Active and passive immunity

Last time

Before we start: last time, in Circulation, the heart and exercise, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Why you only get measles once

Most people who catch chickenpox as a child never catch it again, even when they meet it many times later. Their body remembers the pathogen and destroys it before they feel ill. That memory is immunity. Today: how it works, how vaccines use it, and the short-lived kind, passive immunity.

Antigens and antibodies

Every pathogen has antigens on its surface: molecules with a particular shape. Lymphocytes make antibodies, proteins whose shape is complementary to one antigen. Each pathogen has its own antigens, so a different antibody is needed for each one.

What antibodies do

Antibodies bind to the antigens on a pathogen, which marks it for destruction. They can clump pathogens together, or stop them entering cells. Then phagocytes engulf and digest the marked pathogens.

Active immunity

Active immunity is defence against a pathogen by antibody production in the body. It happens after an infection, or after a vaccination. Memory cells are made, so the protection is long-term.

Memory cells

The first time a pathogen enters, lymphocytes take days to make enough antibodies, so you feel ill. Memory cells stay in the blood for years. If the same pathogen returns, they make antibodies much faster and in greater amounts, so you do not get ill.

Vaccination

A vaccine contains a harmless form of the pathogen, or just its antigens. Lymphocytes recognise the antigens and make antibodies, and memory cells. If the real pathogen later infects you, the memory cells respond quickly. When most people are vaccinated, the disease cannot spread, which protects others too: herd immunity.

Passive immunity

Passive immunity is short-term defence by antibodies made by another individual. A fetus receives antibodies across the placenta, and a baby receives them in breast milk. Doctors may inject antibodies, for example after a snake bite. No memory cells are made, so the protection fades within weeks.

Active or passive?

Here is the comparison. Who makes the antibodies, whether memory cells form, and how long it lasts. The key difference: memory cells only in active immunity.