The bursa of Fabricius is a specialized epithelial and lymphoid organ found exclusively in birds. Positioned as an outpouching of the hindgut near the cloaca, it was first described in 1621 by Hieronymus Fabricius, an Italian anatomist, after whom it is named.
The Role in Avian Immunity
The bursa of Fabricius functions as a primary lymphoid organ, serving as the site for the maturation of B lymphocytes, commonly known as B cells. These B cells are a type of white blood cell that are central to the adaptive immune system’s humoral immunity. Their primary role involves producing antibody molecules, which can either be secreted into the bloodstream or embedded in the cell membrane to act as B-cell receptors.
When a B cell encounters a specific foreign substance, called an antigen, it becomes activated. This activation prompts the B cell to multiply and transform into specialized cells, such as plasmablasts or plasma cells, which are responsible for secreting large quantities of antibodies. Antibodies then bind to and neutralize pathogens or mark them for destruction by other immune cells. The “B” in “B cell” directly originates from the bursa of Fabricius, reflecting its discovery in birds and its fundamental contribution to understanding this branch of the immune system.
Lymphoid stem cells migrate from the fetal liver to the bursa during embryonic development. Inside the bursa, these stem cells undergo a process where they acquire the characteristics of mature, immunocompetent B cells. This process involves gene diversification, which is essential for generating a wide array of antibody specificities, allowing the bird’s immune system to recognize and respond to diverse pathogens.
Developmental Changes of the Bursa
The bursa of Fabricius exhibits a distinct lifecycle, being most active and largest in young birds. It typically reaches its maximum size around 8 to 10 weeks of age in chickens. Following this peak, the organ begins to gradually shrink, a process referred to as involution.
Involution signifies that the bird has largely developed its full collection of mature B cells, reducing the organ’s necessity in adulthood. By approximately six months of age, the bursa becomes heavily involuted, often appearing as only scarred remnants. This natural decline in size and function is a normal biological progression once the initial B cell repertoire has been established.
The Human Equivalent
Humans and other mammals do not possess a bursa of Fabricius. However, mammals have an equivalent mechanism for B cell maturation. B lymphocytes primarily mature within the bone marrow.
The bone marrow serves as the primary site for both the formation of blood cells, known as hematopoiesis, and the development of B cells in mammals. This functional similarity means that while the anatomical structure differs, the underlying immunological process of B cell maturation is conserved across vertebrates, albeit in different organs.
Impact of Bursal Diseases
Diseases affecting the bursa of Fabricius can have severe consequences for avian health, particularly in poultry. Infectious Bursal Disease (IBD), also known as Gumboro disease, is a highly contagious viral infection that specifically targets this organ in young chickens. The virus responsible, Infectious Bursal Disease Virus (IBDV), primarily replicates within the immature B cells of the bursa.
When IBDV infects the bursa, it destroys developing B cells, leading to immunosuppression. This damage impairs the bird’s ability to produce antibodies and mount effective immune responses, making it highly susceptible to other infections. The disease can result in mortality and causes reduced weight gain and increased susceptibility to secondary infections, leading to significant economic losses in the poultry industry.