Immune System Response
When the virus passes the body's first line of defence (the skin and mucus membranes) then the second line of defence kicks in. This is called the innate immune system. The infected cell releases interferon proteins that let neighbouring cells know there is a problem so they can create molecules to stop the virus from entering. Also interferon beckons cells such as macrophages in the bloodstream.
As part of this defence system there are two types of white blood cells, phagocytes and lymphocytes. The phagocytes' membrane surrounds the pathogen and engulfs it, breaking it down and destroying it. As phagocytes do this to all pathogens they encounter they are known as 'non-specific'.
The third line of defence is the adaptive immune response. When this is triggered the invading microbe or pathogen is then called an antigen (short for 'antibody generator' and is any substance that can spark an immune response). Antigens are proteins that are found on the surface of the pathogen and are unique to that pathogen.
When the SARS COV-2 virus enters the body a foreign antigen is detected and interferon sends out an alert. Certain white blood cells called lymphocytes recognise these antigen proteins on the surface of pathogens and then either produce antibodies (by the B lymphocytes) which recognise part of the spike protein and bind to it which will stop the spike from attaching to a cell. Interferon also recruits T cells which can destroy any body cells compromised by the pathogen (T lymphocytes). Some B and T cells become memory cells which can identify and fight a future infection by the virus.
The antibodies are Y shaped proteins that are produced by B lymphocytes and are specific and complementary to an antigen. This means that one antigen will perfectly fit the antibody and the antibody will be able to stop it, but that antibody does not work on any other antigen. These antibodies are key for some of the COVID-19 tests.
They are part of a larger family of chemicals known as Immunoglobulins (Ig), which play many different important roles in an immune response. Antibody tests for COVID-19 look for the IgG antibody in the blood sample.
The production of antibodies that recognise particular viral antigens can last for many months as the viral infection will have caused an expansion of the population of lymphocytes that recognise the virus. These are called memory cells and mean that the body will be able to act quickly to fight the virus if the body is exposed to it again.
Long lasting immunity to a virus depends on both antibodies and the memory cells that recognise that virus. However, this does assume that the virus does not mutate too much so the body can still recognise it, but mutation is limited as the virus still has to bind to its host cell so cannot change too much.
It is not known yet how much immunity a person who can show that they have antibodies to COVID-19 has as those antibodies may not be very effective at neutralising the virus. However, for the other severe coronaviruses SARS and MERS people generated antibodies for at least a few years research has shown, however on the other side the strains of coronaviruses that cause the common cold can reinfect the same person within a year.
An overactive immune response is thought to play a role in the disease progression in the severe cases of COVID-19.
Once the virus has infected the lung cells the immune cells identify the virus and produce cytokines as part of the inflammatory response. These cytokines attract more immune cells such as white blood cells which in turn produce more cytokines creating a cycle of inflammation which damages the lung cells.
Damage can occur through the formation of scar tissue (fibrin) that can get in the way of oxygen trying to pass into the blood stream.
Weakened blood vessels allow fluid to seep in and fill the lung cavities which can lead to respiratory failure.