Confused about shellfish allergy

[Augiesma]

Over the years I have experienced an allegeric  reaction to lobster (specifically it appears to be in a lobster roll rather than a whole lobster). The most severe reaction was actually a one time episode, probably 20+ years ago...I broke out in hives and was congested. That was the only time I got hives from lobster, but in recent years I have gotten very congested if I eat too many. I seem to be able to eat one occasionally but last summer I had several in one week and became very congested. Today I saw an allergist who gave me a skin test. She said the tests specifically for lobster and shrimp were negative, but the test for shellfish was positive. My question is, how can I test positive for shellfish but negative for shrimp and lobster? I have never reacted to shrimp, so it is possible to be allergic to lobster but not shrimp? The Dr told me to not eat ANY shellfish. And why can I eat lobster roll occasionally but just not too often? And why did the lobster test come out negative? The Dr could not answe any of these questions. I am planning to get a 2nd opinion.

[PurpleCat]

Second opinion is an excellent idea.

Allergies are not black and white.  Reaction history trumps testing.

In the shellfish/mollusk categories my DD is unusual too!

She tests positive to all shellfish however she eats shrimp regularly without incident.  She can not eat lobster or crab.

She tests negative to all mollusks however she failed a clam food challenge, so she is allergic to those.  Meanwhile she eats scallops without incident.

[Macabre]

Crustaceans are high in histamine as well. It may be that rather an allergy to the food itself.

[lakeswimr]

Many seafood items get cross contaminated with one another while being prepared. I wonder if you are not allergic to lobster but some other seafood that occasionally cross contaminates your lobster.