Which of the following is true about the antigen titre in Widal test?
**Core Concept**
The Widal test is a serological test used to diagnose typhoid fever by detecting the presence of antibodies against the O and H antigens of Salmonella Typhi in the blood. The antigen titre refers to the concentration of these antibodies.
**Why the Correct Answer is Right**
The Widal test measures the titre of antibodies against the O and H antigens of Salmonella Typhi. A rising titre of antibodies against these antigens indicates an acute infection, while a falling titre suggests recovery or a chronic infection. The O antigen is a polysaccharide component of the bacterial cell wall, while the H antigen is a lipopolysaccharide component. The body's immune response to these antigens involves the production of IgM and IgG antibodies, which can be detected in the Widal test.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the Widal test is not a rapid diagnostic test, but rather a serological test that requires a blood sample and a laboratory setting to perform.
* **Option B:** This option is incorrect because the Widal test does not measure the titre of antibodies against all Salmonella species, but rather specifically against the O and H antigens of Salmonella Typhi.
* **Option C:** This option is incorrect because the Widal test is not a rapid diagnostic test, but rather a serological test that requires a blood sample and a laboratory setting to perform.
**Clinical Pearl / High-Yield Fact**
A rising titre of antibodies against the O and H antigens of Salmonella Typhi in the Widal test is a strong indicator of an acute typhoid fever infection, while a falling titre suggests recovery or a chronic infection.
**Correct Answer:**
(There is no correct option provided, please provide the correct option so I can complete the explanation)