In an Indirect ELISA, the enzyme-linked antibody will attach to:

• A. The patient's antigen.
• B. The antigen binding region (F_ab) of the primary antibody.
• C. The constant region (F_c) of the primary antibody.
• D. The wall of the microtiter well.
• E. The constant region (F_c) of the secondary antibody.

Answer: (C)

Verified step by step guidance

1. Understand the structure of antibodies: Antibodies have two main regions, the Fab (antigen-binding fragment) and the Fc (constant fragment). The Fab region is responsible for binding to the antigen, while the Fc region is constant and can bind to other molecules, such as secondary antibodies.
2. Review the Indirect ELISA process: In an Indirect ELISA, the primary antibody binds to the antigen. A secondary antibody, which is enzyme-linked, is then added to bind to the primary antibody.
3. Identify the role of the secondary antibody: The secondary antibody is designed to bind specifically to the Fc region of the primary antibody. This is because the Fc region is constant and allows for the secondary antibody to recognize and bind to it.
4. Consider the options given: The enzyme-linked antibody (secondary antibody) will not bind to the patient's antigen, the Fab region of the primary antibody, the wall of the microtiter well, or the Fc region of another secondary antibody.
5. Conclude the correct attachment: The enzyme-linked secondary antibody will attach to the constant region (Fc) of the primary antibody, allowing for detection and amplification of the signal in the ELISA assay.