1. Basics of antigen-antibody
Antigen are protein, polysaccharide or lipid.
Antibodies are proteins produced and secreted by B cells (lymphocytes) in response to presence of antigen-pathogen
Antibodies are present on the surface of B cells (surface immunoglobulins) serving as antigen receptors or are secreted into the extracellular space where they can bind and neutralize their target antigens.
Each antibody recognize specific antigen. There will likely be multiple antibodies to multiple different binding sites on an antigen such as a virus. Only when 2 different, but similar, viruses have identical structures will cross-reactivity occur. For example, if multiple strains of a coronavirus have maintained regions of nucleic acid that have not undergone mutation, an antibody that targets that region in one may target the identical region in another.
2. Different class of antibody
- Neutralization – Bind and inactivate pathogen/toxin in circulating blood & mucosa. Prevent virus from entering and infecting cells
- Opsonization – Facilitate phagocytosis by phagocytic cells
- Chemostaxis – Activation of complement system
3. Different types of antibody and different antigen
Humans have 5 different class of antibodies (IgM, IgG, IgD, IgA, IgE), and each plays a unique role in immunity.
IgM, IgG, IgA and total antibody(IgA+IgM+IgG) count are the primary targets of COVID-19 serology tests.
IgA
It makes 10-15% of total.
It is primarily responsible for protecting mucosal surfaces & can be found in serum, mucosal secretions, saliva, tears, sweat and breast milk.
SARS-CoV-2 targets the mucous membranes and induces the release of secretory IgA within the first week of symptoms, followed by IgM and IgG in the second week.
IgM
Large, makes 10% of total.
It can be expressed in monomeric form on the surface of B lymphocytes or found circulating in the blood and lymphatic fluid in pentameric form. Less specific. Less affinity but More avidity due to more binding sites due to pentameric structure.
IgG
Small, Most abundant, 70-75% of total.
IgG typically appears later in infection when mature B cells receive signals to switch from production of IgM to IgG. During the secondary immune response, IgG can have many potential roles, including direct neutralization of microbes and targeting of microbes for immune cell-mediated processes. It is transffered to fetus via placenta and conferes immunity to infant until their immune system develops. More specific
Previous studies have shown that IgG antibodies to common coronaviruses peak approximately 2 weeks post infection and return to baseline about a year post exposure.
Spike Protein
Spike protein (S proteins) are unique, mushroom-shaped surface proteins that bind host cells and mediate virus entry. S1 & S2 subunits which facilitate attachment and membrane fusion, respectively.
Nucleocapsid
The nucleocapsid protein (N protein) is a basic RNA-binding protein that plays structural and nonstructural roles in infection.
Receptor Binding Domain (RBD)
RBD represents the portion of the S1 protein that binds angiotensin-converting enzyme 2 (ACE2), the human receptor for SARS-CoV-2.
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Those antibodies that bind specifically to surface structures of SARS-CoV-2, like the spike protein, prevent the virus from interacting with its target cell and are called neutralizing antibodies. These antibodies play an important role in virus clearance because they have the ability to block viral infection and are assumed to protect patients.
Unlike antibodies against the spike protein, antibodies against the nucleocapsid protein do not have a neutralizing effect on SARS-CoV-2 because the target protein is located inside the virus and is therefore not directly accessible for antibodies.
Measuring antibodies to spike will therefore indicate whether there has been a good response to vaccine, whereas measuring antibodies to nucleocapsid would help identify whether the individual had nonetheless become infected. Antibody against S protein have better sensitivity and specificity compared to N protein, more cross reactivity between N protein between different corona virus.
Measuring the different antibodies might also have prognostic value; a report showed that a predominant humoral response to nucleoprotein is associated with poor outcome in patients admitted to hospital, compared with that of spike
4. Different methods of testing
Lab based and cassette based.
Lab based requires blood and more sensitive-specific and give titres.
Cassette systems use the principle of lateral flow immunoassay or immunochromatography and require only 2 drops of blood (10-15microlitre). Less costly and ideal for epidemiological survey in high prevalence population.
Lab based – ELISA, CLIA, Neutralization test (Requires BSL-2,BSL-3)
5. Timing & Interpretation
For the interpretation of results in an effort to make meaningful recommendations, there are several factors that must be considered:
1. Is the subject symptomatic or asymptomatic at the time of antibody testing?
2. If symptomatic, how long since symptom onset?
3. IgM positive or negative?
4. IgG positive or negative?
5., Total antibody positive/negative?
Both IgM and igG negative – no past infection and no immunity against infection , It does not ruleout if subject is recently exposed to virus / symptomatic within 7 days.
igG positive and igM negative – likely past infection >2 week ago. Need PCR to ruleout active sheding if symptomatic recently.
igM positive and igG negative – Infectious, >5-7 days since onset
IgM and igG both positive – Likely >10-14 days since onset
>100 kits are ICMR approved till now for diagnosis of COVID-19 including molecular tests. Different kits have different cut off . So comparison between various lab using different kits can not be done and it needs to be validated.
Titre : Low titre in mild/asymptomatic cases and more titre in severe cases. Different kits have different cut off , so titre can not be compared between different labs.
Indications:
- Epidemiological – Serosurveillance (in high prevalence population)
- Identify potential plasma donor
- Evaluate vaccine response
- Diagnosis in later phase when pcr negative or not readily available
Excellent tool if used along with molecular test and clinical correlation is done.
When not to be used:
- To diagnose acute or recent cases of COVID-19.
- To establish protective immunity
- To establish infectivity/diagnose active infection
It’s important to consider the possibility that not everyone will develop neutralizing antibodies to SARS-CoV-2. When neutralizing antibodies are developed we still donÂ’t know whether they are fully protective, what titer is needed to have full protective immunity or how long the titers will persist.