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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Scanning electron micrograph of a human T lymphocyte (also called a T cell) from the immune system of a healthy donor. Photo: CC BY 2.0 NIAID
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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.
String-like Ebola virus particles are shedding from an infected cell in this electron micrograph. Photo: CC BY 2.0 NIAID
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
Associate Professor Jo Kirman is an immunologist at the University of Otago. Photo: University of Otago
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.
“It is very specific,” says Jo. “It will be tailored not only to a particular virus, but to a particular protein in a particular virus – and even a particular part of that protein.”
The immune system, she says, is everywhere but its cells develop in different part of your body. T cells, for instance, develop in the thymus.
“The thymus is basically where T cells go to school and learn how to be a good T cell.”
So what makes a good T cell?
“You’ve got to have a receptor on your surface,” says Jo, “that is able to recognise parts of an invader.”
Jo says that only about 10 percent of T cells graduate from the thymus; the rest end up dying.
The T cell graduates spend their day wandering the body, and they circulate between the blood and the lymph as well as the body’s tissues. They are joined by B cells, which develop in the bone marrow.
T cells are the part of the human immune system best suited to tackling viruses such as the new SARS-CoV-2 virus that causes COVID-19.
There are several different kinds of T cells.
CD8 T cells are also known as Killer T cells. They recognise cells that are infected with a virus and kill the cell before it can spread the virus. However, each T cell only recognises one kind of virus or pathogen, so the protection is very tailored.
Helper T cells, or CD4 T Cells have several roles. They are the command centre for the immune system, and they do this by producing cytokines, which Jo says are basically chemical communicators, able to send messages from one cell to another.
“So those cytokines might say to a T cell ‘proliferate and make more of yourself because there’s a virus here’,” says Jo. “Or they might come up to a macrophage and say ‘activate your antiviral responses because there’s a virus coming’.”
“Or, they might go up to a B cell and say ‘make more antibody and make more of this particular kind of antibody’.”
The B side
Jo says that “B cells are highly critical, because antibodies are really important. They are able to bind to disease-causing organisms - whether that’s bacteria or viruses – and they can completely neutralise them and stop them infecting other cells.”
“If you’ve had a cold or the flu you might have noticed you’ve got swollen glands or swollen lymph nodes,” says Jo. “And that’s your B cells starting to proliferate.”
To find out more about the human immune system, how it fights invaders such as the coronavirus that causes COVID-19 and what this all means for developing a vaccine against the coronavirus, listen to the full interview with Jo Kirman.
Our Changing World has been providing scientific context to the COVID-19 pandemic:
Virus 101 - the science of viruses.
Our immune system vs coronavirus – ‘I think of it as an orchestra’