In immunological dose–response, exposure to very low doses of antigen is most likely to induce which phenomenon?

Introduction:
Immune responsiveness depends on antigen dose, timing, and context. At extremes of dosage, the immune system can become unresponsive rather than activated. This question targets the concept of low-zone tolerance—unresponsiveness induced by very small amounts of antigen, contrasted with high-zone tolerance at very high doses.

Given Data / Assumptions:

• Very low antigen doses may fail to provide adequate costimulation or T-cell help.
• Tolerance can be peripheral (e.g., anergy) or central; here we focus on dose-dependent peripheral effects.
• Outcome measures include clonal deletion, anergy, or regulatory cell engagement.

Concept / Approach:
When antigen levels are too low, lymphocytes may receive insufficient signals (signal 1 without adequate signal 2), driving anergy rather than productive activation. This phenomenon is termed low-zone tolerance and is antigen-specific. It is distinct from immunological ignorance (lack of encounter) and from hypersensitivity (pathologic over-reaction).

Step-by-Step Solution:

Verification / Alternative check:
Experimental systems show bell-shaped dose–response curves: strong immunity at intermediate doses, tolerance at very low or very high doses (low-zone and high-zone tolerance, respectively).

Why Other Options Are Wrong:

• Hypersensitivity: requires prior sensitization and is not dictated purely by tiny doses.
• Immunological ignorance: implies lack of encounter, not low-dose signaling.
• Low-zone immunity: contradicts tolerogenic outcome.
• Polyclonal activation: typically driven by mitogens or superantigens, not trace antigen doses.

Common Pitfalls:
Confusing low-zone tolerance with lack of exposure, or assuming any dose is stimulatory if repeated often (context and costimulation matter).

Final Answer:
Low-zone tolerance (antigen-specific unresponsiveness)