Africanized honeybees (AHBs), commonly called "killer bees," are hybrids between wild African and domesticated European honeybees (EHBs). The AHBs are descendants of 26 Tanzanian queen bees accidentally released from Southern Brazil where they were being interbred with European bees in order to develop a species that was better adapted to the Southern Brazilian tropical climate. When the AHBs were noted to be especially defensive, the wild African queens were purposefully kept from being released into the wild. However, in 1957, the queen bees were accidentally set free and since then have been interbreeding with EHBs and expanding their range northward averaging between 330-500 km (200-300 mi) each year. In the United States, the AHBs first arrived in southern Texas in 1990. As of January 2009, the bees are found in California, Nevada, Arizona, New Mexico, Texas, Oklahoma, Arkansas, Louisiana, Florida, and Utah.

AHBs are similar to the domesticated EHBs. They look nearly identical and have the same venom. The danger associated with AHBs is that they have a heightened defensive reaction. AHBs are wild bees that do not like being around humans or animals and will defend their hives more aggressively than the domesticated bee. As a result, AHBs sense threats at greater distances, become more defensive with less reason, and will sting in much greater numbers. When disturbed, large numbers of AHBs will swarm and follow a person or animal up to hundreds of meters from the colony (3 times the distance of EHBs).

Africanized honeybees sting defensively. The sting separates from the abdomen allowing the venom to continue to pump into the wound while alarm pheromones are released to attract other bees. The principal components of honeybee venom include the peptides melitin, mast cell degranulating peptide, apamin, and adolapin. Melitin, a strongly basic peptide, damages cell membranes and is responsible for the pain associated with the sting. Mast cell degranulating peptide causes histamine release and an inflammatory reaction. Apamin is a neurotoxin that acts principally on the spinal cord, and adolapin has anti-inflammatory activity. The two main enzymes are phospholipase A and hyaluronidase. Phospholipase A and melitin are thought to be the major venom allergens and account for much of the acute lethality.

The lethal dose of honeybee venom is approximately 15 stings/kg, but one sting in a sensitized person can be fatal due to anaphylaxis. Most deaths related to honeybee stings are due to anaphylaxis. However, given the aggressiveness of the AHBs, the victim typically is stung multiple times resulting in increased morbidity and mortality from systemic envenomation and direct venom toxicity. Approximately 50 simultaneous stings can cause systemic envenomation, whereas 500 stings can cause death by direct toxicity. There have been over 400 documented fatalities associated with AHB attacks.

Four possible reactions have been seen after a honeybee sting: local reactions, regional reactions, anaphylaxis and, rarely, delayed-type hypersensitivity. In addition, multiple stings can cause systemic envenomation and death from direct venom toxicity.

Local sting reactions include immediate pain, swelling, and redness at the sting site that often subsides within a few hours.

Regional reactions (exaggerated local reactions) occur in some individuals with extended swelling that can last 2-7 days. These reactions are not allergic in origin.

Anaphylactic reactions cause diffuse urticaria, pruritus, angioedema, bronchoconstriction, respiratory distress, hypotension, loss of consciousness, and cardiac arrhythmias. Typically within 10 minutes of the sting, onset of life-threatening, anaphylactic signs will occur.

Delayed-type hypersensitivity can manifest as serum sickness, vasculitis, neuritis, encephalitis, nephritis, and coagulation defects occurring days to weeks following the sting.

Systemic envenomation from multiple stings may cause vomiting, diarrhea, generalized edema, dyspnea, hypotension, tachycardia, acute myocardial infarction, atrial fibrillation, cardiovascular collapse, and cerebral infarction. Widespread necrosis of skeletal muscle with hyperkalemia, acute tubular necrosis with renal failure, hepatorenal syndrome with hemolysis, acute pancreatitis, and disseminated intravascular coagulation have been reported.