When a specific allergen (penicillin) is injected directly into the circulation, the allergen can react with basophils of the blood and mast cells in the tissues located immediately outside the small blood vessels, if the basophils and mast cells have been sensitized by attachment of $IgE$ region. This is called anaphylactic shock. An occasional person who experiences this reaction dies of circulatory shock within a few minutes unless treated with epinephrine to oppose the effects of histamine.