In the case of rabies treatment, if the patient has not received the rabies pre-exposure vaccination then they must receive a dose of immunoglobulin (antibodies against the rabies virus) in addition to the rabies vaccine. Without it, the patient would die before their immune system would be ready to fight the virus. Receiving the dose of immunoglobulin gives the immune system enough time to start producing its own antibodies.

For this reason, travellers to countries where immunoglobulin may not be available are often advised to get the rabies pre-exposure vaccination before they go.