How a Silkworm Fights Infection

No Antibodies, and Still Effective

An insect has no adaptive immune system. No antibodies, no memory of past infection, none of the machinery a vertebrate relies on. And yet a silkworm survives in an environment thick with bacteria and fungi, because it has a different system that works on different principles.

The Three Parts

The humoral immune response of the silkworm — the part carried in the haemolymph rather than by cells — runs on three components:

• PRRs, pattern recognition receptors. These detect molecular signatures common to whole classes of microbe rather than to one specific strain. The insect does not learn what a pathogen looks like; it is born knowing what bacteria in general look like.
• AMPs, antimicrobial peptides. Short proteins produced in response, which attack microbes directly.
• ROS, reactive oxygen species. Chemically aggressive molecules deployed against invaders.

Recognise a class, flood the system with something that kills it. Fast, broad, and requiring no prior exposure.

The Gut Problem

This creates a difficulty an insect has to solve. A silkworm's gut is full of microbes it needs, and the immune system has no antibodies with which to distinguish a resident from an invader.

The answer is regulation rather than recognition: the same pathogen-restriction mechanisms that fight infection also keep the gut microbiome in balance. Immunity and digestion are not separate systems here; they are the same system held at a working equilibrium.

Why This Gets Studied in Silkworms

Because insect immunity is broadly conserved, and the silkworm is the lepidopteran everyone already knows how to keep, breed and sequence. Work on how a caterpillar handles a bacterial challenge feeds into using the same animal to screen antibiotics — you cannot interpret whether a drug worked without knowing what the animal would have done unaided.

Related: the silkworm genome · the four silkworm diseases