• Serological tests

Molecules of parasitic origin (viral particles, cell-walls of micro-organisms ...) can act as antigen by inducing, in the tissues of warm-blooded animals, the formation of antibodies with which they react specifically. The serological reaction of antibody fixation can be observed in vitro by introducing a suspension containing the antigens of the pathogen agent and the antiserum. The specificity of the reactions antibody-antigen makes it a very efficient tool for disease diagnosis.

The use of monoclonal antibodies is widely spread in phytopathology because of their specificity to a single antige (epitope), their homogeneity and their continuous and homogeneous production in unlimited quantities.

The main serological techniques are:

Immunoprecipitation

This technique is based on the formation of aggregates bacteria/antibodies that can be observed when a tissue extract is mixed with a specific antiserum. Those aggregates are insoluble and can be seen by the naked eye or with a microscope.

• Done on a slide, in a tube or a Petri dish.
• Rapid but not a very sensitive method

ELISA

Method that has revolutionized the field of disease diagnosis. Its principle consists in revealing the interaction antibody/antigen by marking the imunoglobulins with enzymes involved in a coloured reaction.

• Widely used method
• Simple to use, not expensive, sensitive and specific (depending upon the antibodies used)

Two frequently used methods :

Das-Elisa (Double Antibody Sandwich) :

The Elisa Protocol, the most classical where the antigen is blocked between two layers of antibodies. The intensity is measured through a spectrophotometer at 405 nm.

• This test presents a good sensitivity : (1 à 10 ng of virus, 107 bact / ml)

Tas Elisa :

A secondary antibody is directed against a primary antibody.

• · Method equally sensitive (10pg/?l virus, 106 bact/ml)

Immunofluorescence, method combining microscopy and immunology :

• Method widely used in bacteriology
• Sensitive and specific (105 bact/ml) but relatively time concuming and not always easy to interpret the results

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