Page 712 - Veterinary Immunology, 10th Edition

VetBooks.ir TABLE 22.2

Approximate IgA Levels in the Serum and Various Secretions
of Domestic Animals

SECRETION (mg/dL)
Species Serum Colostrum Milk Nasal Mucus Saliva Tears
Horse 60-350 500-1500 50-1000 160 140 150
Cow 10-50 100-700 10-50 200 56 260
Sheep 10-50 100-700 5-12 50 90 160
Pig 50-200 1000 300-500 2-14 25-75 —
Dog 20-150 500-2200 110-620 — — —
Cat 30-150 150 240-620 — 54 —
Chicken 50 — — — 20 15

To undergo a class switch and thus produce IgA, mucosal B cells
must also receive signals from other cells as well as the microbiota
and retinoic acid from dietary vitamin A (Chapter 21). Some IgA
production is T cell independent and so requires cytokines from
dendritic cells and epithelial cells. Other B cells require help from

Th2 cells to make the switch to IgA production.
T-dependent IgA production mainly occurs in Peyer's patches.
Dendritic cells take antigen from M cells and use it to generate Tfh

cells. CD40-ligand (CD40L) and IL-21 from these Tfh cells induce
expression of activation-induced cytidine deaminase (AID). This
promotes IgA class switch recombination. This pathway is
stimulated by bacteria such as the SFBs attached to the enterocytes.
The B cells generated by this pathway are persistent. They can re-

enter germinal centers and undergo somatic mutation. Thus this
IgA may be of very high affinity.
T-independent IgA production mainly occurs in the lamina

propria and isolated lymphoid follicles under the influence of BAFF
and APRIL. ILC3 cells also contribute to this process. The IgA
produced is usually of low affinity and is directed primarily at the
microbiota.
Follicular dendritic cells secrete soluble B cell–stimulating

cytokines including BAFF and APRIL (Chapter 15). The dendritic
cells themselves are activated by intestinal neuropeptides, such as
vasoactive intestinal peptide (VIP). Eosinophils are required for the

maintenance of plasma cells in the lamina propria. They are also a
major source of APRIL. As a result, they directly support the B cell
class switch and are required for both T-independent and T-
dependent generation of IgA. These signals, together with CD40-

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