I’m sorry to read about the reactions you and the fellow forum members are having due to your chemotherapy treatment.
You have asked a specific question about what causes drug reactions, so here’s my simple explanation for you. Please bear in mind it is much more complex than this.
A human body has a built in defence system called the immune system, which you can read about here. When we introduce a foreign substance into our body like a medication it recognises that this is harmful (although the drug is being given for good reason). So, our immune system is triggered and responds. The response is a chemical reaction within the body and it’s the releasing of these chemicals which cause the ‘allergic reaction’. This happens in varying degrees in each person.
These reactions can occur with any medication, but it is known they can occur more commonly with certain types of drugs. The medical team prescribing treatment will know of the drugs that are commonly associated with allergic reactions, so they will usually give preventative medication to reduce and prevent these reactions happening.
I also wanted to add that whilst the immune system deals with the foreign substance it also produces antibodies. Antibodies are a form of protein which identify and can disable foreign substances. So, when you have the drug again, the antibodies automatically recognise it and direct the immune system to deal with the foreign substance. So, this is why you may have repeated reactions after.
It is way more complex than this and there are also different types of immune reactions, but hope this has helped explain why its happens.
I’m sorry your last cycle freaked you out, and I can appreciate that you may be worried and nervous about the next cycle. It sounds like from what you have said that your oncologist is already thinking of how to help reduce this happening again. So hopefully this will help, but do talk to them again and explain how concerned you are about these reactions. If you have a specialist nurse you could also speak to them in the meantime.
I hope this has been helpful. You are more than welcome to get back to us if you have any more questions or call us on 0808 800 4040. We are here from Monday to Friday, 9 am to 5 pm.