EPITHELIA

Skin
Vitamin A deficiency causes increased keratinisation of the skin, which then becomes thicker (hyperkeratosis). However, the barrier function against pathogens is normal.

Mucosa
Vitamin A deficiency destroys the barrier function of the mucous membranes in conjunctiva, airways, gastrointestinal tract and urogenital tract due to loss of mucus-producing goblet cells and reformation of the epithelia to 'squamous metaplasia' (see figure Infected mucosal epithelium) - especially in the airways following infection.

Incomplete repair of epitelial tissue after infection

Charles B Stephensen: VITAMIN A, INFECTION, AND IMMUNE FUNCTION Annu. Rev. Nutr. 2001. 21:167–92

Figure: Infected mucosal epithelium
Vitamin A deficiency impairs regeneration of normal mucosal epithelial barriers during infection. Normal mucosal epithelium (top) prevents adherence of many potentially pathogenic bacteria by trapping them in mucus and moving them out of the airways or down the gastrointestinal tract. In vitamin A deficiency, mucosal epithelium that is damaged by infection (and the associated inflammation) does not regenerate normally, and foci of squamous metaplasia (in the respiratory tract) or increased necrosis (in the gut) develop at sites of inflammation. Such foci allow greater adherence of potentially pathogenic bacteria and may allow increased bacterial translocation across the mucosal surface, into lymphatic vessels, and to local lymphoid tissues and beyond, resulting in invasive disease.