The term 'immunoglobulin' refers to what type of biological molecules?

Immunoglobulins are specifically classified as proteins. They are a crucial part of the immune system and function primarily as antibodies, which help to identify and neutralize foreign objects like bacteria and viruses. Each immunoglobulin has a complex structure, typically consisting of multiple protein chains, which allows for the diverse range of immune responses that the body can employ against various pathogens.

Understanding the role of immunoglobulins as proteins also highlights their importance within the biological systems of mammals, as they are synthesized by B cells and play a pivotal role in adaptive immunity. The structure and functionality of proteins enable immunoglobulins to interact specifically with antigens, which is vital for effective immune defense. This distinction underscores why the correct answer identifies immunoglobulins as proteins.

The other options, such as carbohydrates, lipids, and nucleic acids, do not encompass the nature and function of immunoglobulins, differentiating proteins from these other classes of biological molecules.