Communication within the body can take one of three different forms:

Autocrine signaling
A form of signaling where cells can communicate with themselves. In this pathway, the cell directly affects its own function by secreting substances which can act on the cellular receptors.

Paracrine signaling
Cells can also communicate with cells in the immediate environment. Paracrine signaling is especially important in the local immune response.

Endocrine signaling
Signaling at a distance. Occurs through the presence of hormones. Hormones are biologically active substances which are secreted into the bloodstream. Many of the regulators of white cell growth and development, for example, are hormones.