Getting to know

Allergic reactions to insect stings occur in a limited number of people who are genetically predisposed to develop allergy. The immune system of these people “overreacts” to the venom injected by the insect. At the first sting they produce a particular type of antibody (IgE antibody) that reacts to proteins found in the insect venom or saliva. As the production of antibodies is a quite laborious process that takes hours, days or even weeks to complete, these people do not manifest any unusual reaction to the first sting (in rare cases, they can exhibit normal reactions to the first few stings). However, if the person is stung again by the same species of insect, IgE antibodies produced after the first sting react with the venom proteins. This triggers activation of specific types of immune cells which release a large number of substances that act on blood vessels and nerves causing allergic symptoms. 
To learn more about the role of IgE and how the allergic response develops please visit the section  What is Allergy? 

Allergy to insect stings can take various forms, from mild, local reactions to extensive ones which involve body organs distant from the site of the sting.