Every day, scientists perform various tests on cells (known as in vitro tests) to evaluate numerous concepts, such as drug efficiency and nanoparticle toxicity. Unfortunately, these tests do not always provide the best or most accurate results since the human body is made up of different types of cells, which interact and cause robust responses that cannot be mimicked or studied precisely with individual cells. Therefore, current techniques are employed to mimic the complexity of the human body by using more than two cell lines combined together. Previous techniques used single cell types, or what is commonly known as a monoculture system to study cellular responses. This is often used for simplicity and to study individual cellular responses rather than cellular interactions. It is useful for studying the general response that cells may have under experimental conditions, such as testing toxic materials. It is also useful for studying the mechanisms through which the toxic material exerts its effects. Unfortunately, it is not useful for measuring cellular responses that follow cell signaling mechanisms. For example, one type of cell may not be damaged by a toxic particle, while a different type of cell may be damaged or even killed following exposure. This response is due to the different responsibilities of different cells. Some cell types are capable of controlling toxicity, while others are not designed to handle toxicity and typically rely on other cells to protect them. This has led to inaccurate measurements of nanoparticle toxicity compared to toxic exposures in animals or humans. This is due to the complex cellular system of living things compared to the simplistic monoculture model. For example, if the scientists... in the center of the paper... signal each other to change the answer. This model provides a more accurate representation of how the human body would respond to a toxic nanoparticle as it is more complex and allows for cellular interactions and robust responses. This model is also useful if scientists are interested in studying animal responses to the nanoparticle, as the measured results are more accurate than monoculture models. Monocultures have long been used for toxicity testing. Although they provide many benefits, their ability to represent what may happen in humans or animals is not sufficient. Co-cultures are better able to represent complex systems because the bodies have more than one cell type that would respond following exposure. This complexity allows for cellular interactions that can more closely represent human and animal responses.