Pharyngeal lymphoid tissue: gatekeeper or showstopper?

The research article examines the role and importance of the mucosal immune system, specifically the mucosal-associated lymphoid tissue (MALT), in establishing immunity against various disease-causing agents and maintaining a balance between necessary and harmful immune responses. Significant emphasis is cast on secretory IgA, which is a key component in this system, responsible for offering protection against bacteria and viruses. Certain equine diseases that primarily cause respiratory infections showcase the effectiveness of a mucosal immune response. The pharyngeal tonsil and intestinal Peyer’s patches are viewed as primary examples of MALT, which is a complex system dedicated to maintaining a robust defense mechanism and handling harmless antigens effectively.

Mucosal Immune System and Its Significance

• The mucosal immune system shields the body by maintaining surveillance across the extensive external surface area of the body. Immune responses initiated in one location can be transferred throughout the mucosal immune system. Secretory IgA, a prominent immunoglobulin manufactured by the mucosal immune system, protects the body against pathogenic invasions by physically blocking these invaders from attaching to mucosal surfaces.
• The paper refers to existing research showing the importance of mucosal IgA in immunity against equine diseases like the equine influenza virus and Streptococcus equi. It also suggests that the mucosal immune response may be the most effective immune protection for several equine diseases, especially those causing viral respiratory infections.

Mucosal-Associated Lymphoid Tissue (MALT)

• Tightly coordinated defensive and regulatory roles of the mucosal immune system are centrally governed by MALT. It serves as the linchpin for differentiating between harmful and innocuous antigens, orchestrating the intricate functions of the immune system.
• MALT primarily comprises B cells enclosed in lymphoid follicles, surrounded by interfollicular T cells areas, and an overlying follicle-associated epithelium (FAE). The FAE is specialized for antigen sampling, with M (microfold) cells playing a vital role by aiding in antigen uptake and presentation.
• Upon identification of antigens, specific IgA-positive B-cells are triggered and subsequent trafficking of memory B cells results in the dissemination of the local mucosal IgA responses throughout the immune system.

Equine Respiratory Immune System

• The article mentions prior studies by Mair et al. that have extensively described the distribution of MALT within the equine respiratory tract. Among all the lymphoid tissues in the respiratory tract, the nasopharyngeal tonsil was found to be the largest, with IgA-producing plasma cells dominating in the upper airways.
• A more recent study also revealed that the same cells predominate in the pulmonary humoral immune system. These findings have been complemented by a new study by Kumar et al. which identifies the structure and composition of the equine nasopharyngeal tonsil, echoing the similarity of these tissues across different species.