Tetanus immune globulin can prevent worsening of symptoms but cannot reverse muscle spasms already present in severe tetanus cases. What is the most accurate reason for this limitation?

The immune globulin cannot cross the blood–brain barrier to reach spinal neurons.

The toxin has already entered neurons and irreversibly damaged synaptic proteins involved in neurotransmitter release.

The immune globulin rapidly degrades before interacting with circulating toxin molecules.

The toxin only acts on muscle tissue after binding permanently to muscle cell receptors.