The physical structure of an antibody allows it to bind to a specific antigen, such as bacterial or viral proteins, to form a complex. Because antibodies are highly specific in what they bind, tests can detect specific antibodies by replicating the antigen which that antibody binds to. Assays can likewise detect specific antigens by replicating the antibodies that bind to them. If an antibody is already bound to an antigen, that antibody and that antigen cannot bind to the test. Antibody tests therefore cannot detect that specific antibody molecule. Due to this binding, if the amounts of antigen and antibody in the blood are equal, each antibody molecule will be in a complex and be undetectable by standard techniques. The antigen, which is bound as well, will also be undetectable. The antibody or antigen is only detectable in the blood when there is substantially more of one than the other. Standard techniques require a high enough concentration of antibody or antigen to detect the amount of antibody or antigen; therefore, they cannot detect the small amount that is not bound during<unk> . The immune system may take several days or weeks to detect antigen in tissue, begin to create antibodies, and ramp up the production of antibodies to counter the antigen. As a result, the antigen molecules outnumber the antibody molecules in the early stages of an infection. Because there are more antigen molecules than antibody molecules, the majority of the antibody molecules are bound to antigen. Thus, tests at this stage are unable to detect sufficient unbound antigen. On the other hand, there may be unbound antigen that can be detectable. As<unk>  progresses, the amount of antibody in the blood gradually rises. Eventually the amount of antibody outnumbers the amount of antigen. At this time, the majority of the antigen molecules is bound to antibodies, and the antigen is undetectable. Conversely, there is a substantial amount of unbound antibodies, allowing standard techniques to detect these antibodies.
Seroconversion
Reverse tolerance
Cross-tolerance
Sensitization 
Recrudescence
Peripheral tolerance
Antigenic escape
Cross-resistance