system
Given the following post, respond as if you're commenting on Reddit:
human
Can someone teach me about the COVID19 antibody test? What types of biomarkers are we looking for and what does that tell us?


I’ve been reading all about the effort for antibody testing and learned that there are different types of antibodies such as IgM, IgG. What are the different types? What can they tell us? Do you need a certain level of each antibody to correlate to a 95% immunity against the next infection?
gpt
An antibody is a Y-shapes protein with two binding sites for some “antigen”, which hopefully are foreign particles. This is called the variable fragment or sometimes the Fab. If this binds to self proteins, this is what leads to autoimmune reactions. The two binding sites on an antibody are identical, which means that both of the Fab need to bind to the same antigen. There are different isotypes of antibodies, which are used by different parts of the immune response. IgG is the most common in the blood, and it has the typically Y shaped structure. IgM actually is like 5 IgGs put together so there are actually 10 antigen binding sites, which we say has an avidity of 10. However, since the avidity is higher, the affinity is usually lower. This is because IgM does not go through affinity maturation, which is a refining process that IgGs go through. There are other isotypes of antibodies, is such as IgE (important in allergic responses/parasite detection) and IgA (important in mucosal surface protection). 

When an unknown particle (antigen) from the virus appears in our body, the objective is for those particles to end up binding with high affinity to immune cells in the spleen or lymph nodes typically (although this can occur in other places such as mucosal layers MALT). After an immune cell binds, there will be various processes that will lead to the proliferation of that cell so that the antibody it created can be used to fight the antigen.