True about exotoxins –
**Core Concept**
Exotoxins are a type of bacterial toxin produced by certain bacteria, such as Clostridium tetani, Corynebacterium diphtheriae, and Bacillus anthracis. These toxins are secreted into the environment and can cause a range of diseases in humans. Exotoxins are highly potent and can be lethal in small quantities.
**Why the Correct Answer is Right**
Exotoxins are produced by bacteria as a means of defense against predators and competing microorganisms. They work by binding to specific receptors on host cells, which triggers a series of downstream effects that can lead to cell death or tissue damage. For example, the exotoxin produced by Clostridium tetani, tetanospasmin, blocks the release of neurotransmitters in the nervous system, leading to muscle spasms and rigidity.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because endotoxins, not exotoxins, are associated with the cell wall of Gram-negative bacteria.
* **Option B:** This option is incorrect because antitoxins are antibodies that neutralize the effects of exotoxins, but they are not exotoxins themselves.
* **Option C:** This option is incorrect because exotoxins are not produced by viruses.
**Clinical Pearl / High-Yield Fact**
Exotoxins can be neutralized by antitoxins, which are antibodies that specifically bind to the toxin and prevent it from binding to host cells. This is the basis for post-exposure prophylaxis and treatment of exotoxin-mediated diseases.
**Correct Answer:** D.