A substance with antitumor, antiviral, and immunoregulatory effects is _____

Interferon is a naturally occurring protein produced by the host cells in response to infection or in the presence of tumor cells. It plays a critical role in the immune response by enhancing the antiviral effects, modulating the activity of immune cells, and exhibiting antitumor properties. Interferons fulfill these roles by regulating gene expression in target cells, increasing the recognition of infected cells by the immune system, and promoting the activation of immune cells like macrophages and natural killer (NK) cells.

While cytokines are involved in cell signaling in the immune response and can have similar effects, they are a broader category that includes a variety of proteins, not limited to those with antiviral or antitumor properties. Immunoglobulins and antibodies, meanwhile, are specific types of proteins produced by B-cells that primarily function to recognize and neutralize foreign pathogens, but they do not have the same wide range of effects as interferon in terms of direct antiviral and antitumor action.

This specificity of action, along with the multifunctional role that interferons play in immune regulation and their application in clinical treatments for various cancers and viral infections, clearly identifies interferon as the most appropriate answer to the question.