The term "foreign substances" refers to things that are not normally found or belong in a particular place or thing. They are materials or elements that are unfamiliar or do not naturally occur in a specific environment.
The simplest markers show concentrations of foreign substances in blood or fat. (discovermagazine.com)
Due to a weak immune mechanism, your body at times is unable to resist the attack of foreign substances like the bacteria, parasites and viruses. (fitnesshealthzone.com)
Your baby's immune system is still very immature, meaning it is less able to distinguish harmful foreign substances from harmless protein. (enfamil.com)