Humoral immunity basics: which soluble proteins mediate the classical humoral response? Identify the proteins primarily responsible for antigen-specific humoral immunity.

Introduction / Context:
The humoral immune response refers to antibody-mediated immunity, distinct from cell-mediated mechanisms. Clarifying which soluble molecules execute this response is central to understanding serology, vaccine correlates, and passive immunization.

Given Data / Assumptions:

• B cells differentiate into plasma cells that secrete immunoglobulins.
• Antibodies circulate in blood and mucosal secretions.
• Specificity is encoded in variable regions recognizing epitopes on antigens.

Concept / Approach:
Antibodies neutralize toxins and viruses, opsonize pathogens for phagocytosis, activate complement, and mediate antibody-dependent cellular cytotoxicity. While cells initiate and regulate humoral responses, the effector molecules that define “humoral” are antibodies.

Step-by-Step Solution:
Identify soluble, antigen-specific proteins: immunoglobulins meet this criterion.Differentiate from cellular effectors (T cells, NK cells), which are central to cell-mediated immunity.Select “Antibodies” as the defining molecules of humoral immunity.

Verification / Alternative check:
Serum transfer experiments confer protection via antibodies; monoclonal antibody therapies demonstrate humoral effector function without cellular transfer.

Why Other Options Are Wrong:
“Macromolecules without specificity” contradicts antigen-specific recognition. “Lymphocytes only” are cellular, not soluble proteins. Cytokines modulate responses but are not the primary humoral effectors.

Common Pitfalls:
Equating presence of antibodies with protection in every context; affinity, isotype, and concentration determine protective capacity.

Final Answer:
Antibodies.