Immunity is a bird's ability to fight disease. If the bird has been on a bad diet for a period of time, had infections - fungal, viral or bacterial - their immune system will not be sufficient. If a bird has a metabolic disease - liver disease, heart problems, kidney issues..... - their immune system will be suppressed.
"Immune suppression describes a loss of
immune function. It
can occur for many reasons, including disease, medication, surgery, age or genetics."
en.wikipedia.org
.
Overview
The physiology and immune system of birds resembles that of other animals. The lymphomyeloid tissues develop from epithelial or
mesenchymal anlages that are full of haematopoetic cells.
[3] The bursa fabricus,
thymus,
spleen and
lymph nodes all develop when haematopoetic stem cells enter the bursal or thymic anlages and become competent B and T cells.
[4] The avian immune system is divided into two types of immunity, the
innate and
adaptive ones. The innate immune system includes physical and chemical barriers,
blood proteins and
phagocytic cells. In addition,
complement serum proteins, which are a part of the innate immune system, work with antibodies to lyse target cell. Adaptive immunity, on the other hand, kicks in when the innate system fails to stop invading pathogens. The adaptive response includes targeted recognition of specific molecular features on the surface of the pathogen. Birds, like other animals, have
B cells,
T cells and
humoral immunity as part of their adaptive response.
[5]