Simplify the following text for a Grade 11 native English-speaking person.
Original: The main difference between professional and non-professional phagocytes is that the professional phagocytes have molecules called receptors on their surfaces that can detect harmful objects , such as bacteria , that are not normally found in the body .
Simplified: The reason they are called professional phagocytes is because they have receptors on their surfaces that can detect harmful objects , such as bacteria .