The human immune system is a complex system where T cells, B cells and antibodies battle invaders such as bacteria and viruses such as the one that causes COVID-19.

Associate Professor Jo Kirman thinks of our immune system as "an orchestra."

"And there's lots of different instruments playing and they all work together to make the music."

The instruments in the immune orchestra have names like phagocytes, T cells and B cells, and together they help the human body repel invaders such as bacteria and viruses, including the new coronavirus that causes COVID-19.

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When we're born, says Jo, we have a very under-developed immune system of our own, with only a small number of instruments. But as babies we're not without protection, because we inherit antibodies and protection from our mother, via the placenta before we're born and through breast milk.

"But then we develop our own orchestra and are able to fight infection," says Jo, who is an immunologist at the University of Otago..

"We've got two arms of the immune system," says Jo. "We've got our innate immune system which is very fast-acting but it's not particularly specific. And then we've got our highly specific adaptive immune system."

The first line of defence against infection, is of course our skin, mucus and ear wax, which prevent microbes gaining entry - but if that fails, then our immune system kicks in.

First responders

The first responders against infection from bacteria and viruses - the paramedics, if you like - are from the innate immune system. "They're there, they're ready," and they respond in a couple of minutes.

Jo says that many of the cells are phagocytes.

"Phagocytic cells like to eat things. They're like the vacuum cleaners of the immune system - they go around sucking things up."

There are a number of different kinds of phagocytes. Macrophages - their name basically means 'big eater' - "gobble up" infected cells or bacterial cells, and then digest and destroy them.

Neutrophils, monocytes and mast cells are also phagocytes, and collectively they are also known as white blood cells or leukocytes.

Good immunity takes time

Jo says the adaptive system is a slower-acting response that can take hours and even days to kick into action. It's the immune equivalent of going to hospital for more complex treatment if the paramedic hasn't been able to solve the problem…

Go to this episode on rnz.co.nz for more details