**2.5 Fifth line of defense: antibacterial peptides**

Hence their vast Golgi and endoplasmic reticulum system Paneth cells known for their cytoplasmic eosinophilic granules, [33], which are exclusively found in the crypts of the small intestine and they the secretory cells of the intestinal mucosa specialized in the secretion of the antimicrobial peptides, which are crucial modulators of innate immunity, as well as with neuropeptides and hormones [14, 34].

α-defensin, a ubiquitous antimicrobial peptide found in the intestine, along with β-defensin, has bactericidal activity, particularly against both the gram-negative and gram-positive bacteria, and any disturbance in the existence of these peptides is shown to increase predisposition to inflammatory bowel disease [14, 34].

Apart from defensins, other antimicrobial proteins such as phospholipase A2 and lysozyme are also secreted from Paneth cells in case of an interaction with bacteria or bacterial antigen [34].

Besides these Paneth cell-driven antimicrobial activities, immunoglobulin A (IgA), which is secreted by plasma immune cells through lamina propria (transcytosis) [5, 14, 35–37], also contribute to the fifth line of defense in the IB. IgA either binds directly to the potential pathogenic microorganism and toxins to counter the unwanted colonization or epithelial injury or binds and interacts with immune complexes within the lamina propria to clear out these complexes to soothe the systemic inflammatory responses [14]. It also pumps the already translocated bacteria and antigens in the lamina propria back into the lumen [38].

The previous experimental studies on the B cell-deficient mice (as the IgA secreting plasma cells are differentiated from B cells) and the polymeric immunoglobulin receptor (pIgR) knockout mice (as the transcytosis of IgA to the intestinal lumen is conducted by pIgR) noted intestinal inflammation due to faded IgAoperated adaptive immune response is seen in both mice groups [14, 39].

#### **2.6 Sixth line of defense: immune barrier as "the largest lymphoid organ"**

As the largest immunological organ [40], the gut's innate and adaptive immune system is in continuous interaction with the biological barrier to prevent the conversion of intestinal microbiota into pathobiota. The immune barrier consists of different types of cells, which are members of the gut-associated lymphoid tissue (GALT) like intraepithelial lymphocytes (IELs), natural killer cells (NK cells), innate lymphoid cells (ILCs), mast cells, and M cells (**Table 2**) [9, 12, 41, 42].

*Intestinal Barrier Dysfunction, Bacterial Translocation and Inflammation: Deathly Triad… DOI: http://dx.doi.org/10.5772/intechopen.99554*


**Table 2.**

*Intestinal immune cells: Different types of the cells of "the largest lymphoid organ" contribute to IB function [9, 12, 21, 41–46].*

These cells reside in the intestinal mucosal lymphoid structures of GALT, including the intestinal epithelium, isolated lymphoid follicles (responsible for the local IgA response), and mesenteric lymph nodes the lamina propria and Peyer's patches (5–10% of the follicle-associated epithelial cells) [21, 43, 44, 47] (for a recent review, *see* reference [48]).

ILCs, innate lymphoid cells; ILEs, intraepithelial lymphocytes; M cells, microfold cells; NK, natural killer; Th-17 cell, T helper-17 cell.
