Well, not knowing the exact details of the antibody test, I can merely surmise that it detects extremely small amounts, ie ineffective amounts.
The infectious cycle starts when you are exposed to a single (usually more than one) virus. At first it simply penetrates your bodily cells and the viral DNA/RNA turns them into a factory for producing more viruses. In an exponential growth, this happens rapidly among the 35 trillion cells in the body.
Somewhere along the line, it tries to infect the wrong cell, usually a lymph node cell, where instead of more viruses, it produces the correct antibody to kill the infection. Between these two events you are normally asymptomatic. Some viruses simply produce 2 or 4 copies of themselves. Some turn host cells into factories producing thousands of virus phages. I am not familiar with the mechanism of growth for Corona Virus. Â Anyway, in between, there is some finite time period that the patient is asymptomatic, and, producing antibodies. The first couple days a human body might not produce any antibody, for the next 5-11 days it will produce atibodies at an ever increasing rate, even when you are over the disease, it will likely over-produce antibodies. But, a certain level of antibodies is needed to maintain immunity to the disease. Eventually your body will decrease productioin of the disease specific antibody intill you get a partial infection again…then you might get the “sniffles” for a day while your body ramps up production, again. We have all experienced days where we don’t feel all that well, but the next day feel fine…left over immunity, often a virus. Sometimes the “immunity” can cause interactions with other “immunities” as in the case of “mumps” and “shingles”. You cannot get shingles without the antibody for mumps. Anyway… “immunity” can lead to other “not-immunity” to other diseases. I believe this answers the question of how long immunity will last. Again, it is statistical, but usually well weighted to recovery based on how soon you have recovered from the disease. It is possible to be immune (not have any virus in your system,) catch a disease, (have a virus in your system) and have the body kill it 100% within 24 hours. No one has this type of immunity with COVID19 that we have found.
The production of antibodies means one of two things: you have had the disease and are over it, or, you currently have the disease. There are several mechanisms for antibody production, and action, I won’t get into here. Anyway, tests can detect proteins very easily by type, so a really small amount is detectable in a test. But they need your history to determine which phase you are in. Some people react badly, ie, produce the wrong antibody…usually fatal.
Different individuals have different reactions to being infected. You have 35trillion cells in your body, it takes an antibody producing cell to be infected to start producing antibodies (resistance) to any disease so it is entirely statistical. This answers the question of how much immunity anyone has…it is expressed as a bell curve normalized to the infectious rate of the disease…think of a normal gausian curve offset over a population to the regular infectios cycle of a disease. This is like saying how infectious is a disease? Note a fatality is NOT cured for the purposes of normalization.
Herd immunity?? Not a real thing. This is only a sociological term to express a 50% exposure of a population. We WILL get there. But it is mixing topics, ie not apples and oranges. It is never 100% safe to allow exposed people back into the general population because they “could” always carry small amounts of the disease at any time. But, the chances of picking it up is very low as long as they have a minimum level of antibodies and have already “gotten over” the disease. No, I don’t know what a “minimum level” is. No, I don’t know what the definition for “gotten over” a virus is…it may have become permanently embedded into your genetic makeup for life. I don’t do trumpisms well, I guess. Except he would never say this because it would take a full 18 months at the current rate. He would say NOW it is safe to ignore the virus because I have antibody doners and I don’t care how many I kill off and the upper 10% of the age group of the world doesn’t pay me anyway and I don’t understand all this shit.