A humanised antibody is one in which the
1.antibody heavy chain is from human and light chain is from mouse
2. antibody heavy and light chains are from human
3.complementarity-determining regions (CDRs) are from mouse and the rest is from human
4. antibody light chain is from human and heavy chain is from mouse

Detailed Explanation:
Correct Answer:
3. Complementarity-determining regions (CDRs) are from mouse and the rest is from human

What Are Humanised Antibodies?

Humanised antibodies are a class of monoclonal antibodies that have been engineered to improve their therapeutic potential, particularly in humans. The concept of humanising antibodies involves taking an antibody originally derived from a non-human species (usually a mouse) and modifying it to resemble more closely a human antibody. This helps reduce the immune response that could be triggered by the administration of foreign antibodies, improving their safety and efficacy in humans.

Structure of a Humanised Antibody:

The process of humanisation involves several key changes, but the primary focus is on the complementarity-determining regions (CDRs) of the antibody. The CDRs are the parts of the antibody that directly interact with the target antigen and are responsible for the specificity of the immune response.

1. CDRs from Mouse:
   In a humanised antibody, the CDRs, which are responsible for recognizing the antigen, are typically derived from the mouse antibody. These regions are the most variable part of the antibody and dictate its specificity to a particular antigen.

2. Framework Regions from Human:
   The remaining portions of the antibody, which provide structural support and stability to the antibody molecule, are taken from a human antibody. These framework regions are less variable and are therefore replaced with human counterparts to reduce the potential for an immune reaction in humans.

Why the Other Options Are Incorrect:

1. Antibody heavy chain is from human and light chain is from mouse (Option 1):
   This description is not accurate for humanised antibodies. The heavy and light chains in humanised antibodies are both derived from human sources, except for the CDRs, which are from the mouse.

2. Antibody heavy and light chains are from human (Option 2):
   While this option may describe fully human antibodies, it does not describe humanised antibodies. Humanised antibodies still contain some mouse-derived elements (the CDRs), which are not found in fully human antibodies.

3. Antibody light chain is from human and heavy chain is from mouse (Option 4):
   This is incorrect because humanised antibodies maintain both the heavy and light chains from human sources, except for the CDRs.

Conclusion:

Humanised antibodies are engineered to incorporate mouse-derived CDRs while using human framework regions for the rest of the structure. This approach helps in creating antibodies that are highly specific to the target while reducing the likelihood of an immune response in the human body. Therefore, complementarity-determining regions (CDRs) are from mouse and the rest is from human is the correct answer.