The first is always ready to go and leaps into action as soon as any foreign invader is detected in the body. It is known as the innate immune response and includes the release of chemicals that cause inflammation and white blood cells that can destroy infected cells.
But this system is not specific to coronavirus. It will not learn and it will not give you immunity to the coronavirus.
Instead you need the adaptive immune response. This includes cells that produce targeted antibodies that can stick to the virus in order to stop it – and T cells that can attack just the cells infected with the virus, called the cellular response.
This takes time – studies suggest it takes about 10 days to start making antibodies that can target the coronavirus and the sickest patients develop the strongest immune response.
If the adaptive immune response is powerful enough, it could leave a lasting memory of the infection that will give protection in the future.
It’s not known if people who have only mild symptoms, or none at all, will develop a sufficient adaptive immune response.
Understanding of the role of T-cells is still developing, but a recent study found people testing negative for coronavirus antibodies may still have some immunity.
For every person testing positive for antibodies, it was found two had T-cells which identify and destroy infected cells.
Have people caught it twice?
There were early reports of people appearing to have multiple coronavirus infections in a short space of time.
But the scientific consensus is that testing was the issue, with patients being incorrectly told they were free of the virus.
Researchers conclude reinfection is uncommon but still possible and say people must continue to follow current guidance, whether they have had antibodies or not.
Experts say re-infection isn’t surprising, but it’s likely to be rare, and larger studies are needed to understand why this might happen.
If I have antibodies am I immune?
This is not guaranteed and that is why the World Health Organization is nervous about countries using immunity passports as a way out of lockdown.
The idea is if you pass the antibody test then you are safe to go back to work. This would be particularly valuable for staff in care homes or hospitals who come into contact with those at risk of developing severe symptoms.
Another issue is that just because you might be protected by your antibodies, it doesn’t mean you cannot still harbour the virus and pass it onto others.
Why does immunity matter?
It matters for obvious personal health reasons and whether you will get Covid-19 multiple times and how often.
Immunity will also affect how deadly the virus is. If people retain some, even imperfect, protection then it will make the disease less dangerous.
Understanding immunity better could help ease lockdown if it is clear who is not at risk of catching or spreading the virus.
If it is very difficult to produce long-term immunity, then it could make a vaccine harder to develop. Or it may change how the vaccine needs to be used – will it be a once a lifetime or once a year like the flu shot.