Which test(s) are based on the principle of toxin neutralization in medical microbiology?

Introduction / Context:
Toxin neutralization assays assess whether specific antibodies can inhibit the biological activity of a bacterial toxin. Two classic examples are Nagler’s reaction (plate-based antitoxin neutralization of lecithinase) and the Schick test (intradermal diphtheria toxin challenge neutralized by preexisting antitoxin).

Given Data / Assumptions:

• Nagler’s reaction: colony lecithinase halo is abolished adjacent to antitoxin.
• Schick test: a negative skin reaction indicates circulating antitoxin that neutralizes the diphtheria toxin dose.

Concept / Approach:
Neutralization occurs when antibodies bind toxin epitopes, blocking enzymatic or receptor-binding steps. In plates (Nagler), antitoxin diffuses and prevents opaque precipitation of digested lecithin. In vivo (Schick), toxin elicits erythema if not neutralized; prior immunity yields no lesion.

Step-by-Step Solution:

Verification / Alternative check:
Standard references describe both tests as demonstrations of functional neutralization by specific antitoxins.

Why Other Options Are Wrong:

• Widal: Agglutination test for Salmonella antibodies, not toxin neutralization.
• “None”: contradicted by the well-known mechanisms of Nagler and Schick tests.

Common Pitfalls:

• Confusing agglutination or precipitation assays (antigen–antibody lattice) with true functional neutralization tests.

Final Answer:
Both (a) and (b)