Page 340 - The Veterinary Laboratory and Field Manual 3rd Edition

Serology and immunology 309

following antigen re-exposure. In the secondary Agglutination
response IgG reaches higher levels than in the
primary response and usually remains detect- Agglutination is the clumping together of bio-
able in serum for several years. In mammals, logical material, such as red blood cells (RBC)
the pregnant female is able to pass on some or bacteria, suspended in liquid, usually in
of this ‘immunity’ to the newborn in the first response to a particular antibody. The reaction
milk (colostrum). Intake of colostrum contain- between an antigen and specific antibody can be
ing antibodies provides ‘passive immunity’ to visualized more clearly by attaching a coloured
the newborn but this protection does not last dye to antigen or antibody to highlight the reac-
long and immunity needs to be boosted to tion. RBC and latex beads can also be used.
ensure ongoing protection. Colostrum is crucial
for newborn mammals because most species Complement fixation
receive no passive transfer of immunity via the
placenta before birth, so any antibodies that they Complement fixation is the binding of active
need have to be ingested. This oral transfer of serum complement to an antigen–antibody pair.
immunity can occur because the newborn’s gut This is the basis for various diagnostic tests
is porous. This means that large proteins (such to detect the presence of a specific antigen or
as antibodies) can pass through the gut wall. The antibody. The complement cascade is an impor-
newborn animal must receive colostrum within tant chemical mediator of the inflammatory
6 h of being born for maximal transfer of anti- response. Activation of guinea pig complement
bodies to occur. In birds, the IgY is passed to the in an in vitro system which contains sensitized
chick via the egg yolk. red blood cells (complement fixation test [CFT])
Before we move on to consider specific tests is often used to test for the presence of antibody
it is important to define some commonly used in a test sample.
terms.
Enzyme-linked-immunosorbent assay
(ELISA)
definition and use of terms
ELISA tests are available in various formats but
Sensitivity most involve specific antibody or antigen in the
Sensitivity is the ability of a test to detect known test system that is fixed to a plastic plate or dish.
positive samples (positive reactions). This Test serum or other material is added. Antibody
should be considered in the context of analytical or antigen in the test material binds to bound
sensitivity as well as diagnostic sensitivity. The antigen or antibody. The reaction is visualized
latter relates to the performance of a test in the (coloured) by addition of an enzyme linked to
field whereas the former relates to the detection antibody or antigen and its specific substrate.
limits of the test in the laboratory. Results can be measured by eye or using a color-
imeter. There are now a lot of pen side ELISA kit
tests available for preliminary serological screen-
Specificity
ing. These have been widely used for influenza A
Specificity is the ability of a test to distinguish screening as well as in the screening of a num-
between true positive samples and ‘cross’ reac- ber of small and large animal viral infections and
tions which may give false positive results. Ideally are available in both antibody or antigen capture
a test should be both highly sensitive and specific. formats (see Figure 4.29).

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