This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Systemic Bacteriology (Gram Positive Bacilli, Gram Negative Bacilli), Subject name: Microbiology
[{'content': 'Which of the following is true about the antigen titre in Widal test? \nSelect and explain. \nA. O-antigen titre remains positive for several months & reaction to it is rapid., B. H-antigen titre remains positive for several months & reaction to it is rapid, C. Both remain positive for several months & reaction to both is rapid, D. None', 'role': 'user'}, {'content': "Answer: B \nExplanation: H-antigen titre remains positive for several months & reaction to it is rapid. Option 1, 3 - O Abs appear early and disappear early and indicate recent infection. H Abs appear late and disappear late. Option 2 Widal test: it can be done in second week and third week. Principle: It is an agglutination test where H and O antibodies are detected in the patient's sera by using H and O Antigens. Antigens used: Four antigens are used: O antigen of S.Typhi (TO) H antigens of S.Typhi (TH) S.Paratyphi A (AH) S.Paratyphi B (BH) - H agglutinins persist longer than O agglutinins. Four-fold rise in antibody titre in paired sera at 1 week interval is more meaningful than a single high titre. - After 1 week if titre rises- indicates true infection. - After 1 week if titre falls- indicates anamnestic responses.", 'role': 'assistant'}]