**Question:** The protective titre of antitoxin to diphtheria following vaccination is:

A. 8
B. 10
C. 16
D. 30

**Core Concept:**

Antitoxins play a crucial role in neutralizing the toxic effects of various poisons or toxins produced by pathogens. In the context of diphtheria, antitoxin is a critical component of the vaccine, which helps the immune system develop immunity against the diphtheria toxin produced by the bacterium Corynebacterium diphtheriae.

**Why the Correct Answer is Right:**

The protective titre is the minimum level of antitoxin required to neutralize the toxin effectively. In the case of diphtheria, the protective titre corresponds to the minimum concentration of antitoxin required to neutralize at least 99% of the toxin. The correct answer, **16**, represents the minimum antitoxin titre that provides protection against diphtheria toxin. This titre ensures the neutralization of 99% of the toxin, thus offering adequate protection against the disease.

**Why Each Wrong Option is Incorrect:**

A. 8: A titre of 8 indicates less than 99% toxin neutralization, making it insufficient to provide adequate protection against the disease.
B. 10: Although this titre is higher than the correct answer, it still falls short of the 99% toxin neutralization required for protection against diphtheria.
C. 30: Similarly, a titre of 30 is still below the required 99% toxin neutralization, rendering it inadequate for providing protection against diphtheria.
D. 30: As mentioned above, a titre of 30 is insufficient for providing adequate protection against diphtheria.

**Clinical Pearl:**

The protective titre of 16 emphasizes the importance of vaccinations in achieving immunity against life-threatening infections like diphtheria. Maintaining a high vaccination rate is crucial to control the spread of diseases and protect vulnerable populations. In addition, understanding titres helps healthcare professionals determine the appropriate dosage of antitoxin for therapeutic purposes in cases of diphtheria poisoning. Monitoring titres after vaccination also allows healthcare providers to assess the immune response and adjust future vaccination schedules if necessary.