Widal test, a widely used serological test for enteric fever, is a type of

Concept:

• The Widal test is widely used in diagnostic laboratories to diagnose enteric fever (Typhoidal or Paratyphoidal fever) serologically. 
• The O and H antigens of Salmonella typhi and A, B, and C antigens of Salmonella paratyphi are the targets of this assay, which assess the level of serum antibodies. 
• O and H antigens are present in Salmonella typhi on the cell wall and flagella, respectively. 
• While AH and BH antigens are present on the flagella of Salmonella paratyphi.
• In this test, the O antigen of Salmonella paratyphi is not used because they react with the O antigens of Salmonella typhi.
• These antigens are immunogenic, which means that when they are presented to a host's immune system, they cause the development of particular antibodies. 
• Serum levels of salmonella antibodies begin to show up around the end of the first week and rapidly increase during the third week of endemic fever. O agglutinins in acute typhoid fever can typically be seen 6–8 days after the commencement of the illness and H agglutinins 10–12 days later.
• To show an increasing antibody titre, it is advisable to analyse two sera samples at intervals of 7–10 days.
• Slide and tube procedures can be employed with salmonella antigen suspensions.

Explanation:

Option 1: Precipitation test 

• Precipitation is a reaction that leads to the formation of insoluble solid mass from the chemical reactions between ions (ionic compounds) present in the solution. 
• In the Widal test, insoluble solid moas are not the result of ions reaction.
• Hence, this is an incorrect option. 

Option 2: Agglutination test

• Agglutination is a reaction that leads to the formation of solid mass from aggregating suspended particles in the solution. 
• In the Widal test, suspended particles are antigens and antibodies that lead to the formation of a solid mass.
• Hence, this is the correct option.

Option 3:Complement fixation test

• The complement fixation test is one of the most important tests used to determine the Ag-Ab complex present in the testing sample.
• The complement molecules cannot bind to antigens alone or antibodies alone, they can only bind to the Ag-Ab complex. Once, it is bonded to the complex, the complement is fixed and a further reaction takes palace.
• In the Widal test, we do not use any complement to bind to the Ag-Ab complex.
• Hence, this is an incorrect option. 

Option 4: Immunofluorescence detection

• Immunofluorescence is a vital immunochemical technique which allows the localization and detection of antigens in the tissues of different cell preparation. 
• In this process, antibodies are tagged with fluorophores. 
• In the Widal test, fluorophores antibodies are not used for the detection of antigens.
• Hence, this is an incorrect option. 

Hence, the correct answer is option 2 i.e. agglutination reaction.