Blood groups are identified on the basis of antigen-antibody reactions.it is studied under

Options

A. serology
B. immunity
C. pathology
D. microbiology

Explanation

**Step-by-Step Solution:**

1. **Understanding Blood Groups**: Blood groups in humans are classified into different types based on the presence of specific antigens on the surface of red blood cells. The main blood group system is the ABO system, which includes types A, B, AB, and O.

2. **Role of Antigens and Antibodies**: Each blood group has specific antigens (A and B) and corresponding antibodies in the plasma. For example, type A blood has A antigens and anti-B antibodies, while type B blood has B antigens and anti-A antibodies.

3. **Antigen-Antibody Reactions**: The identification of blood groups is based on the reactions between these antigens and antibodies. When blood is mixed with specific antibodies, agglutination (clumping) occurs if the corresponding antigen is present.

4. **Branches of Study**: The question asks which branch of science studies these antigen-antibody reactions related to blood groups. The options provided include:
- Immunology (study of immune responses)
- Pathology (study of diseases)
- Microbiology (study of microbes)

5. **Identifying the Correct Branch**: While immunology deals with immune responses, it does not specifically focus on blood grouping. Pathology and microbiology are also not directly related to blood group identification. The correct branch that studies blood groups and their reactions is **Serology**.

6. **Conclusion**: Therefore, the study of blood groups, particularly the ABO blood group system, is conducted under the branch of **Serology**, which focuses on the study of serum and immune responses.

**Final Answer**: The study of blood groups is done under the branch of **Serology**.

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