Viruses are much smaller than cells. In fact, viruses are basically just capsules that contain genetic material. To reproduce, viruses invade cells in your body, hijacking the machinery that makes cells work. Host cells are often eventually destroyed during this process.
Viruses are responsible for causing many diseases, including:
Antibiotics designed for bacteria have no effect on viruses.
In type I allergy, allergen-specific IgE antibodies are formed upon initial (symptomless) contact.These bind to the surface of mast cells (histamine-rich cells that are present in all mucous membranes). The reaction occurs on second contact within seconds to a few minutes. In the case of allergies, antigens are free/dissolved molecules that are incorrectly viewed as threatening by the immune system.
This is a test for Allergy screening small.