Types of immunity and vaccination

MDCAT Biology · Biology · Immunity. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Ask two questions about any case of immunity. First, who made the antibodies? In active immunity the person's own lymphocytes make them, so memory cells form and protection lasts for years. In passive immunity ready-made antibodies are received, so protection is immediate but lasts only weeks. Second, did it happen naturally or by medical treatment (artificial)?

A vaccine contains a harmless form of an antigen. It causes a primary response and leaves memory cells. If the real microbe enters later, the secondary response is faster, larger and longer-lasting.

The immune system can also go wrong, as in allergies and autoimmune diseases.

Rules to remember

Natural active: recovering from an infection. Artificial active: vaccination
Natural passive: antibodies across the placenta or in breast milk (colostrum). Artificial passive: an injection of antiserum or antitoxin
A vaccine may be a live weakened (attenuated) microbe, a killed microbe, a toxoid (harmless toxin) or a single antigen
Primary response: slow start, small amount of antibody, short-lived. Secondary response: quick, much more antibody, long-lasting
Allergy = an excessive response to a harmless antigen (allergen); mast cells release histamine
Autoimmune disease = the immune system attacks the body's own tissues, e.g. rheumatoid arthritis, type 1 diabetes, multiple sclerosis
Edward Jenner introduced the first vaccine, against smallpox, in 1796

Common mistake

An injection is not always a vaccine. An injection of antibodies (antiserum) gives artificial passive immunity and leaves no memory cells; only an injection of antigen gives active immunity.

Worked example

A girl has measles at the age of five and recovers. She meets the virus again at fifteen but does not fall ill. What type of immunity does she have, and why did she not fall ill?

Her own lymphocytes made the antibodies, so the immunity is active.
She met the antigen through an ordinary infection, not a vaccine, so it is naturally acquired.
Memory cells from the first infection give a fast, large secondary response that destroys the virus before symptoms appear.

Answer: Naturally acquired active immunity; memory cells produce a rapid secondary response.

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