**Core Concept**
The Direct Coomb's test is a laboratory assay used to detect the presence of antibodies or complement proteins that are bound to the surface of red blood cells (RBCs). This test is essential in diagnosing autoimmune hemolytic anemias, where the immune system produces antibodies against the patient's own RBCs.

**Why the Correct Answer is Right**
The Direct Coomb's test specifically detects antibodies that are attached to the RBC surface, indicating an immune-mediated destruction of these cells. This is achieved through the addition of complement proteins, which bind to the antibodies and mark the RBCs for destruction. The test is typically performed using a combination of antiglobulin serum and complement, which helps to identify the antibodies bound to the RBC surface. The presence of these antibodies can lead to the activation of the complement system, resulting in the lysis of the RBCs.

**Why Each Wrong Option is Incorrect**
**Option A:** Antigen in serum - The Direct Coomb's test does not detect antigens in the serum; it detects antibodies or complement proteins attached to the RBC surface.

**Option C:** Antigen in RBC surface - The Direct Coomb's test is not designed to detect antigens on the RBC surface, but rather the antibodies that bind to these antigens.

**Option D:** Antibodies in serum - While the test indirectly detects the presence of antibodies in the serum, it specifically looks for these antibodies bound to the RBC surface, not in the serum alone.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the Direct Coomb's test can be positive in cases of autoimmune hemolytic anemia, where the immune system produces antibodies against the patient's own RBCs. This can lead to the destruction of these cells, resulting in anemia.

**✓ Correct Answer: B. Antibodies in RBC surface**