivity reaction), if you look at the adverse reactions sections of the package inserts for most drugs, you'll see a listing of the events that have been reported in clinical trials and the incidences at which they occurred.

Your immune system, in particular antibodies, recognizes and attacks things based on the presence of these things called antigens. Things like viruses, bacteria, or even pollen (i.e. allergies) possess antigens which your antibodies can detect via a receptor. Antigens come in all forms of shapes, sizes, and chemical compositions whether it's proteins, peptides, saccharides, etc., but what triggers the antibody to attack is when an antigen binds to the antibody's receptor. If the antibody receptor does not bind, the antibody does not attack. This is why your body doesn't usually destroy itself (except for autoimmune diseases), as a normally functioning immune system will only attack things that trigger it (anything else will not be attacked). Think of it as a reverse IFF (Identification Friend or Foe), where your immune system assumes everything is friendly except for when it detects a threat.

Modern pharmaceuticals are formulated so that when used in treatment, they will be effective. If it's not effective, it's not considered medicine - it's as simple as that. The biotech engineering takes into account things like delivery method (ex. oral ingestion, IV, etc.), potential interactions (ex. drug to drug), and how well tolerated it will be when taken (i.e. whether it causes an adverse reaction and if so, the extent of it). Thus, the first filter (reason) why drugs work is because they are specifically engineered to work and won't be passed off as medicine until it has passed drug trials and has been approved by whatever regulatory body your country has.

Secondly, drugs/medicine can have an effect on your body that is not limited to immune responses - these are usually referred to as side-effects. In regards to specifically to drugs activating an immune response though, certain drugs may or may not illicit an immune response, and if they do, it could either be a big response, or a small response, or they could even shut down a response all together. It is extremely varied and it all depends on the context. For example, certain drugs, such as chemotherapy meds, obliterate your white blood cells and weaken your immune response as an unintentional effect of killing cancer. Other drugs can trigger an immune response if the drug's antigen is bound and your immune system can actively try to rid itself of the drug, such as in the case of Bococizumab (a cholesterol control drug developed by Pfizer, which failed its trial as while the drug was initially effective, the trial participants started developing antibodies against it, which is why drug trials happen in the first place, to see what happens). This is also one of the reasons why certain drugs lose effectiveness over time (another reason is tolerance, such as in the case of opioids).

Thirdly, certain drugs are formulated to intentionally tone down an immune response, such as antihistamines or steroids to control inflammation. This category of drugs are commonly referred to as immunosuppressants, which are usually used to treat autoimmune diseases. Drugs, like man, are not made the same. The mode of action is a consideration.

In summation:

a) Your immune system will not attack anything that doesn't have a detectable trigger (an antigen) and this function is not limited to drugs and thus pathogens (ex. bacteria, viruses, etc.) which can evade detection will also evade destruction.

b) Drugs are engineered specifically to be effective and well-tolerated. Any drug that is not effective won't make it to market due to regulatory bodies.

c) The assertion that drugs don't provoke an immune response is based on an incorrect parameter. Many drugs do in fact, provoke an immune response and can be expressed as a side-effect, though side-effects are not limited only to immune responses.

d) Certain drugs are specifically formulated to tone down or disable an immune response (immunosuppressants).

To add to what others have said, your immune system isn't responsible for handling changes in bodily chemistry, be it through hormones or otherwise. They can overreact to more complex molecules such as proteins because bacteria, viruses, and other cells use proteins for receptors. And really, that's mostly the class of molecules they look for. Proteins and lipids.

So what is responsible for handling drugs, toxins, and other molecules? Your liver. There are other organs involved but anything that isn't reduced to salts or other excretable molecules goes through the liver to be handled. The liver itself has a limited capacity to handle substances, so what drugs effectively do is exceed the liver's capacity to break them down in time before they can act on your body in the intended way. With multiple circulations of your blood, your liver will eventually catch the rest.

The immune system has a number of checks and conditions that should be met before an immune response is elicited. Things like anaphylaxis and atopy are what occur when this system fails.

But obviously you don't react violently to everything you eat, or everything that touches your skin, so we shouldn't expect drugs, especially oral ones, to do the same. Others have mentioned other limitations, such as molecule size. But also there are things called PAMPS and DAMPS. Pathogen/Damage Associated Molecular Patterns. These are molecules that are typically found in an infection, or during injury, that alert the immune system that previously unseen antigens may be worthy of provoking an immune response.

The regulation of the immune system against unnecessary activation is why inactivated vaccines tend to need an 'adjuvant', something that tells the immune system to take notice. injecting a syringe is a very clean and non injurious insult to the body, especially if you compare it to something natural like an actual wound with active pathogens invading the tissue.

"Drugs " and "Immune response " are a little too vague to give a comprehensive response. The immune system is passive and active .....so do you mean a change in the status quo homeostasis?

Any molecule could be described as a drug if it changes the function of cells, organs...,etc

Allergies are a immune response but some might say our resident bacteria throughout our bodies is a continuous response against hostile organisms.

Antivenom created to protect us from venom is sometimes toxic if it has no venom to counteract.

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They do provoke an immune response thats why some people get paranoid when they smoke weed, its the body freaking out and getting a massive histamine overload, not anaphylactic but still a stress response.

Everyone ive met who gets paranoid from weed will get hives if you rub fresh ground dry weed really hard into their inner forearm skin, while others who dont get paranoid from smoking do not get hives.

Anti-histamines make you sleepy and relaxed, the opposite is true, just straight up histamines make you amped up and anxious.