Immunology & Serology Flashcards

A fourth-generation HIV screening assay detects which analytes?

Answer: HIV-1/2 antibodies and HIV-1 p24 antigen

Fourth-generation combination assays detect both antibodies and p24 antigen, which shortens the window period.

In the CDC HIV testing algorithm, what follows a reactive fourth-generation Ag/Ab screen?

Answer: HIV-1/HIV-2 antibody differentiation immunoassay

A reactive screen is followed by an HIV-1/2 differentiation immunoassay, and HIV-1 NAT is used if that result is negative or indeterminate.

In the Davidsohn differential test for infectious mononucleosis, heterophile antibodies are:

Answer: Absorbed by beef erythrocytes but not guinea pig kidney

IM heterophile antibodies are absorbed by beef RBCs but not by guinea pig kidney, unlike Forssman antibodies.

Which EBV serologic profile indicates acute (primary) infection?

Answer: VCA IgM positive, EBNA negative

VCA IgM appears early, while antibody to EBNA does not develop until weeks to months after infection.

An elevated antistreptolysin O (ASO) titer supports a recent infection with which organism?

Answer: Streptococcus pyogenes (Group A strep)

Streptolysin O is produced by Group A streptococci, and a rising ASO titer supports post-streptococcal sequelae such as rheumatic fever.

When acute and convalescent sera are tested in parallel, which finding best indicates a current infection?

Answer: A fourfold or greater rise in titer

A fourfold (two-tube dilution) or greater rise between paired sera drawn 2-3 weeks apart indicates active infection.

Nephelometry quantifies immune complexes by measuring:

Answer: Light scattered at an angle from the incident beam

Nephelometry detects light scattered by antigen-antibody complexes, usually at an angle such as 70-90°.