Clumping of RBC may occur when blood of one person is mixed with serum or blood of another person. This is due to

Options

A. antigen-antibody reaction
B. antitoxin-antibody reaction
C. antigen-antigen reaction
D. antibody-antibody reaction

Answer

To answer the question regarding the clumping of red blood cells (RBCs) when blood from one person is mixed with the serum or blood of another person, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Blood Composition**:
- Red blood cells (RBCs) have a membrane that contains specific proteins called antigens. The two main types of antigens present on RBCs are A and B.

2. **Blood Group Classification**:
- Based on the presence or absence of these antigens, blood is classified into four main groups:
- Type A (has A antigens)
- Type B (has B antigens)
- Type AB (has both A and B antigens)
- Type O (has neither A nor B antigens)

3. **Role of Antibodies**:
- The plasma of the blood contains antibodies that are specific to these antigens. For instance:
- Type A blood has anti-B antibodies.
- Type B blood has anti-A antibodies.
- Type AB blood has no antibodies against A or B.
- Type O blood has both anti-A and anti-B antibodies.

4. **Mixing Blood**:
- When blood from one person is mixed with blood from another, if the antigens on the RBCs of one blood type are recognized by the antibodies in the plasma of the other blood type, an immune reaction occurs.

5. **Antigen-Antibody Reaction**:
- This immune reaction leads to the clumping (agglutination) of RBCs. For example, if Type A blood (with A antigens) is mixed with Type B plasma (which contains anti-A antibodies), the antibodies will bind to the A antigens, causing the RBCs to clump together.

6. **Conclusion**:
- Therefore, the clumping of RBCs occurs due to the antigen-antibody reaction, where the antibodies in the serum react with the corresponding antigens on the RBCs.

### Final Answer:
The clumping of RBC may occur when blood of one person is mixed with serum or blood of another person due to **antigen-antibody reaction**.

---