**Question:** Substance present in both serum and plasma: September 2007

**Core Concept:** Serum and plasma are the two primary components of blood, which are derived from the same source but have some differences due to the presence of certain substances. Serum is the liquid portion of blood obtained after removal of cells and platelets through centrifugation, while plasma is the liquid portion of blood without additives, which means it may contain certain substances present in the original blood.

**Why the Correct Answer is Right:** The correct answer refers to a substance that is common to both serum and plasma. Serum and plasma share many common components, including proteins, electrolytes, and various enzymes. However, they differ in the presence of certain substances, such as clotting factors, antibodies, and glucose.

**Why Each Wrong Option is Incorrect:**

A. **Option A (Glucose):** Although plasma contains glucose, serum does not contain it because glucose is removed during the process of serum preparation by centrifugation.

B. **Option B (Clotting Factors):** Clotting factors are specific proteins involved in blood coagulation. Plasma contains these factors, while serum does not, as they are removed during serum preparation.

C. **Option C (Antibodies):** Antibodies are proteins produced by the immune system in response to foreign substances. They are primarily present in plasma, not serum, as serum is derived from plasma after removal of cells and platelets.

D. **Option D (Hemoglobin):** Hemoglobin is a protein found in red blood cells (RBCs). Serum does not contain hemoglobin as RBCs are separated during serum preparation via centrifugation.

**Clinical Pearl:** Understanding the differences between serum and plasma is crucial for medical professionals, as it enables accurate interpretation of laboratory test results involving serum and plasma samples. For example, the presence of hemoglobin in a serum sample may indicate hemolysis (RBC lysis) or a contamination issue.

**Correct Answer:** Correct Answer: Option C (Antibodies)

**Explanation:** Antibodies are proteins produced by the immune system in response to foreign substances. They are primarily present in plasma, as plasma contains various components derived from cells, including RBCs, platelets, and white blood cells, which are removed during serum preparation through centrifugation. Consequently, the correct answer is antibodies, as they are primarily found in plasma and remain in serum due to the removal of cells during serum preparation.