Antiviral defenses — What protein is produced by host cells after viral infection to warn neighboring cells?

Introduction:
Cells mount intrinsic defenses against viruses by secreting warning signals. This question targets the key cytokine family induced by viral infection that establishes an antiviral state in neighboring cells.

Given Data / Assumptions:

• Virus infects host cells.
• Host responds by producing soluble proteins.
• Goal: identify the correct protein name.

Concept / Approach:
Virus-infected cells produce type I interferons (IFN-alpha and IFN-beta) that act in autocrine and paracrine fashion to upregulate antiviral genes, enhance antigen presentation, and modulate innate and adaptive responses. These are distinct from antibodies (antiserum), pre-exposure vaccines, and inflammatory mediators like histamine.

Step-by-Step Solution:

Verification / Alternative check:
Clinically, recombinant interferons have been used as antivirals and immunomodulators, reflecting their natural role in antiviral defense.

Why Other Options Are Wrong:

• Histamine: mediator of allergy and vasodilation, not antiviral signaling.
• Antiserum: exogenous antibody preparation; not produced by infected cells as a cytokine.
• Vaccine: preventive preparation; not a host cytokine.
• Complement C3: innate opsonin produced mainly by the liver; activation can follow infection but is not the specific antiviral warning protein.

Common Pitfalls:
Confusing antibodies (humoral products) with cytokines (cell-secreted signaling proteins).

Final Answer:
Interferon.