Microbiology, 2021

15.4 • Virulence Factors of Eukaryotic Pathogens 645
To survive within the host long enough to perpetuate their often-complex life cycles, helminths need to evade
the immune system. Some helminths are so large that the immune system is ineffective against them. Others,
such as adult roundworms (which cause trichinosis, ascariasis, and other diseases), are protected by a tough
outer cuticle.
Over the course of their life cycles, the surface characteristics of the parasites vary, which may help prevent an
effective immune response. Some helminths express polysaccharides called glycans on their external surface;
because these glycans resemble molecules produced by host cells, the immune system fails to recognize and
attack the helminth as a foreign body. This “glycan gimmickry,” as it has been called, serves as a protective
cloak that allows the helminth to escape detection by the immune system. 11
In addition to evading host defenses, helminths can actively suppress the immune system. S. mansoni, for
example, degrades host antibodies with proteases. Helminths produce many other substances that suppress
elements of both innate nonspecific and adaptive specific host defenses. They also release large amounts of
material into the host that may locally overwhelm the immune system or cause it to respond inappropriately.
CHECK YOUR UNDERSTANDING
• Describe how helminths avoid being destroyed by the host immune system.
11 I. van Die, R.D. Cummings. “Glycan Gimmickry by Parasitic Helminths: A Strategy for Modulating the Host Immune Response?”
Glycobiology 20 no. 1 (2010):2–12.