MLPAO MLPAO Immunology and Serology 2 — Questions and Answers
Question 1: In a Western blot (immunoblot), what is the purpose of the electrophoresis step?
- To denature all proteins so antibodies cannot bind
- To separate proteins by molecular weight before antibody probing (Correct answer)
- To amplify specific DNA sequences
- To concentrate antigens onto nitrocellulose
Correct answer: To separate proteins by molecular weight before antibody probing
Electrophoresis separates denatured proteins by molecular weight in a gel matrix before transfer to membrane, allowing identification of specific proteins by size when probed with antibodies.
Question 2: Which serological test is based on the principle that patient antibodies, if present, will prevent complement-mediated lysis of sensitized red blood cells?
- Latex agglutination
- Complement fixation test (Correct answer)
- Indirect immunofluorescence
- Immunodiffusion
Correct answer: Complement fixation test
The complement fixation test measures whether patient antibodies bind antigen and 'fix' complement, thereby preventing it from lysing the indicator system of antibody-sensitized sheep red blood cells.
Question 3: ANA (antinuclear antibody) testing by indirect immunofluorescence using HEp-2 cells: a homogeneous staining pattern is most commonly associated with antibodies to:
- Centromere proteins
- Ribosomes
- Double-stranded DNA or histones (Correct answer)
- Sm antigen
Correct answer: Double-stranded DNA or histones
A homogeneous (diffuse) ANA pattern on HEp-2 cells results from antibodies to double-stranded DNA or histones, and is commonly associated with systemic lupus erythematosus.
Question 4: The MHC class II molecules present antigen to which subset of T lymphocytes?
- CD8+ cytotoxic T cells
- Natural killer cells
- CD4+ helper T cells (Correct answer)
- Regulatory T cells only
Correct answer: CD4+ helper T cells
MHC class II molecules on antigen-presenting cells present exogenous peptide antigens to CD4+ helper T cells via the CD4 co-receptor, initiating the adaptive immune response.
Question 5: Which serological marker is the first to appear after hepatitis B infection and indicates active viral replication?
- Anti-HBs (hepatitis B surface antibody)
- Anti-HBc IgG
- HBeAg (hepatitis B e antigen)
- HBsAg (hepatitis B surface antigen) (Correct answer)
Correct answer: HBsAg (hepatitis B surface antigen)
HBsAg is the first serological marker to appear after HBV infection, detected before symptoms and before antibody development, indicating active hepatitis B virus infection.
Question 6: In latex agglutination tests, patient antibodies cause visible clumping because:
- Latex beads coated with antigen cross-link with specific antibodies (Correct answer)
- Antibodies activate complement on the latex surface
- Latex particles bind complement and precipitate
- Antibodies dissolve the latex polymer matrix
Correct answer: Latex beads coated with antigen cross-link with specific antibodies
Antigen-coated latex beads agglutinate when specific patient antibodies bridge multiple beads simultaneously, forming visible aggregates that indicate a positive result.
Question 7: A four-fold or greater rise in antibody titer between acute and convalescent serum samples is considered diagnostically significant because it:
- Rules out a prior infection with the same organism
- Indicates a current or recent active infection rather than past exposure (Correct answer)
- Confirms the patient has developed immunity
- Demonstrates a class switch from IgM to IgG
Correct answer: Indicates a current or recent active infection rather than past exposure
A four-fold rise in titer between paired acute and convalescent samples provides strong evidence of active infection, distinguishing it from pre-existing antibodies from past exposure or vaccination.
MLPAO MLPAO Immunology and Serology 2
In a Western blot (immunoblot), what is the purpose of the electrophoresis step?