PRINCIPLES OF MODERN MICROBIOLOGY
Mark Wheelis
ANSWERS TO STUDYQUESTIONS
Chapter 18
Host Defenses against Microbial Infection
2.
The normal flora is commonly considered a mutualistic symbiosis, that benefits the host by
protecting it from colonization by other, less benign, microbes. However, members of the normal
flora can cause disease if they get into the wrong place, or if the local ecology is disturbed.
4.
Both PMNs and macrophages are phagocytic cells that function as a defense against invading
bacteria. They move by amoeboid motion.
PMNs are found mainly in blood and at sites of infections, to which they migrate from the blood.
They lack mitochondria and are thus fermentative, and they are short-lived. They are adapted to
phagocytose bacteria in damaged tissue that may be poorly oxygenated.
Macrophages have mitochondria and are oxidative. They are long-lived cells, found in large
numbers in lymph and in the tissues, particularly the lungs and the brain.
6.
The complement system functions in two major ways. First, it induces the inflammatory system
when low levels of C3b binds to bacterial cell surfaces. This induces increased hydrolysis of C3,
leading to high levels of C3a and C3b. The C3a induces inflammation, and the bound C3b acts as
an opsonin.
The other major role of the complement system is as an adjunct to the immune response. When
antibody binds to an antigen, it stimulates a cascade of reactions, beginning with hydrolysis of C1,
and resulting in bound C3b on the surface of the antigen. This bound C3b stimulates another
cascade of reactions that results in a membrane attack complex that forms a pore through the
membrane of mycoplasmas, gram negative bacteria, enveloped viruses, and protozoal pathogens.
8.
Antibody binding to a foreign cell or protein protects in a number of ways. It stimulates the
complement system, which can lead to lysis of the cells or enveloped viruses to which the antibody
is bound. It also facilitates phagocytosis, as bound antibody acts as an opsonin. And it can interfere
sterically with the interaction of viruses or toxins with host cells, preventing their entry.
10. MHC I and II are very similar proteins with very different function. Both bind short peptides and
present them on the cell surface, and both interact with T-cell receptors. MHC I is found in the
membrane of virtually all cells in the body, and it presents peptides derived from the turnover of
proteins within the cell. Thus it usually presents peptides that do not evoke a response by the
immune system, because they are native and tolerance has eliminated any immunological response
to them. But if a cell is infected with a virus, some of the peptides presented in MHC I will be viral.
These are then recognized by epitope-specific T-cell receptors on the surface of CTLs, and the
CTLs then kill the infected cell.
MHC II, on the other hand, is found in the membranes of only a few types of cells, including Bcells. It presents peptides derived by digestion of material that was bound by antibodies on the
surface of the B-cell, and then phagocytosed. These foreign peptides are recognized by epitopespecific TH cells, which then stimulate the B-cells to divide.