Final answer:
Cytotoxic drugs are used as both immunosuppressants and antineoplastic agents, and they inhibit cell division to perform these functions.
Step-by-step explanation:
The medication type that is used as both an immunosuppressant and an antineoplastic agent is cytotoxic drugs (option C). These drugs work by inhibiting cell division, which is essential for both suppressing the immune response in conditions where it's overactive, such as in organ transplantation, and combating cancer cells which are characterized by their rapid and uncontrolled division. Some cytotoxic drugs are specifically designed to reduce the activity of the immune system to prevent it from attacking the body's own tissues, or in the case of transplants, the new organ. Other cytotoxic drugs are formulated to target and kill cancer cells.
In the context of type III hypersensitivity reactions, common treatments include anti-inflammatory steroid treatments as they help reduce inflammation and modulate the immune system's response. Antihistamines, on the other hand, are not typically used for this type of reaction.
Antiviral drugs such as interferon are also noteworthy in the context of treatment for viral infections, as they help the body's immune system to combat viruses. However, they are not the correct answer to this specific question, as they are not used as immunosuppressants or antineoplastics.