Agglutination of dyed bacterial cells represents which type of reaction?

• A. Direct agglutination
• B. Passive agglutination
• C. Reverse passive agglutination
• D. Agglutination inhibition

Answer: (A)

Direct agglutination occurs when antibodies bind to antigens on intact cells and cross-link multiple cells, producing visible clumps. Here, the dyed bacterial cells themselves carry the surface antigens, so the antibodies in the test bind across many cells and link them together, making agglutination observable without any carrier particles or soluble antigens involved. This distinguishes it from passive agglutination (where a carrier particle is coated with soluble antigen), reverse passive agglutination (where carriers are coated with antibody and react with soluble antigen), and agglutination inhibition (where soluble antigen blocks clumping).