The chicken immune system has been studied for many years and these studies have contributed substantially to our understanding of the fundamental concepts of immunology and the development of different immunoglobulin classes. It is thus surprising that only a small fraction of the antibodies presently used in laboratories are of avian origin. A laying hen produces more yolk antibodies than a rabbit can produce during the same time period, and the animal care costs are lower for the chicken compared to the rabbit. Chicken antibodies offer many advantages to the traditional mammalian antibodies when used for the detection of mammalian antigen. Due to the evolutionary difference chicken IgY will react with more epitopes on a mammalian antigen, which will give an amplification of the signal. Chicken antibodies can also be used to avoid interference in immunological assays caused by the human complement system, rheumatoid factors, human anti-mouse IgG antibodies (HAMA) or human and bacterial Fc-receptors. The antibodies can be purified in large amounts from egg yolk, making laying hens highly efficient producers of polyclonal antibodies.