During cell signaling, cells communicate with others all over the body. There are several different types of hormone signaling: endocrine, paracrine, and autocrine signaling. Endocrine signaling is the movement of hormones through the bloodstream, traveling long distances to reach the target cells. When a hormone travels to a neighboring cell within the same tissue, the response is called a paracrine response. When the hormone acts only on the cell from which it is secreted, it is called an autocrine response. A paracrine is a type of hormone that serves as a chemical messenger to neighboring cells. They are released from the producing cell through simple diffusion, so they stay very close to their point of origin. They fail to increase in concentration in the blood because enzymes quickly deactivate them.
Some examples of paracrine molecules include neurotransmitters and cytokines. A neurotransmitter is a chemical released from the ends of a nerve cell that carries the nervous impulse across the gap found between cells called a synapse. The neurotransmitter, such as acetylcholine, carries the nerve signal from the axon of one cell to the dendrites of the next cell. Cytokines are chemicals that are secreted by white blood cells to help regulate immune responses. They control inflammation and increase the activity of the cells that produce antibodies. They have also been shown to have a slight effect on cell division and growth, as well as on the healing of damaged tissue from wounds. While cytokines, like other paracrines, act very close to their sites of origin, they can also travel in the blood to other sites to help produce a much larger response. They are responsible for the all-over aching and tiredness a person feels when they have a very bad cold.
A third type of paracrine communication happens with chemical messengers called eicosanoids. These are lipophilic, but are not officially classified as hormones because of their behaviors. Eicosanoids act very locally and are quite diverse in their impact on body functions. Prostaglandins, for example, are formed in the testes and enhance sperm transport in both the male and female reproductive systems. Additionally, eicosanoids work on the respiratory, circulatory, and immune systems. Leukotrienes are another form of eicosanoid. These are produced from white blood cells and serve as localized messengers involved with the inflammation that occurs in the lung passageways during an asthma attack.Autocrines, hormones that work within the producing cell itself, often produce changes within that cell. The autocrine is secreted outside the cell and binds to the cell that produced it in order to initiate a response from organelles within the originating cells. Autocrines may initiate growth, which may be normal or abnormal growth patterns. The growth of tumors is an abnormal reaction to growth initiated by autocrines. For example, T-lymphocytes, cells that are part of the body's immune response, are encouraged to replicate themselves when a foreign invader has been detected. By producing more T-lymphocytes, the body is better able to fight off the infection. A negative impact of autocrines occurs in cancer. Here, autocrines damage the regulatory mechanisms that turn off cell division. As such, cells duplicate uncontrollably, forming into a tumor.