In the January Issue of SciAm (discussing Haemophilia):
When damage occurs to blood vessels, exposure of the blood to collagen in the cell walls and material released by the cells triggers the activation of clotting factors.
I read the original in print, but it is available online here.
This seems to imply that animal cells (in this example, those of humans) have cell walls. I sometimes see similar implications in other resources. However, in elementary biology, one is taught that animal cells never have cell walls.
Therefore, my question: Are references to animal cell cell walls (such as the above, for human animal cells) simple mistakes--or are they exceptions to a generalization?