All diseases that affect the immune system cause a decreased immune response.

• A. True
• B. False
• C. Depends on disease
• D. Cannot be determined

Answer: (B)

Immune-system diseases can affect how the immune system behaves in different ways, not just by reducing its activity. Some conditions cause immunodeficiency, where the response is weakened and infections are more likely. But other diseases involve an overactive or misdirected immune response, leading to inflammation and tissue damage rather than a simple decrease in defense. Autoimmune diseases are a prime example, where the immune system attacks the body's own tissues, triggering ongoing inflammation. Allergic reactions and hypersensitivities show an exaggerated response to harmless substances. In lab animals, you can see both ends of the spectrum: immunodeficiencies with poor pathogen defense, and autoimmune or inflammatory disorders with heightened immune activity. So, diseases that affect the immune system do not always cause a decreased immune response; they can cause increased or dysregulated responses as well.