AS & A Level Biology 9700: Antitoxin, Toxoid Vaccine and Neutralisation

Clostridium tetani bacteria can infect a deep wound and release a toxin that damages the nervous system, causing the disease tetanus. A patient who has never been vaccinated against tetanus receives a deep, dirty wound. Doctors immediately give the patient an injection of tetanus antitoxin, which contains ready-made antibodies against the toxin, produced in another organism. They also recommend that the patient later be vaccinated with tetanus toxoid, which is an inactivated (harmless) form of the toxin.

(a) Explain how the antibodies in the antitoxin injection protect the patient from the harmful effects of the toxin. Use the term neutralisation in your answer. [3]

(b) State and explain the type of immunity provided by the antitoxin injection. [2]

(c) Explain why vaccination with tetanus toxoid, unlike the antitoxin injection, can give the patient long-term protection against tetanus. [3]

(d) Suggest one reason why doctors give the patient both the antitoxin injection and, later, the toxoid vaccine, rather than the toxoid vaccine alone. [2]

Part (a): Neutralisation of the toxin

The antitoxin contains antibodies whose antigen-binding sites are complementary in shape to the active (toxic) part of the tetanus toxin molecule. When these antibodies bind to the toxin, they physically cover or distort its active region, so the toxin can no longer bind to and damage the patient’s nerve cells. This blocking of a toxin’s harmful action by antibody binding is called neutralisation. The resulting antibody–toxin complexes are then harmless and can be removed from the body, for example by phagocytes.

Part (b): Type of immunity from the antitoxin

Because the antibodies in the antitoxin were produced in another organism and simply given to the patient, rather than being made by the patient’s own B-lymphocytes, this is passive immunity. The patient’s immune system was not itself stimulated to respond, so no memory cells specific to the tetanus toxin are formed as a result of this injection.

Part (c): Why the toxoid vaccine gives long-term protection

The toxoid is a chemically or heat-inactivated version of the tetanus toxin: it keeps the antigenic shape recognised by the immune system, but has lost its ability to damage the nervous system. When injected, it acts as an antigen and triggers a genuine primary immune response in the patient. B-lymphocytes with a complementary receptor are selected (clonal selection), divide by mitosis (clonal expansion), and differentiate into plasma cells, which secrete antibodies against the toxin, and into memory cells.

The memory cells remain in the body for years. If the vaccinated patient is later exposed to the real toxin, these memory cells allow a rapid secondary response, producing large amounts of neutralising antibody quickly enough to prevent harm. Because this protection is generated by the patient’s own immune system and includes memory cells, it is active immunity, and it lasts far longer than the passive protection from the antitoxin.

Part (d): Why both treatments are given together

A primary immune response to the toxoid vaccine is not instant: there is a lag period of days to a few weeks before enough antibody and memory cells are produced to give reliable protection. However, a dirty wound infected with Clostridium tetani is an immediate risk, since the bacteria can begin releasing toxin straight away. Giving the antitoxin provides immediate, ready-made antibody to neutralise any toxin produced right away (passive protection), covering the patient during the period before the toxoid vaccine has had time to stimulate the patient’s own long-term active immunity.

Final answers

(a) Antibodies in the antitoxin bind to and block the toxin’s active region (neutralisation), preventing it from damaging nerve cells.
(b) Passive immunity. The antibodies were made by another organism, not the patient’s own B-lymphocytes.
(c) The toxoid triggers the patient’s own primary response, forming plasma cells and long-lived memory cells, enabling a fast secondary response and long-term active immunity later.
(d) The antitoxin gives immediate passive protection while the slower-developing active immunity from the toxoid vaccine builds up.