Antibodies are proteins produced by the immune system to identify and neutralize foreign substances like bacteria and viruses. They are a key component of adaptive immunity, learning to create specific antibodies to eliminate threats and provide protection. Immunoglobulin G (IgG) is one type of antibody, particularly relevant for understanding past infections or immune responses to vaccinations, especially concerning COVID-19.
Understanding IgG Antibodies
Immunoglobulin G (IgG) is the most common type of antibody found in human blood and extracellular fluids, making up about 75% of the total antibodies in serum. These Y-shaped proteins consist of four peptide chains: two identical heavy chains and two identical light chains, all held together by disulfide bonds. This structure allows IgG to bind specifically to antigens, which are unique markers on pathogens.
IgG antibodies neutralize pathogens by binding to them and preventing them from entering or damaging cells. They also mark pathogens for destruction, making them easier for other immune cells, such as phagocytes, to recognize and eliminate. A unique property of IgG is its ability to cross the placenta, providing passive immunity from a mother to her unborn child, thereby offering protection during the initial months of life.
IgG’s Role in COVID-19 Immunity
IgG antibodies are generated in response to SARS-CoV-2, the virus that causes COVID-19, whether from a natural infection or through vaccination. After natural infection, the immune system produces IgG antibodies targeting various parts of the virus, including the spike protein, which is crucial for the virus to enter human cells. The presence of these antibodies indicates a past exposure to the virus.
Vaccination also stimulates the production of IgG antibodies, typically focusing on the spike protein of SARS-CoV-2. These vaccine-induced antibodies help prepare the immune system to fight off future encounters with the virus. While IgG antibodies contribute to protection against subsequent infection or severe disease, their presence primarily confirms a prior immune response rather than guaranteeing complete immunity. Individuals with prior infection often show higher antibody levels after vaccination, indicating a robust “hybrid immunity.”
Detecting IgG Antibodies
An IgG antibody test for COVID-19, also known as a serology test, determines if a person has previously been infected with SARS-CoV-2 or has developed antibodies from vaccination. This test typically involves a blood sample, which can be obtained through a finger prick or by drawing blood from a vein.
Results are reported as positive, negative, or sometimes indeterminate. A positive result indicates IgG antibodies were detected, suggesting a past infection or vaccination, while a negative result means these antibodies were not found. Antibody tests do not detect active infection, as IgG antibodies take 1 to 3 weeks to develop after exposure. False negatives can occur if tested too soon, and false positives might arise due to cross-reactivity with other coronaviruses or test quality issues. These tests indicate antibody presence but do not quantify protection or predict future immunity.
Duration of IgG and Long-Term Protection
The duration of IgG antibodies against SARS-CoV-2 varies among individuals and depends on factors like disease severity and vaccination status. After natural infection, IgG antibodies can persist for several months, with some studies detecting them for up to 9 to 12 months or even longer. Antibody levels tend to peak around one to three months post-infection or vaccination and then may gradually decline over time.
Vaccination also induces a sustained IgG response, which can last for at least 4 to 6 months, though levels may wane. Despite this decline, the immune system retains a memory of the virus through memory B and T cells, which can quickly produce new antibodies and mount a defense upon re-exposure. While the presence of IgG antibodies indicates past exposure or vaccination, the exact level of antibodies required for long-term protection against re-infection or severe disease is still an area of ongoing research.