Hormones are a part of the endocrine system in the body. They are small chemical molecules that are secreted by different glands or organs that are designed to invoke a chemical response within another specific type of cell in the body. In this way they can be thought of as cell to cell chemical messengers. Hormones travel through the body though the bloodstream. Once they reach their target destination, they interact with specific cells through highly specific chemical interactions. Different cells have different protein based receptor molecules on or embedded in their plasma membranes. These protein receptors are very specific with highly evolved shapes. Specific hormones are designed to interact with these specific proteins and invoke a series of chemical reactions within the cell to produce the desired response. Some hormones are capable of slipping through the plasma membrane and interacting with proteins within the body of the cell, but most limit their interactions to the surface. These highly specific hormone/receptor chemical interactions (called molecular recognition) are what determines exactly which cells a certain hormone will affect.