A nurse is planning to give a preschool child an immunization consisting of bacterial cells that have been modified. what is the substance called?

Vaccine.

A vaccine is a biological preparation that provides immunity to a specific disease. Vaccines are usually made from weakened or killed forms of a microorganism (e. g. bacteria and viruses), its toxins or its surface proteins. When administered, they stimulate and "train" the body's immune system to recognize, attack, and destroy these microorganisms when the body encounters them in the future.