Microbiology, 2021

17.2 • Chemical Defenses 697
Figure 17.10
Autocrine, paracrine, and endocrine actions describe which cells are targeted by cytokines and how far the cytokines must
travel to bind to their intended target cells’ receptors.
Three important classes of cytokines are the interleukins, chemokines, and interferons. The interleukins were
originally thought to be produced only by leukocytes (white blood cells) and to only stimulate leukocytes, thus
the reasons for their name. Although interleukins are involved in modulating almost every function of the
immune system, their role in the body is not restricted to immunity. Interleukins are also produced by and
stimulate a variety of cells unrelated to immune defenses.
The chemokines are chemotactic factors that recruit leukocytes to sites of infection, tissue damage, and
inflammation. In contrast to more general chemotactic factors, like complement factor C5a, chemokines are
very specific in the subsets of leukocytes they recruit.
Interferons are a diverse group of immune signaling molecules and are especially important in our defense
against viruses. Type I interferons (interferon-α and interferon-β) are produced and released by cells infected
with virus. These interferons stimulate nearby cells to stop production of mRNA, destroy RNA already
produced, and reduce protein synthesis. These cellular changes inhibit viral replication and production of
mature virus, slowing the spread of the virus. Type I interferons also stimulate various immune cells involved
in viral clearance to more aggressively attack virus-infected cells. Type II interferon (interferon-γ) is an
important activator of immune cells (Figure 17.11).