Review Questions.


1.Describe four ways that antibodies help to block infection.


A.Neutralization - simply binding to a toxin, virus, or bacteria can block its binding to host cells.

B.Agglutination - antigen-antibody complexes can become extremely large, forming an aggregate that is more readily cleared by other immune cells.

C.Macrophage engulfment.Macrophages have Fc receptors that facilitate the engulfment of antibody covered pathogens.

D.Complement.Binding of antibody to a bacterial surface initiates binding of a set of proteins collectively called complement.Some of these complement proteins cause a pore to be made in the pathogen, killing it.


2.Why is the cellular arm of the acquired immune system needed?How does it function?What is the end result?


The cellular arm is needed because the humoral arm (the antibody-producing arm) is ineffective against pathogens that have already entered host cells.Infected cells present portions of foreign proteins on their cell surfaces, complexed with MHC proteins.These foreign proteins are detected by the Tc receptor of T killer cells.If the T killer cell (also called a cytotoxic T cell or a CD8 T cell) finds the proteins to be foreign, it will release proteins called perforins and granzymes that kill the infected cell, thereby blocking further spread of the pathogen.


3.Why is vaccination effective?


Vaccines mimic the primary infection, so that the patient, when infected by a particular pathogen for the first time, can respond with a secondary immune response rather than a primary immune response.The secondary response is stronger and more rapid, such that the pathogen may be cleared from the body without any disease symptoms, or with very mild symptoms.


4.Describe 3 types of vaccines, and the benefits and drawbacks of each.


A.Live attenuated pathogens.These are either weakened versions of the pathogen (as in the Sabin polio vaccine) or a related microbe that does not cause serious disease (such as vaccinia virus).Such vaccines can be extremely effective, but can also be somewhat dangerous.Polio virus can mutate back to the original, virulent form, while vaccinia virus can (rarely) cause serious problems, especially with immune compromised individuals.


B.Killed whole cell vaccines.These are vaccines made from fully virulent pathogens that have been killed by heat, formaldehyde.These are safer, since they can never cause disease, but are sometimes less effective than live vaccines.Also, in the case of the older Bordetella pertussis vaccine, whole cells were used, and the endotoxin (lipopolysaccharide) from these dead cells were too reactogenic, causing toxicity in a small percentage of vaccinated individuals.Ū


C.Acellular vaccines.These are made from purified proteins such as (inactivated) toxins or adhesins.These avoid the problem of the older Bordetella vaccine in that they do not cause toxicity.They are still in many cases less effective than attenuated pathogens.


5.Describe two clinical tests for HIV infection.


Both tests try to determine whether a person is žseropositiveÓ for HIV, that is, whether the person has antibodies against HIV.The presence of these antibodies indicates a prior exposure to the virus.The first test is conducted in microtiter plates.The 96 well plates are coated with HIV proteins.The patientŪs serum (made from blood simply by removing the red blood cells by centrifugation) is added to a well so that any anti-HIV antibodies can bind to the HIV proteins.The wells are then washed to remove unbound antibodies.›› Bound antibodies are then detected using a second antibody made in another animal.This secondary antibody can detect all human antibodies, and has an enzyme bound to it, such as alkaline phosphatase, which is easy to detect using a reagent that forms a colored product.


The second test is a western immunoblot.It is conceptually identical to the first test, except that the HIV proteins are first size-fractionated by SDS-PAGE (sodium dodecylsulfate-polyacrylamide gel electrophoresis), and then transferred to a membrane.At they point, anti-HIV antibodies are detected in exactly the same way as described above.

6.How are antibodies useful as diagnostic reagents?


Antibodies can be used to detect antigens in a clinical setting.For example, they can stain pathogens present in blood samples, facilitating their detection.Also, they can stain cells that have a particular surface protein, such as the CD4 protein that is expressed by T helper cells.Such antibodies must themselves be detectable, and are usually bound to a fluorsence dye (for light microscopy) or a gold particle (for electron microscopy) or an enzyme (for other diagnostics).


7.How can the concentration of HIV per unit volume of blood be estimated?


One way is by reverse-transcriptase PCR (RT-PCR, not to be confused with real-time PCR).HIV RNA is purified from serum.It is then converted by DNA by reverse transcriptase (a purchased variety made from another retrovirus, not the native RT of the HIV particle).This DNA is then amplified by PCR and quantitated by gel electrophoresis and staining.


8.Contrast a common source epidemic with a host-to-host epidemic.


Common source epidemics are caused by a common source of contamination, such as polluted water or spoiled food.These epidemics are characterized by rapid onset and recovery.Host-to-host epidemics are spread by repeated infection from one person to another, and are generally spread more slowly through a population.


9.Describe two mechanisms that enhance the rate of evolution of influenza virus.


Antigenic drift.This is the accumulation of point mutations in influenza genes, which can help flu virus evade the host immune system.


Antigenic shift.Since flu has a segmented genome, RNA fragments from more than one virus can be reassorted by coinfection of a single host cell.New flu pandemics can be attributed to antigenic shift.


10.How did HIV likely evolve?How might it be expected to evolve in the future, and how might we be expected to evolve?


It probably evolved from a virus that infects higher apes (SIV).SIVs are not harmful to apes.HIV also does not infect apes.HIV might be expected to become less virulent, as it evolves.Already, we see that HIV 2 has evolved, which is less harmful than HIV 1, and confers some immunity to HIV1.It might therefore overtake HIV1 in the human population.Further attenuation of the virus might be anticipated.Humans also might be expected to evolve resistance to the virus.The mutant form of CCR5 should be selected by HIV, since this mutation confers resistance to HIV.