In article <xzqafbxz9s1.fsf at dali.uni-paderborn.de>,
Axel Boldt <axel at uni-paderborn.de> wrote:
>>central question of all should be: how do macrophages know what to
>engulf? Which soluble proteins, which free viruses, which bacteria?
>How do THEY distinguish self from non-self? And, given this huge
I think the most general answer is "they don't". I think that macrophages
will phagocytose almost anything under a certain size. However, some
things are phagocytosed more efficiently than others. Particulate
material of a certain size range, for example (and I don't know what that
range is) will get sucked up pretty well, while free proteins are less
efficiently taken up (mostly by fluid-phase pinocytosis, as a background
process), and so forth. Others have pointed out that there are various
triggers, including (as I recall, and this is just from the memory of
someone who doesn't work in the field) particular membrane components only
found on bacteria, and other membrane components only seen on apoptotic
cells. And of course there's complement, which similarly 'sees' abnormal
stuff.
So even before any specific component kicks in and provides antibody for
the Fc receptors or cytokines, you have multiple levels of efficiency and
redundancy, I think. You have things with 'foreign' markers (membrane
components, eg), which are phagocytosed really fast. You have particulate
material, which is taken up less efficiently. You have free protein,
which is slowly and inefficiently taken up. And so on.
But even at the low end of efficiency, there's still a low level of
phagocytosis, and if and when this happens to turn up a genuine non-self,
then you get massive amplification of the response because it cranks up
all the specific stuff. At the low-efficiency end, the macrophages as a
group probably depend on a positive feedback.
Hope this helps.
Ian
--
Ian York (iayork at panix.com) <http://www.panix.com/~iayork/>
"-but as he was a York, I am rather inclined to suppose him a
very respectable Man." -Jane Austen, The History of England
