Antibodies can be detected within a few days after infection. Antibodies can be divided into two groups, i.e., 1) neutralising antibodies that are capable of inactivating viruses In vitro by being bound to their antigenic neutralisation determinant on the virion and 2) non-neutralising antibodies which are unable to neutralise the virus simply by being attached to their antigenic determinant. Interactions between antibodies and viruses are diverse and function in different ways in tests demonstrating the virus or antibody. Three antibody tests based on three different neutralisation reactions by antibodies and another two ELISA modifications, one showing a binding reaction and the other a blocking reaction by antibodies to be demonstrated, represent five different antigen-antibody interactions used for demonstrating antibodies to viruses.
These five tests are: 1) the first-order virus neutralisation
test, 2) the virus aggregation neutralisation test, 3) the complement-enriched
virus neutralisation test, 4) the conventional antibody ELISA, and 5) the
blocking antibody ELISA. Basic versions of these tests are evaluated. The
reaction in each test is described. The reacting antibodies in the highest
concentration, measuring the antibody titer and the test sensitivity, are,
where relevant, defined as either neutralising or non-neutralising, and
modifications adjusted to possess an appropriately high sensitivity are
evaluated for routine use and for the potential application as a reference or
even gold standard assay, cf. Definitions.
The reaction in a first-order virus neutralisation test is slowly
progressing and enduring, with increasing reaction periods, and the reacting
antibodies are exclusively neutralising. The sensitivity of this test is both
variable and adjustable. The test sensitivity is temperature-dependent and
directly proportional to the reaction time. The very sensitive 37oC/24h
configuration (reaction at 37oC for 24 hours) is concluded to be the ideal
reference and gold standard assay measuring neutralising antibodies to viruses.
In the virus aggregation neutralisation test, the inactivation of
the virus by aggregation is prompt and short-lasting. All antibodies to the
various antigenic determinants on the virus, neutralising or non-neutralising,
react synergistically. The reaction depends strongly on the antibody
concentration. The test sensitivity is rather low and practically not
adjustable, because of which the test is unsuited for demonstrating antibodies
to viruses. This test is almost identical to a conventional 37oC/1h
neutralisation test, which by many has been considered to be a gold standard
assay for demonstrating neutralising antibodies.
In the complement-enriched neutralisation test, the reacting
antibodies of the highest concentration are non-neutralising. The reaction is
almost instantaneous immediately after the addition of complement due to a
prompt reaction of the C1q component of complement with antigen-antibody
complexes formed, but otherwise of first order with extended increasing
reaction periods, following the first-order binding of non-neutralizing
antibodies to the virus. The sensitivity of a 37oC/24h modification is high and
for IgG antibodies equal to that of a conventional antibody ELISA with identical
reaction conditions. However, it is laborious, because of which the latter will
be a better alternative.
The conventional antibody ELISA is basically a first-order assay,
and the reacting antibodies of the highest concentration, determining the antibody
titer and test sensitivity, are non-neutralising. Virus aggregation is
impossible, and the sensitivity is directly proportional to the length of the
reaction time. A 37oC/24h configuration will be the ideal reference and gold
standard antibody assay for measuring titers of non-neutralising antibodies in
the highest concentration and test sensitivities.
The blocking antibody ELISA is in its basic form, simple, very
sensitive, specific, and applicable for automation. It is well-suited for
routine, large-scale examinations. The sensitivity of the blocking antibody
ELISA depends on the reaction temperature and time, but the reaction is not of
first order. Increasing the reaction time from 1 to 24 hours in a herpesvirus
test raises the sensitivity by approximately a factor of 4. The reaction rate
is decelerating with increasing reaction time, but still, the sensitivity is
relatively high with extended reaction. With herpesviruses, the sensitivity of
a 37oC/24h test is twice that of the 37oC/24h first-order neutralisation test,
although 4 times lower than that of the conventional antibody ELISA. In its
basic configuration, it is the test best suited for large-scale antibody
examinations in diagnosing and controlling viral infections. The reactants can
be varied to serve different objectives.
The sensitivity and specificity of 37oC/24h modifications of the
three antibody tests found of special value are regularly over 99 per cent when
undiluted serum/plasma samples are examined. A particular subnormal level of
antibodies associated with infection of young individuals having maternal
antibodies capable of reducing the stimulus to antibody formation is described.
This condition underlines that when the diagnosis of an earlier infection is
decisive, e.g. in connection with control of herpesvirus infections in the
veterinary medical field, a high test sensitivity is crucial. This study also
addresses SARS-Cov-2 antibody testing.
Author (s) Details
Viggo Bitsch
The State Veterinary Serum Laboratory, Copenhagen and The Department of
Cattle Diseases, The Danish Dairy Board, Aarhus, Denmark.
Please see the book here:- https://doi.org/10.9734/bpi/mbrao/v3/5447
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