Do anaphylactic reactions always occur immediately or can they happen hours or days after exposure?

"Type I hypersensitivity is also known as immediate or anaphylactic hypersensitivity. The reaction may involve skin (urticaria and eczema), eyes (conjunctivitis), nasopharynx (rhinorrhea, rhinitis), bronchopulmonary tissues (asthma) and gastrointestinal tract (gastroenteritis). The reaction may cause a range of symptoms from minor inconvenience to death. The reaction usually takes 15 - 30 minutes from the time of exposure to the antigen, although sometimes it may have a delayed onset (10 - 12 hours)."
http://pathmicro.med.sc.edu/ghaffar/hyper00.htm

"Symptoms of anaphylactic shock tend to develop rapidly although the initial presentation can be delayed and/or deceptively mild. The victim may become uneasy, upset and red in the face. They may also develop a rapid heartbeat, prickling and itchiness in the skin, throbbing in the ears, sneezing, coughing and difficulty breathing. Shock may then follow, in which blood vessels become leaky, blood pressure falls and the person becomes cold, clammy and faint."

http://www.anaphylaxis.org/content/whatis/anaphylaxis_is.asp

"In a paper published in the February 2009 Journal of Allergy and Clinical Immunology, UVA researchers describe a novel and severe allergic response, which they call delayed anaphylactic shock. The reaction occurs three to six hours after patients eat beef, pork or lamb. Symptoms begin with itching that intensifies as hives develop on the skin's outer and deeper layers. Itching quickly escalates to swelling, intestinal irritation and the alarming, life-threatening symptoms of anaphylaxis: airway constriction, chaotic heart beat, a rapid drop in blood pressure and loss of consciousness.
""Our conventional understanding is that anaphylaxis happens within seconds or minutes of exposure. The notion that it can be delayed for several hours is a paradigm altering discovery," says senior study investigator, Thomas Platts-Mills, M.D., Ph.D., Professor of Medicine and Microbiology at UVA and head of the Division of Allergy and Clinical Immunology."
"...The UVA study yielded other paradigm-challenging findings. First, the allergist's key diagnostic tool, the skin prick test, proved ineffective in detecting red meat allergy in study patients. (As part of the UVA research effort, Commins developed a skin testing technique to identify this allergy.) Second, most patients began experiencing symptoms as adults, defying the common belief that food allergies rarely develop after childhood."
http://www.healthsystem.virginia.edu/internet/news/archives09/anaphylaxi...