Seroconversion

In immunology, seroconversion is when a specific antibody becomes detectable in the blood, and the corresponding antigen becomes undetectable. During an infection or immunization, antigens enter the blood and the immune system begins to produce antibodies in response. Seroconversion is the point in time when the amount of antibody in the blood exceeds the amount of antigen, and the antibody becomes detectable. Before seroconversion, the antigen is detectable, but the antibody is not.

Mechanism

The physical structure of an antibody allows it to bind to a specific antigen to form a complex. Because of this binding, if the amounts of antigen and antibody in the blood are equal, each molecule will be in a complex and be undetectable. The antibody or antigen is only detectable in the blood when there is more of one than the other.

Early in an infection, the antigen molecules outnumber the antibody molecules, and there will be unbound antigen that may be detectable, while all the antibody molecules are bound. After seroconversion, there is more antibody than antigen, so there is a detectable amount of free antibody, while all the antigen is bound and undetectable.

Terminology

During seroconversion, when the amounts of antibody and antigen are very similar, it may not be possible to detect free antigen or free antibody. This may give a false negative result when testing for the infection. This time is referred to as the window period.

Serology (testing for antibodies) is used to determine if specific antibodies are in an organism's blood. Serostatus is a term denoting the presence or absence of particular antibodies in an individual's blood. Before seroconversion, the blood test is seronegative for the antibody; after seroconversion, the blood test is seropositive for the antibody.

The word seroconversion is often used in reference to blood testing for anti-HIV antibodies. In particular, "seroconverted" has been used to refer to the process of having "become HIV positive".

Seroreversion is the opposite of seroconversion. This is when the tests can no longer detect antibodies in a patient’s serum.^[1]

Background

The immune system maintains an "immunological memory" of infectious pathogens to facilitate early detection and to confer protective immunity against a rechallenge. This explains why many childhood diseases never recur in adulthood (and when they do, it generally indicates immunosuppression or failure of a vaccine).

In the initial (primary infection) phase of the infection, immunoglobulin M (IgM) antibodies are produced and as these levels drop (and become undetectable) immunoglobulin G (IgG) levels rise and remain detectable. Upon reinfection, IgM antibodies usually do not rise again but IgG levels will increase. Thus an elevated IgM titre indicates recent primary infection, while the presence of IgG suggests past infection or immunization.

References