Your immune system is like your own personal army: a vast network of cells, tissues and organs that coordinate your body's defences against illness and disease.

Yet how the immune system responds to coronavirus — and whether it mounts any lasting defence — is one of the biggest unknowns and most urgent questions facing researchers and health authorities right now.

"Like most other viruses, we're going to mount an immune response and in the short term that should mean that we're protected [from reinfection]," said Larisa Labzin, an immunologist from the University of Queensland.

"What we don't understand yet is exactly what that protection is, and how long it's going to last."

With less than four months of data — the outbreak only emerged in late 2019 — questions about immunity, especially in the long term, are difficult to answer.

Here's what we know so far.

How your body mounts a response

When your body detects an foreign invader like SARS-CoV-2, the new coronavirus that causes COVID-19, your immune system quickly triggers a series of responses to try to identify and remove it.

The first line of defence is what's known as the innate immune response, which is effectively "an amping up" of the immune system, said Dr Labzin.

"We tend to think of it as the non-specific response ... able to respond to anything that's invading and presenting a danger to the body," Dr Labzin said.

The innate immune response usually helps to slow down the infection before the adaptive immune response kicks in, which consists of antibodies made by B-cells and antiviral cell-killing T-cells.

Armed with more information about the infection — including the unique proteins found on the virus's surface called antigens — the body produces T-cells that go in search of infected cells and kill them.

Meanwhile, B-cells produce special proteins called antibodies which latch onto antigens like a lock and key. The first type of antibody to appear is called immunoglobulin M, or IgM.

To target and destroy the infection, the immune system refines this antibody into a second type called immunoglobulin G, or IgG, which can specifically recognise and neutralise the virus, and stop it entering new cells.

It typically takes a week or two for these antibodies to appear.

After your body has cleared the virus infection, a subset of antibodies and memory cells stick around — sometimes for years — ready to protect you if you encounter the same virus again.

What we know about COVID-19 immunity

Infection with COVID-19 activates your immune system, but some people will mount a better immune response than others, said virologist Bill Rawlinson.

"There have been papers looking at antibody [response] and other forms of immunity and we know that both types of antibody — the IgG type and IgM type — come up in a couple of weeks," said Professor Rawlinson from the University of New South Wales.

Antibodies are proteins produced by B cells that bind to foreign substances that invade the body.

This response, it may be assumed, will offer some protection against the virus in the short- to medium-term. But it's too early to say how long that immunity will last.

"What you might do is look at the other four coronaviruses … that cause the common cold every year," Professor Rawlinson said.

"We know that [this immunity] lasts for sort of months to years, so we could expect that SARS-CoV-2 immunity would not be forever."

Some viruses, such as polio and measles, generate antibodies that can provide lifelong immunity. For others it may be just a few years, as is seen in Severe Acute Respiratory Syndrome (SARS).

"We think [COVID-19] is likely to be similar, because the virus is so similar."

Questions around antibodies

Small, early studies in animals suggest COVID-19 immunity sticks around for a few weeks at least.

Chinese researchers found a small group of monkeys that had recovered from COVID-19 produced enough antibodies to be protected from re-infection two weeks later.

But a recent study of 175 recovered coronavirus patients in Shanghai — which has not yet been peer-reviewed — found nearly a third of people had unexpectedly low levels of neutralising antibodies, and ten people had no detectable traces at all.

While this research raises questions about the human body's ability to produce certain antibodies in response to COVID-19 infection, Dr Labzin said the data did not mean the patients had built no lasting immunity.

"For a lot of viruses, we produce antibodies against them, and the majority of those antibodies are neutralising … they're able to stop the virus from entering cells," she said.

"But antibodies can have more functions … and we don't know for coronavirus whether neutralising antibodies are that important for overall immunity."

Everybody's immune response is slightly different and depends on their age, genetic make-up, and overall health.

One of the many unanswered questions is what is going on with the immune response of people who have the virus don't show any symptoms (asymptomatic) and those who only have very mild symptoms.

Andrew Taylor-Robinson, a professor of infectious disease immunology at Central Queensland University, said people infected with COVID-19 who were asymptomatic probably had a "very robust immune response".

He said even mild cases were likely to generate antibodies and other protective mechanisms.

But Dr Labzin said it was possible asymptomatic people built a "smart immune response" rather than a strong one, which meant they produced fewer antibodies.

"Maybe in asymptomatic people, the immune response was in there early and managed to control things, so that they didn't develop severe symptoms," she said.

"But in terms of measuring absolute levels of antibodies or immune cells, it might be a lot smaller than someone who showed strong symptoms, just because their immune response wasn't as smart or quick … so there was a lot more virus to fight."

What about reports of reinfection?

There have been several reports of people who have tested positive for COVID-19 after apparently recovering from the disease, fuelling concern that infection doesn't guarantee immunity, even in the short-term.

But most experts agree these cases don't represent reinfections, and are more likely to reflect the imperfect nature of testing.

One possibility is a false negative test: where the virus never actually left the person's body, but temporarily dipped below detectable levels, incorrectly indicating a "negative" result, said Professor Taylor-Robinson.

But the opposite problem can also occur, Professor Rawlinson said, when a very sensitive test returns a positive result even when a person has recovered from COVID-19 and is no longer ill.

"In that setting … there is virus RNA there," he said.

"That RNA is not actually able to grow and cause infection, it's just residual, but it takes a while for the body to get rid of it."

Warnings against immunity tests

On Friday, the World Health Organisation (WHO) cast doubt on the use of antibody tests — officially known as serologic tests — as a reliable measure of COVID-19 immunity.

WHO emergencies expert Mike Ryan said the WHO has no evidence that antibody tests can show an individual is immune or protected from reinfection with COVID-19, despite several countries proposing to roll out such testing to measure immunity.

Dr Ryan said that even if antibodies were effective, there was little sign that large numbers of people had developed them and were beginning to offer so-called "herd immunity" to the broader population.

"A lot of preliminary information coming to us right now would suggest quite a low percentage of the population have seroconverted (to produce antibodies)," he said.

"The expectation that … the majority in society may have developed antibodies, the general evidence is pointing against that, so it may not solve the problem of governments."

Raina MacIntyre, professor of global biosecurity at the University of New South Wales, said it would be premature for governments to plan strategies around assumed immunity — based on serologic tests — when we don't know enough about the immune response to the virus.

"We only have three months of data from the earliest cases," she said.

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