TLRs which is short for Toll Like Receptors, function in recognizing a large variety of different ligands found on pathogens. There are ten human TLRs and each one recognizes a specific type of pathogen. For example TLR9 recognizes unmethylated CpG-rich DNA in the endosomes. TLR8 and TLR7 recognizes single stranded viral RNAs in the endosomes. TLR5 on the other hand is found on the plasma membrane and recognizes the Flagellin of bacteria.
TLRs were first discovered in a mutated Drosophilla. It was found that the TLR in the Drosophilla were able to recognized fungal pathogens. This led to the further understanding and research of other TLRs.