1. (5 points) Name the parts of the body where epithelia act as barriers to infection. List and
describe the three main ways/mechanisms by which epithelia function as barriers.
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Epithelia acts as barriers to infection on our skin, gut, lungs, urogenital tract,
eyes/nose/oral cavities.
First, they have tight junctions, longitudinal flow of air/fluid in the skin and gut, production
of mucus in the lungs and nostrils, and tears in the eyes. Antimicrobial peptides are
produced by all these parts. Fatty acids are produced by the skin, the gut and vagina are
acidic, and antimicrobial enzymes are found in the gut, tears and saliva. Also, microbiota
exist in all of these parts which are beneficial to our health.
2. (5 points) What are the main differences between innate and adaptive immunity?
● The innate immune response is immediate (within hours), nonspecific with a limited
number of specificities, and has a constant response. The adaptive immune response is
slow (may take days), specific, variable with a high number specificities, and improves
after the first response.
3. (2 points) How can antibiotics upset the barrier function of intestinal epithelia? Give a specific
example.
● Antibiotics can attack the beneficial microbiota in the gut, which protect against
pathogens. They use up our nutrients, can make vitamins and help us digest among
other things. If antibiotics attack these “good” microbiota along with the bad ones, it will
be bad for our immune system. A specific example is Clostridium difficile, which can gain
a foothold after antibiotic treatment.
4. (3 points) Define clonal selection and clonal expansion. How do they shape the adaptive
immune response?
● Clonal selection is the stimulation of pathogen-specific lymphocytes to be proliferated as
an adaptive immune response. Clonal expansion is the actual proliferation and
differentiation of the chosen lymphocytes. Because of these processes, lymphocytes can
become highly specific to the pathogen, which strengthens the adaptive immune
response.
5. (3 points) Explain the steps that take place after a bacterium is opsonized via C3b:CR1
interaction between the bacterium and a resident macrophage in the tissue.
● After this, receptor-mediated phagocytosis is induced. The bacteria is engulfed by the
macrophage. The macrophage membranes fuse and form a phagosome, which attracts
the lysosome. The phagolysosome is formed and degrades the bacteria.
6. (4 points) Describe the two different domains of TLRs and their respective functions. Explain
why TLRs can detect many different species of microbes despite the limited number of different
TLR proteins.
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The variable extracellular domain recognizes the pathogen, while the conserved
cytoplasmic signaling domain which communicates to the interior of the cell. An example
is the TIR (Toll interlukin-1 receptor).
TLRs can detect many microbes because many pathogens possess common features
common to different groups of pathogens, so only a small number of TLRs are required
to sense the molecular patterns shared by pathogens.
7. (3 points) Explain how the alternative C3 convertase on pathogen cells surfaces is (A) formed
and (B) stabilized.
● The first step of the formation of alternative C3 convertase is when C3 is cleaved into
C3a and C3b. C3b then gets attacked by an hydroxyl group (R-OH), and binds to the
pathogen surface. Then Factor B binds to C3, which attracts Factor D to cleave B into
Bb and Ba. C3bBb attracts more C3 to the binding site, causing a positive feedback
loop. Alternative C3 convertase, once bound to the pathogen, is stable and cannot
diffuse away like iC3Bb. Properdin binds to it, further stabilizing it.