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Toll like receptors (TLRs), macrophage
scavenger receptors, PAMPs and PRRs fit into this category - as they are germ
line encoded and not variable, the genome cannot afford to make many different
receptors. Note that the intracellular signalling part of the IL-1 receptor
share close homology with the same part of the Toll Like Receptor (TLR). IL-1 is
produced intracellularly during the process of apoptosis and, when spilled into
the extracellular space, it is strongly pro-inflammatory. It seems likely,
therefore, that the pathway that leads to cell debris clearance/ingestion was
linked at some time in evolution to the pathway that senses micro-bacterial
ingestion. When it first appeared, it seems (to me) likely that the TLR system
was linked to food acquisition rather than primarily as a defence against
infection (both absorption of dead and dying cells and microbes represent
important resources to promote "negative entropy").
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