Immunoglobulin A (IgA) is primarily produced by specialized cells called plasma cells, particularly in areas associated with the body's mucosal surfaces.
IgA is a critical antibody playing a key role in the immune defense at mucosal sites, such as the gut, respiratory tract, and urogenital tract, as well as in secretions like saliva, tears, and breast milk. Understanding its production location is essential to appreciating its function in protecting these vulnerable areas.
The Producers of IgA: Plasma Cells
Antibodies, including IgA, are proteins generated by immune cells known as plasma cells. Plasma cells are terminally differentiated B lymphocytes. When B cells encounter their specific antigen and receive help from T helper cells, they proliferate and differentiate into plasma cells capable of secreting large amounts of antibodies.
Where IgA Production Takes Place
IgA exists in two main forms: monomeric IgA and polymeric IgA.
- Monomeric IgA: This form typically circulates in the bloodstream and is produced by plasma cells mainly found in the bone marrow and spleen.
- Polymeric IgA: This is the most abundant form and is crucial for mucosal immunity. As highlighted in the provided reference, Polymeric IgA (mainly the secretory dimer) is produced by plasma cells in the lamina propria adjacent to mucosal surfaces. The lamina propria is a layer of connective tissue directly beneath the epithelium lining these surfaces.
This strategic location ensures that IgA is produced precisely where it is needed to neutralize pathogens and toxins attempting to enter the body through these barriers.
Key Sites of Polymeric IgA Production
- Gastrointestinal Tract: The gut-associated lymphoid tissue (GALT), particularly the Peyer's patches and scattered lymphoid follicles, is a major site. Plasma cells migrate from these lymphoid structures into the lamina propria to secrete IgA.
- Respiratory Tract: Similar to the gut, lymphoid tissues in the airways contribute plasma cells to the lamina propria.
- Other Mucosal Tissues: This includes the urogenital tract, salivary glands, mammary glands, and lacrimal glands (which produce tears).
The Journey to the Mucosal Surface
After being produced by plasma cells in the lamina propria, polymeric IgA undergoes a process to be transported across the epithelial barrier onto the mucosal surface. The reference mentions the initial step: it binds to the pIgR (polymeric immunoglobulin receptor) on the basolateral surface of epithelial cells and is taken up into the cell via endocytosis. Within the epithelial cell, it is processed and then secreted onto the lumenal side as secretory IgA (SIgA), which includes a piece of the pIgR called the secretory component.
Summary of IgA Production Locations
| IgA Form | Primary Production Site | Main Location Found |
|---|---|---|
| Monomeric | Bone marrow, spleen (plasma cells) | Bloodstream |
| Polymeric | Lamina propria adjacent to mucosal surfaces (plasma cells) | Secretions (gut, tears, etc.) |
In summary, while monomeric IgA is found in serum, the critical secretory form of IgA that protects mucosal surfaces is uniquely produced by plasma cells situated right next to these surfaces, ensuring a localized and effective immune response.
