Final answer:
It is true that homes with more cats have higher levels of cat allergens. Allergies and autoimmune diseases both stem from an overactive immune system, while the virus that causes human colds often doesn't affect cats and dogs due to differences in biological receptors.
Step-by-step explanation:
The statement Homes with more than one cat have higher levels of cat allergens than homes with one cat is generally considered true. Cat allergens are proteins found in cat skin cells, urine, and saliva. These allergens can become airborne and are found in higher concentrations in homes with more cats. Since each cat produces allergens, having multiple cats can increase the total amount of allergens present.
Regarding the immune system, it is true that both allergies and autoimmune diseases are the result of an over-reactive immune system. In the case of allergies, the immune system responds to a substance that is generally harmless, such as cat dander. In autoimmune diseases, the immune system mistakenly attacks the body's own cells.
Finally, there's a difference in how viruses affect different species due to receptor compatibility. The virus that causes colds in humans commonly does not find the same receptors in dogs and cats, explaining why they do not tend to catch colds from humans. This is due to the specific biological receptors that viruses need in order to infect cells.