What is a disadvantage of infusing cells or cell substitutes?

Infusing cells or cell substitutes into the body indeed comes with several challenges, and one of the major disadvantages is that cells may be cleared by the immune system. This immune response can lead to the rapid elimination of the infused cells, thereby compromising the effectiveness of the treatment. Additionally, while infusion avoids the need for major surgery, which is a significant advantage, the immune system's role means that the cells may not survive long enough to provide the intended therapeutic benefits.

Moreover, another potential disadvantage can arise related to the functionality of the infused cells or substitutes over time. Without adequate protection from immune rejection, even cells designed to secrete beneficial substances can become non-functional as they are attacked by the body’s defense mechanisms.

Overall, both the clearing of the infused cells by the immune system and the potential in vivo challenges represent significant disadvantages, making D a comprehensive choice that encapsulates these important concerns in the context of tissue engineering and cell therapy.