Immunity is the ability of our body to resist disease or an infection. Immunity to viral infection is a combined outcome of both cellular and humoral (antibody) immune responses. While the cellular immune response is often difficult to measure, antibody production serves as a hallmark of the humoral immune response.
The immunity in COVID-19 is due to specific antibodies that prevent the virus from getting attached to the host cell. These are called ‘neutralizing antibodies’ and they are mainly against spike protein (S1 and S2).
These neutralizing antibodies are a subset/part of the IgG and IgM antibodies that most labs are measuring. To measure the real extent of the immunity, therefore, it is important to measure this specific antibody.
These antibodies are not reflected in the estimation of the total antibodies test but are reflected in the amount of neutralizing antibodies present in the bloodstream. So, for an individual who may have a high amount of IgG antibodies in their blood, it is not necessary their Neutralizing antibodies would also be high.
The Neutralizing antibody can be measured actively in the lab in patients who have recovered from a previous COVID 19 infection and also who are currently infected.
It is imperative to estimate this antibody level so that we are aware of the state of our immunity even after the vaccination. Also, post vaccination, if the level of neutralizing antibodies is low, one may need to need to get booster doses.
HOW DO NEUTRALIZING
ANTIBODIES PROTECT US FROM COVID-19?
The virus causing COVID-19, SARS-CoV-2, uses the Spike protein (S) to bind to the receptor on host cells to trigger cell entry and infection.
S protein consists of S1 and S2 subunits, and S1 interacts with the host cells via the Receptor Binding Domain (RBD).
Monoclonal antibodies to S1 protein that exhibit neutralizing activities are being developed as potential therapeutics for COVID-19.
Neutralizing antibodies to the S protein are the key active ingredients of convalescent plasma used to treat severe COVID-19 patients.
In addition, almost all the COVID-19 vaccines currently under development target the S protein with the goal to induce neutralizing anti-S antibodies.