The Weil–Felix reaction relies on antigenic cross-reactivity between rickettsiae and which bacterial antigens?

Introduction / Context:
The Weil–Felix test is a historical agglutination assay used to provide presumptive evidence of rickettsial infections by exploiting cross-reactive antigens shared with certain Proteus strains. Although largely replaced by more specific tests, it remains a classic concept in infectious-disease serology.

Given Data / Assumptions:

• Test principle: heterophile cross-reactions.
• Key Proteus strains: OX2, OX19, OXK.
• Outcome: serum antibodies agglutinate Proteus antigens if anti-rickettsial antibodies are present.

Concept / Approach:
Surface antigens of some rickettsiae share epitopes with Proteus strains. Patient serum containing anti-rickettsial antibodies can agglutinate Proteus cells in vitro. Titer patterns may suggest certain rickettsial groups (e.g., OXK with scrub typhus).

Step-by-Step Solution:

Verification / Alternative check:
Classic microbiology texts summarize the historical utility of Weil–Felix and its limitations (low sensitivity/specificity).

Why Other Options Are Wrong:

• Sheep RBCs/EBV and mycoplasma/human RBCs: Unrelated to Weil–Felix.
• Brucella/Salmonella Vi: Different organisms and antigens with no role in this assay.
• None of these: Incorrect because Proteus cross-reactivity is the key principle.

Common Pitfalls:

• Overinterpreting titers; confirmatory specific serology is preferred now.

Final Answer:
Rickettsial antigens and Proteus strains (OX2, OX19, OXK)