Measuring humoral immunity — Which laboratory metric detects a humoral response to influenza virus?

Introduction:
Humoral immunity refers to antibody-mediated protection. This question asks which laboratory measurement best reflects a humoral response after exposure or vaccination against influenza virus.

Given Data / Assumptions:

• Influenza infection or vaccination context.
• Standard clinical and research assays.
• Goal: detect antibody presence/levels.

Concept / Approach:
Humoral responses are quantified by measuring antigen-specific antibodies in serum (e.g., ELISA, hemagglutination inhibition titer, neutralization assays). These correlate with protection and vaccine efficacy in many settings.

Step-by-Step Solution:

Verification / Alternative check:
Vaccine trials commonly use hemagglutination inhibition titers against influenza antigens as correlates of protection, confirming their role as humoral markers.

Why Other Options Are Wrong:

• Cytotoxicity assays: measure cell-mediated killing by CTLs or NK cells.
• Dividing T cells: indicates cellular proliferation, not antibodies.
• Plasma cytokines: nonspecific; do not directly quantify antigen-specific antibodies.
• TCR repertoire: characterizes T-cell diversity, not humoral titers.

Common Pitfalls:
Assuming any immune assay reflects humoral function; only serum antibody measurements directly quantify the antibody arm.

Final Answer:
serum antibody titer.