Antibodies are the functional unit of humoral immunity. They are secreted by plasma cells, a type of B lymphocyte. When on the surface of a B cell, these molecules are immunoglobulins, following secretion they are termed as antibodies. Antibodies are found in bodily fluids and tissue spaces and are most effective in eliminating extracellular pathogens. As compared to mammalian antibodies, IgY offers various advantages for the targeted extraction of antibodies and their application in bioanalysis. Since the antibodies are extracted from the yolks of laid eggs, the method of antibody production is non-invasive. Thus, no blood must be taken from the animals for the extraction of blood serum. Because of its economic importance most research on immunity implied in the reactivity of the body were performed on domestic birds. Direct consequence of this research is the understanding of fundamental and applied concepts of immunity in birds. Comparing to the mammalian, avian immune system appears insignificant. However, it currently provides important information regarding the fundamental immunological mechanisms. It also provided a significant number of firsts, especially in vaccinology.