Page 715 - Veterinary Immunology, 10th Edition

to the cell surface. Once in the intestinal lumen, the pIgR is cleaved
VetBooks.ir secretory component and serves to protect the IgA from
from the cell and remains bound to the IgA. In this state it is called

degradation.

IgA is not bactericidal and does not activate complement. It can
neutralize viruses and some viral and bacterial enzymes. IgA does
not agglutinate bacteria within the gut. However, it cross-links

daughter cells preventing their separation after division. As a
result, the bacteria are chained together and their clearance
accelerated. IgA-antigen complexes bind to monocytes and

macrophages, neutrophils, and eosinophils through a low-affinity
receptor, FcαR1 (CD89). When IgA-opsonized particles bind to this
receptor, they trigger superoxide production, opsonization,
antibody-dependent cell-mediated cytotoxicity, and the release of
inflammatory mediators. Its most important function is to prevent

the adherence of bacteria and viruses to epithelial surfaces—
immune exclusion. If bacteria or viruses cannot adhere to
enterocytes, they simply pass along with the intestinal contents and

are expelled without doing any harm.
The presence of the gut microbiota also drives IgA production.
Different commensals differ in their ability to induce secretory IgA.
The gut microbiota is constantly being sampled at the inductive
sites and this triggers the generation of new IgA-secreting plasma

cells. Thus the IgA system is constantly responding to the
microbiota. Conversely, most, but not all commensals are coated
with IgA, and this IgA affects their composition. It is likely that the

IgA-coated organisms are potential mucosal pathogens. For
example, it preferentially coats the Enterobacteriaceae while
ignoring the Bacteroides and Lactobacilli. SFBs are especially potent
at triggering IgA responses.
Because IgA is transported through enterocytes, it can also act

inside these cells (Fig. 22.14). Thus IgA can bind to newly
synthesized viral proteins inside these cells and interrupt viral
replication. In this way, the IgA can prevent viral growth before the

integrity of the epithelium is damaged. This is a unique example of
antibodies acting in an intracellular location. The second unique
function of intracellular IgA is to excrete foreign antigens. Thus IgA
can bind to antigens that have penetrated to the submucosa. Once

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