Microvilli

Celiac disease is a food allergy disease associated with gluten, but if gluten can kill the microvilli in the intestine, then anything you are allergic to will do the same. And depending on how bad the microvilli are in the intestines, just not eating the allergic food ...

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... for cells to overcome this barrier, such as cytoplasmic streaming, infolding of the plasma membrane and the presence of vacuoles in the cell. The microvilli of the cells lining the gut is an example of a way to overcome this problem.

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... olfactory epithelium. The olfactory lobe is farther away, kind of a projection of the brain, and just interacts with the neurons. There are also microvilli on the epithelium probably to aid capture of airborne molecules.

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There's so much more membrane on a microvilli-covered surface; if materials are moving across that surface, in or out, they'll move faster.

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... area needs to increase in order for absorption to take place at a high enough rate. A protein transporter can only work so fast... More about how microvilli form here:http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mboc4.figgrp.3025

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