Question 1

Why is testing for Cryptococcal antigen preferred over testing for antibodies?

Accepted Answer

Testing for Cryptococcal antigen is preferred over testing for antibodies because:

A) This is usually not a humoral response to infection - Cryptococcal infection usually elicits a cellular immune response rather than a humoral immune response.
B) Immunocompromised patients do not produce antibody - Immunocompromised patients may not be able to produce a sufficient amount of antibodies to be detected by antibody-based tests.
C) Subclinical infection can be detected - Antigen-based tests can detect the presence of Cryptococcus even in cases of subclinical infection where there may not be enough antibodies present to be detected by antibody-based tests.

Question 2

Which statement best describes antigenic variation seen in parasites?

Accepted Answer

Parasites can periodically change their surface antigens as a means of evading the host immune response. This is known as antigenic variation and is a common strategy among parasitic organisms. By changing their surface antigens, parasites can avoid detection by the host's immune system, allowing them to persist and continue to cause disease. This makes it more difficult for the host immune system to respond quickly or effectively, and contributes to the chronic nature of many parasitic infections.

Question 3

All of the following may be drawbacks of antibody testing for fungal infections EXCEPT:

Accepted Answer

False-positive results do not occur often in patients with functioning immune systems. False-positive results are more common in patients with cross-reacting antibodies or previous exposure to other pathogens. A, B, and C are all valid drawbacks of antibody testing for fungal infections.

Question 4

What would be an advantage of using the tube precipitation test for diagnosis of infection with Coccidioides immitis?

Accepted Answer

The tube precipitation test is particularly useful for the early detection of IgM antibodies, which are indicative of a recent or ongoing infection. In the case of Coccidioides immitis, early detection can be especially important for timely treatment and management of the infection. The other options (A, C, and D) are disadvantages or limitations of the test.

Question 5

When is antibody testing for Cryptococcal antibodies most useful?

Accepted Answer

Antibody testing for Cryptococcal antibodies is most useful in determining patient prognosis.

Question 6

The most likely reason for the development of an IgE antibody response in humans is:

Accepted Answer

IgE antibodies play a significant role in protecting humans against helminth infections. They trigger the release of histamines and other mediators that induce spasms in the smooth muscles of helminths, which eventually expel the parasites out of the body. IgE antibodies do not provide protection against bacterial or viral infections. However, IgE antibodies can also bind to allergens, leading to the release of histamines and other mediators that cause symptoms of allergies.

Question 7

A host's immunological response to parasites includes all of the following EXCEPT:

Accepted Answer

Sequestering the parasite in host cells is not a part of the immune response to parasites. The other options, such as formation of antibodies, cytokine production, and gamma-interferon production by natural killer cells, are all part of the host's immunological response.

Question 8

A 25-year-old pregnant female has serologic testing to determine if she has a toxoplasmosis infection.A low level of both IgM and IgG is found.On repeat testing 2 weeks later,the results are the same.What can be concluded from these results?

Accepted Answer

A low level of both IgM and IgG suggests that the infection may have occurred in the past and the individual now has some level of immunity to the disease. The repeat testing with consistent results supports this conclusion. An active infection would generally show higher levels of IgM antibodies. The presence of low levels of IgG does not necessarily indicate serious central nervous system damage in the baby. The absence of antibodies would suggest no exposure to toxoplasmosis, but that is not the case here.

Question 9

A 30-year-old male patient with AIDS exhibits pulmonary symptoms that include coughing and some difficulty breathing.The patient reported that he had previously worked in a bakery.His white blood cell count was normal,indicating that the infection is not bacterial in nature.Tests for the presence of fungal antigens were performed with the following results: ouchterlony immunodiffusion negative for antibodies to Aspergillus; negative latex agglutination test for Candida antigen,and positive EIA test for the galactomannan antigen of Aspergillus.What conclusion can be reached from these results?

Accepted Answer

The negative results for antibodies to Aspergillus and Candida antigen indicate that these infections are not present, while the positive result for the galactomannan antigen of Aspergillus suggests the presence of Aspergillus infection. Therefore, the best conclusion is that the pulmonary symptoms are likely caused by an Aspergillus infection. The possibility of a false-positive result for the galactomannan antigen test cannot be ruled out entirely, but the combination of clinical symptoms, negative bacterial and fungal antigen tests, and positive galactomannan antigen test suggest Aspergillus infection as the most likely cause.

Question 10

In an individual with AIDS,what is the most likely reason for a toxoplasmosis infection that invades the central nervous system?

Accepted Answer

In individuals with AIDS, toxoplasmosis is most likely caused by reactivation of latent cysts in the tissue. This is because the weakened immune system, as seen in AIDS, cannot effectively control or eliminate the dormant cysts that can persist in the host for the lifetime. These cysts can reactivate, and the parasites can invade the central nervous system, leading to toxoplasmosis. The other options such as antibody production, ingestion of contaminated water, or acquisition of new surface proteins are not as relevant in the context of a reactivated latent infection in AIDS patients.

Quiz 20: Serologic and Molecular Detection of Bacterial Infections