When your immune system calls the shots

Bali Pulendran, PhD

A tiny invader, perhaps a virus or a microbe, enters the body, and our ancient immune system responds. But how does it know what kind of invader has landed? And once it knows, how does it decide what kind of immune response it should launch?

In humans, the immune system consists of two parallel systems working with one another to fend off invaders. One is the innate immune system, the other the adaptive immune system.

Immunologist Bali Pulendran studies how those two systems work together to identify and respond to all kinds of intruders including pathogens, viruses and microbes.

It’s the innate immune system’s job to recognize the first signs of infection—that is, the moment a pathogen enters the body. “In a sense they act as smoke detectors if you will,” says Pulendran. “Little alarms.”

And it’s the adaptive immune system that identifies what kind of pathogen has crossed the immune system’s threshold. In other words, the immune system can recognize nearly any antigen in the universe. What’s more, the adaptive immune system has a long memory.

“There’s immune memory,” says Pulendran. “And immune memory is the essence of vaccination. A few decades down the line if there are any re-exposures to that same virus, the immune system will respond much more quickly and rapidly to control the virus.” Much like the immune system does after a vaccination.

Pulendran and his colleagues are now trying to understand how vaccines, especially highly successful ones, work and what makes them effective. “Almost all vaccines have been made empirically. In other words, we don’t really know how these vaccines work. We don’t know the mechanisms by which they work. What’s happened in the last five years or so as we’ve begun to look at the highly successful vaccines, for example, the yellow fever vaccine, which is a live virus.”

What researchers have found is that the strength of the yellow fever vaccine is attributable to its ability to identify not only individual pathogens but unique components of those pathogens and then act on them.

“So, we’re getting closer to reverse engineering some of our best vaccines,” says Pulendran. “Understanding the rules of immunology by which they work. So, armed with this insight we hope to be calm and more rational in designing new vaccines against HIV, malaria, tuberculosis and other emerging infections.”

To listen to Pulendran’s own words about the immune system, access Emory’s new Sound Science podcast.

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