Cells, the building blocks of living organisms, are able to communicate with each other predominantly using three different strategies: juxtracrine, paracrine and endocrine signaling. When adjacent cells are in contact with each other, they communicate through components of their plasma membrane, the complex protective layer that surrounds each cell. This particular strategy is termed “juxtacrine signaling”. Alternatively, cells can secrete molecules that act as signals targeting neighboring cells. This type of communication, called “paracrine signaling”, includes for instance neurotransmitters – which are vital for us to perceive the surrounding environment, feel pain and think rationally. Other well-known signals include estrogens, which are locally produced by the ovary. Cells can also induce a response in their neighbors by secreting cytokines, which are molecules necessary to elicit an immune response.
When these molecules are released into the bloodstream and act as long-range stimuli, we classify this type of communication as “endocrine”, and the secreted molecules are generally referred to as hormones.