The chemotactic factor is:
**Core Concept**
The chemotactic factor is a substance that attracts white blood cells to the site of infection or inflammation, playing a crucial role in the immune response. This process is mediated by chemokines and cytokines, which bind to specific receptors on the surface of white blood cells.
**Why the Correct Answer is Right**
The correct answer is **C5a**, a fragment of the complement component C5 that acts as a potent chemotactic factor. C5a binds to the C5a receptor on the surface of neutrophils, monocytes, and macrophages, triggering a cascade of signaling events that lead to chemotaxis, or movement of these cells towards the site of inflammation. This process is essential for the recruitment of immune cells to the site of infection, where they can phagocytose pathogens and initiate an inflammatory response.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is not correct because while it may be related to inflammation, it is not a chemotactic factor.
**Option B:** This option is not correct because it is a different type of immune response mediator, not a chemotactic factor.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that chemotaxis is a critical component of the immune response, and chemotactic factors like C5a play a key role in recruiting immune cells to the site of infection. This process is essential for preventing the spread of infection and initiating an effective inflammatory response.
**Correct Answer: C. C5a**