Flea

Different types of fleas prefer specific animals as hosts, but will infest humans if their specific hosts are unavailable. Fleas can carry plague (the Black Death) and murine typhus. They are also thought to be vectors in several other diseases. (Courtesy of the Centers for Disease Control.)

Flea Bite - Close-Up

Fleas are blood-feeding insects. Pain and itching results from an allergic reaction to the materials that the fleas inject into the skin at the time of the bite.

Antibodies

Antigens are large molecules (usually proteins) on the surface of cells, viruses, fungi, bacteria, and some non-living substances such as toxins, chemicals, drugs, and foreign particles. The immune system recognizes antigens and produces antibodies that destroy substances containing antigens.