Active and passive immunity: fast but fading, or slow with memory.

Antibody level after a vaccine (slow rise, memory) vs an antitoxin injection or colostrum (instant, fades, no memory)

Protect a patient bitten today against tetanus toxin within 24 h, and still leave them protected against a second exposure on day 90.

Calling colostrum or placental antibodies active immunity.

• Active immunity: the host is exposed to antigens (living or dead microbes or other proteins) and makes its own antibodies. It is slow and takes time to reach full effect.
• Active immunity follows either deliberate immunisation or natural infection.
• Passive immunity: ready-made antibodies are given directly to protect the body.
• Colostrum, the yellowish fluid a mother secretes in the first days of lactation, is rich in IgA antibodies that protect the infant; the foetus also receives antibodies across the placenta. Both are passive immunity.
• Vaccination rests on immune memory. A vaccine (antigenic proteins of a pathogen, or the pathogen inactivated or weakened) makes the body produce antibodies that neutralise the real pathogen later, and memory B and T cells that recognise it quickly and flood it with antibodies.
• When a quick response is needed against a deadly microbe, as in tetanus, preformed antibodies or an antitoxin (antibodies against the toxin) are injected. The snakebite injection likewise carries preformed antibodies against the venom. This is passive immunisation.
• Recombinant DNA technology makes a pathogen's antigenic polypeptides in bacteria or yeast, allowing large-scale production of vaccines, for example the hepatitis B vaccine made from yeast.

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