Venoms and Toxins: How They Attack the Body

A snake bite, a scorpion sting or a brush against a jellyfish can deliver a chemical cocktail refined over millions of years of evolution. Venoms are some of the most complex mixtures in biology, often containing dozens to hundreds of different proteins and peptides. Each component targets a specific molecule in the victim’s body — a receptor, an ion channel, an enzyme, a membrane. Understanding how they work is a tour of protein chemistry, nerve signalling and blood clotting, and it has given medicine several important drugs.

Venom, poison, toxin: getting the words right

• A toxin is any substance made by a living organism that harms another organism.
• A venom is a toxin mixture that is injected — by fangs, stings, spines or harpoon-like cells.
• A poison is harmful when it’s eaten, swallowed or touched. Poison dart frogs and pufferfish are poisonous; snakes and scorpions are venomous.

A simple rule: if it bites you and you’re harmed, it’s venomous; if you bite it and you’re harmed, it’s poisonous.

What’s in a venom?

Most animal venoms are mainly proteins and peptides — chains of amino acids folded into precise shapes (see proteins explained). They also contain small molecules such as amines, salts and sometimes neurotransmitter-like compounds.

Many venom peptides are held in shape by several disulfide bridges between cysteine residues. This makes them compact and unusually stable: they resist heat and the protein-digesting enzymes they encounter in the victim’s tissues. It’s the same kind of covalent cross-link that stiffens keratin.

Venom components fall into a few broad groups, based on what they attack.

1. Neurotoxins: jamming the nervous system

Nerves communicate using electrical signals along their length and chemical signals across the gaps between cells (synapses). Neurotoxins interfere with one step or another (see neurotransmitters).

Blocking receptors

At the junction between a nerve and a muscle, the nerve releases acetylcholine, which binds to nicotinic acetylcholine receptors on the muscle cell. The receptor is an ion channel; when it opens, positive ions flood in and the muscle contracts.

Some snake venoms, notably from cobras, kraits and sea snakes, contain α-neurotoxins (three-finger toxins) that bind very tightly to these receptors and block them. Acetylcholine can’t get in, so the signal never reaches the muscle. The result is paralysis, and death can follow if the muscles used for breathing stop working. The toxin works just like a competitive antagonist drug — it occupies the binding site without activating it (see pharmacology basics). One such toxin, α-bungarotoxin from kraits, binds so tightly and specifically that scientists use it as a tool to label and count acetylcholine receptors.

Blocking or holding open ion channels

Nerve signals travel as waves of sodium ions rushing into the cell, followed by potassium ions leaving. Many toxins target these channels:

• Tetrodotoxin, found in pufferfish (a poison, not a venom) and also in the venom of blue-ringed octopuses, plugs sodium channels from the outside. No sodium can enter, so no signal can travel.
• Many scorpion toxins do the opposite: they bind to sodium channels and stop them closing properly, so nerves fire repeatedly. This causes intense pain, muscle twitching and, in serious cases, effects on the heart.
• Some cone snail peptides (conotoxins) block calcium channels, potassium channels or receptors with extraordinary precision.

Breaking the release machinery

Some toxins stop nerves from releasing neurotransmitters at all. Botulinum toxin, made by bacteria (a poison, not a venom), is an enzyme that cuts proteins needed for vesicles to fuse with the nerve’s membrane. Black widow spider venom does the reverse: a large protein called α-latrotoxin forms pores and triggers massive, uncontrolled release of neurotransmitters, causing painful cramps.

2. Enzymes that destroy tissue

Many venoms, especially from vipers and rattlesnakes, are rich in enzymes that break down the victim’s own molecules:

• Phospholipase A₂ (PLA₂) hydrolyses the ester bonds in phospholipids, the molecules that make up cell membranes (see phospholipids and membranes). Damaged membranes leak, and cells burst — muscle cells, red blood cells and nerve endings can all be affected. Bee venom and many snake venoms contain PLA₂.
• Metalloproteinases, enzymes that use a zinc ion in their active site, digest the proteins holding blood-vessel walls together. Blood leaks into tissues, causing swelling and bleeding.
• Hyaluronidase breaks down hyaluronic acid, part of the “glue” between cells, helping the other venom components spread. It’s sometimes called a spreading factor.

These enzymes are catalysts, so a small amount of venom can do a great deal of damage: each enzyme molecule acts again and again (see enzymes explained). This is why viper bites often cause severe local swelling, blistering and tissue death around the bite.

3. Toxins that disrupt blood clotting

Blood clotting is a cascade of enzymes, each activating the next, ending with the protein fibrinogen being converted into fibrin threads that form a clot. Venoms interfere at many points:

• Some activate clotting factors, triggering widespread clotting throughout the bloodstream. This uses up all the clotting proteins, so the blood then fails to clot at all — a paradoxical effect called venom-induced consumption coagulopathy. The Australian eastern brown snake and many vipers cause this.
• Some inhibit platelets or clotting enzymes directly, causing prolonged bleeding.
• Some lower blood pressure dramatically by acting on the body’s own blood-pressure hormones.

4. Pore-formers and pain

Bee venom is a small but instructive example. Its main peptide, melittin (26 amino acids), is amphipathic: one side of its helix is water-loving and the other fat-loving. It inserts into cell membranes and forms pores, damaging cells and releasing pain signals. Bee venom also contains histamine and PLA₂. Box jellyfish stings deliver proteins that punch holes in cells too.

Some venoms target pain directly. Certain tarantula and scorpion toxins activate the same receptors that respond to chilli heat (capsaicin receptors), producing burning pain without heat.

How antivenom works

Antivenom is made by injecting small, safe doses of venom into an animal — usually a horse or sheep — over several months. Its immune system produces antibodies against the venom proteins. The antibodies are collected from the animal’s blood plasma, purified, and often cut into smaller fragments to reduce side effects.

When given to a bitten patient, these antibodies bind to venom molecules in the bloodstream, blocking their active sites and marking them for removal. Antivenom works best when given early: it can neutralise toxins still circulating, but it can’t easily reverse damage already done inside tissues. Because venom composition varies between species (and even between regions), antivenoms are often specific to the snakes of a particular area.

From venom to medicine

The precision of venom molecules makes them attractive starting points for drugs (see how drugs are designed):

• Captopril, one of the first ACE inhibitors used for high blood pressure, was developed from peptides in the venom of a Brazilian pit viper.
• Exenatide, used in type 2 diabetes, is based on a peptide from the saliva of the Gila monster lizard; it mimics a gut hormone that boosts insulin release (see insulin).
• Ziconotide, a painkiller for severe chronic pain, is a synthetic version of a cone-snail peptide that blocks calcium channels in pain nerves.
• Eptifibatide and tirofiban, which prevent platelets clumping during heart attacks, were developed from snake venom components.

Common misconceptions

• “Venom and poison are the same thing.” Venom is injected; poison is eaten or touched.
• “All venoms act on the nerves.” Many damage tissue or blood clotting instead.
• “Sucking out venom helps.” It removes very little and delays proper treatment; modern first aid is to keep calm, keep still and get medical help.
• “Antivenom cures everything instantly.” It neutralises circulating venom but can’t undo existing tissue damage.

Key takeaways

• Venoms are injected mixtures of proteins, peptides and small molecules, often stabilised by disulfide bridges.
• Neurotoxins block receptors, plug or jam ion channels, or disrupt neurotransmitter release.
• Enzymes such as phospholipase A₂ and metalloproteinases destroy membranes and blood vessels.
• Some toxins cause blood to clot uncontrollably and then fail to clot.
• Antivenom uses antibodies to neutralise venom; venom peptides have inspired drugs for blood pressure, diabetes and pain.