Microbiology, 2021

698 17 • Innate Nonspecific Host Defenses
Figure 17.11
Interferons are cytokines released by a cell infected with a virus. Interferon-α and interferon-β signal uninfected
neighboring cells to inhibit mRNA synthesis, destroy RNA, and reduce protein synthesis (top arrow). Interferon-α and interferon-β also
promote apoptosis in cells infected with the virus (middle arrow). Interferon-γ alerts neighboring immune cells to an attack (bottom arrow).
Although interferons do not cure the cell releasing them or other infected cells, which will soon die, their release may prevent additional
cells from becoming infected, thus stemming the infection.
Inflammation-Eliciting Mediators
Many of the chemical mediators discussed in this section contribute in some way to inflammation and fever,
which are nonspecific immune responses discussed in more detail in Inflammation and Fever. Cytokines
stimulate the production of acute-phase proteins such as C-reactive protein and mannose-binding lectin in the
liver. These acute-phase proteins act as opsonins, activating complement cascades through the lectin pathway.
Some cytokines also bind mast cells and basophils, inducing them to release histamine, a proinflammatory
compound. Histamine receptors are found on a variety of cells and mediate proinflammatory events, such as
bronchoconstriction (tightening of the airways) and smooth muscle contraction.
In addition to histamine, mast cells may release other chemical mediators, such as leukotrienes. Leukotrienes
are lipid-based proinflammatory mediators that are produced from the metabolism of arachidonic acid in the
cell membrane of leukocytes and tissue cells. Compared with the proinflammatory effects of histamine, those
of leukotrienes are more potent and longer lasting. Together, these chemical mediators can induce coughing,
vomiting, and diarrhea, which serve to expel pathogens from the body.
Certain cytokines also stimulate the production of prostaglandins, chemical mediators that promote the
inflammatory effects of kinins and histamines. Prostaglandins can also help to set the body temperature
higher, leading to fever, which promotes the activities of white blood cells and slightly inhibits the growth of
pathogenic microbes (see Inflammation and Fever).
Another inflammatory mediator, bradykinin, contributes to edema, which occurs when fluids and leukocytes
leak out of the bloodstream and into tissues. It binds to receptors on cells in the capillary walls, causing the
capillaries to dilate and become more permeable to fluids.
CHECK YOUR UNDERSTANDING
• What do the three complement activation pathways have in common?
• Explain autocrine, paracrine, and endocrine signals.
• Name two important inflammation-eliciting mediators.
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