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).