What mechanism protects against a potentially lethal dose of Clostridium tetani after immunization?

The correct answer highlights the importance of the immune response generated by vaccination against Clostridium tetani. After immunization, particularly with the tetanus toxoid vaccine, the immune system produces antibodies that specifically target the tetanospasmin toxin produced by the bacteria. This exotoxin is responsible for the severe muscle spasms and rigidity characteristic of tetanus.

When antibodies bind to the exotoxin, they neutralize its effects, preventing it from binding to motor neurons and entering cells. This neutralization is crucial as it protects the nervous system from damage and the resulting potentially fatal symptoms associated with tetanus.

In essence, the ability of the immune system to generate a targeted response to the exotoxin is vital for providing immunity and protection following exposure to Clostridium tetani, making it clear why the mechanism involving antibody binding to the exotoxin is the correct response.