The Discovery of Immunity

Instructor: Bridgett Payseur

Bridgett has a PhD in microbiology and immunology and teaches college biology.

You know that the immune system helps protect you from infections. But have you ever wondered how you know this? Who were the first people to learn that the body can protect itself from germs?

What is Immunity?

Have you ever gotten a flu shot? A vaccination is a way to train your body what threats to look for. The flu shot, for example, is made of influenza virus proteins. Your body learns that these proteins are bad, and the next time you're exposed to them, your defenses kick in and remove the germs before you even feel sick.

But how did we discover how immunity works? What events led to the concepts and theories we know today?

Stopping Smallpox

The concept of immunity was discovered in a somewhat roundabout way. Rather than trying to figure out where disease comes from, people were mostly interested in stopping it.

The first demonstration of immunity was through a process called variolation. Variolation was first practiced in Asia. It involved taking dry parts of the smallpox scab and having a healthy person inhale them through the nose. The patient would develop a mild case of smallpox, and had a significantly higher likelihood of survival than patients infected naturally with smallpox.

Variolation spread throughout Asia and into Africa. In the early 1700s, Lady Mary Wortley Montagu introduced variolation to Europe. However, rather than putting the dried scabs into the nose, they were placed under on the skin. The process was first tested on prisoners and children, who were then purposely exposed to smallpox. When these tests showed the safety of variolation, others also received the procedure.

Meanwhile, on the other side of the Atlantic Ocean, slaves from Africa introduced the process to America. Cotton Mather learned about variolation from his slave Onesimus. Variolation was utilized during the Boston smallpox epidemic in 1721.

Variolation showed scientists that being exposed to a germ one time could protect a person from the disease in the future. While it was far safer than actual smallpox disease, variolation still had a chance of causing death. It also could potentially spread smallpox disease itself.

By the end of 1700s, English physician Edward Jenner realized that milkmaids did not contract smallpox. He thought this might be due to their exposure to the similar but less dangerous disease, cowpox.

Jenner performed a procedure similar to variolation. He took material from a cowpox scab, and injected it into an 8 year old boy, who was then protected from smallpox. Jenner called this process vaccination.

Germ Theory

Scientists were now able to stop people from catching smallpox, but there was still a big piece of the puzzle missing: how was smallpox transmitted from person-to-person anyway?

It wasn't until the late 1850s that scientists were able to answer these questions. Prior to this time, scientists initially thought that disease was caused by problems with the body's fluids. As microscopic life was discovered, they thought that germs could arise spontaneously. Because of this, no one bothered with hygiene and sterile techniques.

Several scientists were central to the development of germ theory, the idea that small microscopic organisms cause disease. The following scientists all provided evidence showing contagious diseases could be transmitted:

  • Joseph Lister - British surgeon who introduced new practices of cleanliness, radically changing the field by the late 1800s.
  • Robert Koch - German physician who discovered the anthrax disease cycle and the culprits of tuberculosis and cholera.
  • Louis Pasteur - French scientist who brought us pasteurization and later the rabies vaccine.

Microscopic bacteria and viruses, living in the environment or the host, could be passed from person to person. Removing the germs could help stop the spread of disease.


Scientists then took the information garnered from the fight against smallpox and the development of germ theory and combined them.

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