The clinical relevance of IgG food allergy testing through ELISA - Enzyme-Linked Immunosorbent Assay

Townsend Letter for Doctors and Patients, Jan, 2004 by Raymond M. Suen, Shalima Gordon

IgA is the predominant immunoglobulin secreted by the B cells of the gut. Constituting over 70% (38) of all immunoglobulin present in the intestinal mucosa, it obviously plays a key role in immune exclusion of food antigens as a "default" mucosal B cell response. However, its position in oral tolerance is less clear. The GALT is exquisitely sensitive to the residing cytokine milieu of which dysregulation alters mucosal responsiveness. TFG-[beta] and other immunosuppressive cytokines, including those of Th2, interact to maintain intestinal homeostasis and nonresponsiveness to innocuous food antigens. TFG-[beta] in particular, inhibits the proliferation of T and B cells, and decreases the secretion of IgG immunoglobulins, yet at the same time induces isotype IgA class switching. (39) Clearly, local IgA immunity alone is unlikely to account for the absence of food hypersensitivities, but does accompany and serves as a useful backup to other more pivotal immunoregulatory mechanisms.

The Gastrointestinal Mucosa

When we consider the cellular arrangements in the gastrointestinal system it is amazing how the epithelial lining of the mucosa, connected by tight junctions, represents the primary barrier to food antigen entry. The mucosal epithelium, comprised of absorptive cells, mucus producing goblet cells, intraepithelial lymphocytes (IEL's), and a basal membrane, is the interface between the external and internal environments of the body, and permits or excludes entry of various materials, appropriately under ideal conditions.

The gastrointestinal mucosa is the largest surface of about 300[m.sup.2] that is in continuous contact with the external environment. (40) Rightly so, it houses over 60% of measurable immune parameters including; mesenteric lymph nodes, Peyer's patches (PP), isolated follicles, lamina propria lymphocytes, and IEL's. These immune components span the epithelial lining and lamina propria and constitute the gut associated lymphoid tissues (GALT). GALT is the largest lymphoid organ of our immune system comprising 80% of the immunoglobulin producing cells in the body and 75% of the entire T cell population, of which 60% is above the basal membrane. (41)

Antigen presentation in the intestinal mucosa includes; B cells, macrophages, and dendritic cells of which reside primarily in the lamina propria, PP, and mesenteric lymph nodes of the GALT. (42) Not limited to this repertoire, antigen presentation also occurs via; mucosal T cells, IEL's, and intraepithelial cells (IEC's). (43) It is clear from this list that antigen sampling does not solely occur via the M cells overlying PP. All cell types are implicated in the mechanisms of oral tolerance induction. The competency in antigen presentation, the dynamics in T cell trafficking, the dose and type of antigen, and changes in the cytokine milieu of the gut, together influence the antigen-specific T helper pattern activity; either towards down-regulation of the mucosal immune response to facilitate tolerance, or towards untoward inflammation.