Approximately 2 percent of people in the United States are affected by an allergy to shellfish and it is most common in adults. The shellfish family includes crabs, lobster, shrimp clams, octopus and squid.
Reactions to ingestion of one of these can range from very mild symptoms such as eczema and swelling of the lips, face and tongue.  They may also include shortness of breath, vomiting, diarrhea, severe abdominal pains and dizziness.
More severe symptoms include loss of consciousness, a rapid pulse followed by a very large drop in blood pressure and extreme difficulty breathing all of which can lead to shock.
The differences between an allergy to shellfish and the more common adverse reaction to the toxins within shellfish are very obvious.  With an adverse reaction a person only has a reaction to the food that they have eaten that is contaminated on a case by case basis, in fact this is very common with shellfish.  A person who has an allergy to shellfish will have a reaction every time they eat shellfish.
This type of allergy is caused by an immune system malfunction wherein the immune system mistakenly identifies the proteins contained in shellfish as a toxin and releases histamines and other chemicals in your body to deal with them, causing an allergic reaction.
Because there are different kinds of shellfish it is quite possible to be allergic to only a few or even one particular type of shellfish, or you may be allergic to all of them.  To find out which shellfish a person is allergic to the doctor usually will ask a series of questions that are designed to tell him more about their particular problems.
Skin prick test can be conducted to find out which particular shellfish a person is allergic to, whereby a person’s skin is pricked with a small amount of the proteins found within the shellfish.  If the person’s skin reacts by producing a hive then it is safe to assume they have an allergy.  Testing can also be done by drawing blood and sending it off to be tested for the presence of certain antibodies within your bloodstream.
Treatments for an allergic reaction are similar to that of any food allergy, in the case of a mild reaction the use of an anti-histamine is recommended to relieve the symptoms and ease any discomfort.  In the case of a more severe reaction the immediate use of an auto injector with Epinephrine is recommended along with taking the person directly to a doctor or emergency room.