An immunologist has isolated a molecule from the human body to study. It is found to react with antibodies on B cells. Further testing reveals that the molecule is large and chemically diverse. What type of molecule is the immunologist studying?

Antigen.

Explanation:

The immune system is one of the most important system of the body as it protects the body from the pathogens. The antibodies are produced against the specific antigen to kill that antigen and generates an immune response.

A scientist is studying on a substance that can react with antibodies of B cells. The substance must be an antigen because antigen has the ability to binds with specific antibodies. The antigen may be any chemical, bacteria or lipid molecules against which antibodies are produced in the body. Antigen is a large molecule and shows different chemical nature.

Thus, the answer is antigen.