Evolution of the immune systems - origins of innate and adaptive immunity

Most ancient immune defense : antimicrobial peptides

Defensins at bacterial surface

Active against gram -ve and +ve bacteria

Likely that it evolved before plant and animal diverged

Simple complement system - Jules Bordet’s finding

In sea urchin

Toll-like receptors

Toll receptor In drosophila

TLR diversity greatest in echinoderms

TLR diversity decreases towards mammal

Even sponge has TLR?

Adaptive immune system in cartilaginous fish - reason for appearance

In jawed fish and higher vertebrates, adaptive immunity is possible because of a specific event that occurred in some ancestor of the jawed fish in which a transposon carrying the ancestral REG recombinase inserted itself into a gene similar to an immunoglobulin or T cell receptor

What is the alternative adaptive immune system in jawless vertebrates?

Hagfish and lamprey

• Assemble lymphocyte antigen recognition receptor genes through the genomic rearrangement of the leucin rich repeats molecules

• Instead of VDJ they have multiple short stretches of leucine rich repeats next to the single incomplete VLR gene.

• Gene conversion generates one complete VLR gene, gets many possibilities

• Makes both membrane bound and secreted forms