The immune system of a bird enables it to resist and overcome infection.

ANATOMY: The bird's immune system mainly consists of lymphatic vessels and lymphoid tissue. Primary tissues are the thymus, located in the neck along the jugular vein, and the bursa of Fabricius, located adjacent to the cloaca. Secondary lymphatic organs and tissues would be the spleen, bone marrow, mural lymph nodules and lymph nodes. There is also a lymphatic circulatory system of vessels and capillaries that transport lymph fluid through the bird's body and communicate with the blood supply.

FUNCTION: The immune system defends against foreign cells. This may be in the form of an invading organism or an abnormal body cell.

There are three major types of response in a bird's immune system; the specific, consisting of the humoral and cell mediated response, and the nonspecific. The humoral and cell mediated responses need a processed antigen to stimulate response, and their response is to create a specific antibody for each particular antigen. The nonspecific immune system responds to all antigens. The B (bursal produced) lymphocytes are associated with humoral response and the T (thymus produced) lymphocytes are associated with the cell-mediated response. Macrophages, heterophils and thrombocytes are the main cells associated with the nonspecific immune system.

The body's first line of defense is the physical blockade of the skin, cilia and mucous membranes.¹ If these fail, the non-specific immune response comes on-line. Macrophages, heterophils, thrombocytes and Natural Killer (NK) cells converge on the invader, killing it or slowing it down until the components of the specific immune system can join the battle.

For the specific immune system to come on board, the invading antigen must be processed by an antigen presenting cell (APC) such as a macrophage, then appropriately presented to the B and T cells.¹ Once the antigen is presented and in turn processed by the B and T cells, the antibody is produced that will mark it and bind it to be culled from the body by the spleen and liver. This encounter is also filed away in memory cells so response will be faster should this same antigen be confronted again.

NEONATAL IMMUNITY:Birds differ from mammals in the way their immune system develops and becomes diverse enough to meet lifelong challenges, with a unique organ called the bursa and a process called gene conversion.

Baby birds are born with an incomplete immune system. Maternal immunity is passed on in the amniotic fluid and yolk of the egg. It is transferred to the embryo when it swallows amniotic fluid during hatch and in the absorption of the egg yolk after hatch.² These antibodies give the newly hatched chick's immunity a start while their own system is developing.

The bird's immune system begins developing before hatch and is complete by sexual maturity. One of the most important stages of this development happens in the first six weeks of the chick's life, when gene conversion is taking place in the bursa.

GENE CONVERSION is the education of the B lymphocytes, cells that respond to a disease antigen by producing antibodies that then bind the nasty antigen for removal from the blood by the spleen and liver. Unlike mammals, when birds are born they do not have a *library* of genetic information for the B cells to use in their production of antibodies. Birds have only one variable function gene encoded in the germline DNA.² If left this way, the B lymphocytes will not be able to produce the different antibodies needed to resist specific diseases. So for the first six weeks of the bird's life, these B cells go to school in the bursa, where they are rearranged in order to provide the diversity needed to protect against the great variety of potential pathogens. These educated B cells leave the bursa to seed other organs of the immune system.³

Although school is open in the bursa for six weeks, it is the first three that are considered the most critical.² School closes down after six weeks and what diversity has been achieved is what the bird will have for its lifetime. The bursa continues to produce B cells until it involutes at sexual maturity and then the blood marrow takes over the task of producing the B cells. Since the age of sexual maturity varies so across the many different species of psittacines, it seems probable that the time of bursal closing would vary right along with it.

The bursa does continue to play a role in the immune system through the life of the bird, though not as critical a one as it does in the beginning.

References:
1. Wakenell PS. The Function of the Avian Immune System. Proc. Avian Specialty Advanced Program, AAV Conf, 1999; 9.
2. Wakenell PS. The Function of the Avian Immune System. Proc. Avian Specialty Advanced Program, AAV Conf, 1999; 10-11.
3. Schmidt RE. Immune System In Altman, Clubb, Dorrestein, Quesenberry. Avian Medicine and Surgery.Philidelphia,PA:WB Saunders, 1997;647