Every time a child visited a cousin who has two cats, the child's eyes turned red, itched, and began to water. then, the child began to have trouble breathing. it is most likely that the child reacted this way because

(1) normally harmless cat antigens stimulated the immune system
(2) it is difficult for the respiratory system to filter cat antigens out of the inhaled air
(3) cat antigens are a health hazard, since they always cause disease
(4) cat antigens stop the immune system from making antibodies, so bacteria cause these responses

(1) normally harmless cat antigens stimulated the immune system

Explanation:

Some people are allergic to specific antigens. This means that the antigens otherwise harmless for other people initiate the allergic immune responses in these people. The child is allergic to cat antigens. As the child comes in contact with cats, the cat antigens enter the body and start the allergic response.  

The allergy reactions include the production of IgE antibodies that bind to the surface of mast cells and basophils and stimulate the release of certain chemicals from these cells. The mast cells and basophils release histamine, prostaglandins, etc. which in turn cause vasodilation, increase the permeability of blood capillaries, stimulate the contraction of smooth muscle in the airways of the lungs, and trigger the secretion of mucus. As a result, the child experience watery and itchy eyes, inflammatory responses, and difficulty in breathing through the constricted airways.

not sure if this is what you'r looking for eubacteria and archaebacteria