Chicken Secondary Lymphoid Tissues—Structure and Relevance in Immunological Research.

Simple Summary: Birds are highly exposed to pathogens leading to high risk of low egg production, high mortality rates and high costs of veterinary care. Moreover, the intense breeding of avian species for desirable production traits causes immunosuppressive stress. Therefore, line selection requires thorough knowledge not only of genetics but also of the immunology of poultry. The function of secondary (peripheral) lymphoid organs is to activate the immune response through the accumulation of antigens for the activation of naive B and T cells. This review aims to present the morphological characterization of chickens' secondary immune organs along with the latest and more outstanding scientific discoveries and their relevance not only in veterinary medicine but also in human medicine. The authors hope that this review will represent a useful source of information for the design of future experiments with animal models in terms of more tailored protocols and heightened awareness. Recent discoveries have indicated the importance of developing modern strategies for vaccinations, more ethical research models, and effective alternatives to antibiotic treatment in farm animals. Chickens (Gallus gallus) play a crucial role in this context given the commercial and economic relevance of poultry production worldwide and the search for analogies between the immune systems of humans and birds. Specifically, chicken secondary lymphoid tissues share similar features to their human counterparts. Chickens have several secondary or peripheral lymphoid tissues that are the sites where the adaptive immune response is initiated. The more general classification of these organs divides them into the spleen and skin-, pineal-, or mucosa-associated lymphoid tissues. Each of these tissues is further subdivided into separate lymphoid structures that perform specific and different functions along the animal's body. A review summarizing the state of the art of research on chicken secondary lymphoid organs is of great relevance for the design of future studies. [ABSTRACT FROM AUTHOR]