IMMUNE SYSTEM
Birds have both primary and secondary lymphoid organs. The primary organs are the Bursa of Fabricius and the thymus; the secondary ones are the spleen, intestinal lymphoid tissue and bone marrow (Rose 1981; Schmidt 1997).
Thymus
The thymus is found in the neck and consists of three to eight flattened, pale pink lobes lying along the neck, close to the jugular vein. In most birds some tissue persists throughout life although it decreases in size at sexual maturity (Payne 1984; Schmidt 1997). T-lymphocyte precursors are produced by stem cells in the embryo yolk sac and bone marrow and then develop into T lymphocytes in the thymus. These act in cell-mediated immunity, such as delayed hypersensitivity reactions (Rose 1981; Payne 1984).
Bursa of Fabricius
The Bursa of Fabricius is unique to birds. It was first described in the hen by Hieronymus Fabricius in 1621 (Payne 1984). It is a dorsal diverticulum in the proctodeum, which contains folds of lymphoid tissue. In psittacines, galliformes and passerines the bursa is oval or pear shaped, with a central cavity. It has a thick wall in the domestic fowl but is thin and saclike in the psittacines and passerines (King & McLelland 1984; Schmidt 1997). It reaches maximum size before sexual maturity and begins to involute at about 2-3 months. Involution is slower in geese, which do not reach sexual maturity until 2 years of age.
Stem cells in the yolk sac produce B-lymphocyte precursors, which then develop in the bursa. The bursa then seeds other lymphoid organs with B-lymphocytes and is also necessary for normal thymal development. The bursa and dependent lymphoid tissue produce antibodies that are the main defense against invading microorganisms (Schmidt 1997).
CLINICAL NOTE
In ratites the bursa and proctodeum form a single large cavity which has often been mistaken for a urinary bladder (King, AS 1981a; King & McLelland 1984).
Lymphatic system
Lymphatic vessels are less numerous than in mammals and usually follow the blood vessels closely (Rose 1981; Payne 1984). Most species have paired thoracic ducts on either side of the spine that drain the lymph from the hind- limb and abdominal vessels and deliver it into the jugular veins at the base of the neck.
Lymphoid nodules are scattered throughout the digestive tract but especially in the oropharynx, the ceca (called “cecal tonsils”), and small intestine (Payne 1984; Schmidt 1997).
CLINICAL NOTE
Birds do not have lymph nodes, except for ducks and geese, which have two primitive pairs: a cervicothoracic pair near the thyroid gland and a lumbar pair near the kidneys (Rose 1981; King & McLelland 1984; Payne 1984).
Spleen
The brownish red spleen lies to the right of the celom between the proventriculus and ventriculus. It phagocytoses aged erythrocytes, and aids in lymphopoiesis and in the production of antibodies. It does not form a significant blood reservoir, as in mammals, so is relatively small in birds (Rose 1981; Schmidt 1997). It can vary in shape from oval in pigeons and chickens, to triangular in ducks and geese, to elongated in the budgie (Dyce et al. 2002; King & McLelland 1984; Schmidt 1997).
KEY POINTS
• No lymph nodes
• Bursa of Fabricius found in immature birds
• Spleen does not act as a blood reservoir as in mammals