**Core Concept**
Direct Coomb's test is a laboratory test used to detect the presence of antibodies or complement proteins that are attached to the surface of red blood cells (RBCs) in a patient's serum. This is an important diagnostic tool for identifying autoimmune hemolytic anemia (AIHA) and other conditions where the immune system attacks the body's own RBCs.

**Why the Correct Answer is Right**
The Direct Coomb's test detects antibodies or complement proteins that are attached to the RBC membrane through the process of complement activation. This occurs when antibodies bind to antigens on the RBC surface, triggering the complement cascade, which ultimately leads to the destruction of the RBC. The test is performed by adding the patient's serum to a sample of RBCs and observing for hemolysis (RBC destruction) using a microscope or other methods.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as the Direct Coomb's test does not detect antigens on the RBC surface, but rather the antibodies or complement proteins that bind to those antigens.
**Option B:** This option is incorrect as the Indirect Coomb's test is a different test that detects antibodies in a patient's serum that are not attached to RBCs.
**Option C:** This option is incorrect as the Direct Coomb's test is not used to detect the presence of specific RBC antibodies, but rather the presence of antibodies or complement proteins attached to RBCs.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the Direct Coomb's test can be positive in conditions other than AIHA, such as drug-induced hemolytic anemia or RBC transfusion reactions.

**Correct Answer: D. Antibodies or complement proteins attached to the RBC surface.**