The killed bacterial suspension of salmonella carries specific 'o' and 'H' antigen. This will react with immunospecific antibodies which may be present in patient serum and agglutines the antigen to produce agglutination or clumps on the slide. The organism possess 'o' antigen on the cell wall and 'H' antigen on it's flagella against which the host body produces immunospecific antibodies, to counteract the effect corresponding antigens.