Biology Reference
In-Depth Information
of defense against microbes is structural and comprises the exoskeleton, the
peritrophic membrane that lines the gut, and the tracheal linings. In addition,
insects typically maintain a low pH and digestive enzymes and antibacterial lyso-
zymes in their midguts.
Infections induce local immune responses that include the synthesis and secre-
tion of peptides in barrier tissues such as the tracheal and gut epithelium. In
addition, systemic responses are activated that result in encapsulation of patho-
gens by blood cells, melanization of parasites and pathogens, phagocytotic
uptake of pathogens by blood cells, and the production by the fat body of anti-
fungal and antibacterial peptides that are secreted into the hemolymph where
they accumulate to high concentrations (
Khush and Lemaitre 2000
). Many
antibacterial proteins and peptides, such as cecropins, attacins, lysozymes, and
defensins, serve as induced immune responses.
As discussed in Section 4.12, insects are hosts to microbial symbionts. One ques-
tion is, How does an insect discriminate between useful microbial symbionts and
pathogenic microbes? Another question is, How does an insect maintain an appro-
priate level of beneficial microorganisms?
Lazzaro and Rolff (2011)
point out the
immune system is able to maintain homeostasis and regulate microbes that pro-
vide useful services to the insect. This service requires the immune system to dis-
tinguish between cues that are produced by pathogens (danger signals) while
maintaining the beneficial microbes, which are producing molecular signals, as
well. Both types of organisms produce molecular cues. Current data suggest that
the joint presence of signals from microbes
and
the presence of danger signals,
such as the release of collagen or nucleic acids from damaged cells, are required
to elicit an immune response to pathogens. Molecules produced by gut bacteria,
for example, induce expression of host proteins that degrade these molecules,
allowing the insect to regulate the abundance of gut symbionts without eliminat-
ing them. Similar responses occur in the specialized structures such as mycetocytes
inhabited by symbionts in the insect body. The current concept for how insects
can maintain microbial symbionts while responding appropriately to pathogens
is as follows: where microbial signals are present, but no damage is done, the
physiological cost of clearing the infection is not worthwhile, allowing beneficial
microbes to persist. Only if danger signals occur in combination with the molecular
cues produced by the microorganisms will the immune system respond.
Some organisms have evolved to evade the insect immune system. For exam-
ple, an analysis of the immune responses by the mosquito
Culex quinquefas-
ciatus
to West Nile virus, the filarial worm
Wucheria bancrofti
, and an avian
malaria parasite showed that mosquito-borne pathogens have “evolved to
