Avoidance is necessary to prevent this allergy
Shellfish allergies, while often severe, are one of the easiest to avoid. Shellfish is not generally a "hidden" ingredient in staple processed foods, so your job includes less detective work than with most allergies! However, these allergies do tend to be serious (and potentially life-threatening). They also tend to be life long, unlike some allergies that respond well to immunotherapies or that the person can "grow" out of.
If you have life threatening shellfish allergy, your doctor will likely suggest that you carry an Epi-pen. An Epi-pen is a device that allows you to self-administer a dose of adrenaline, in the case of a reaction. This dose of adrenaline is only emergency care; it should allow you enough time to get to a hospital for complete care of your reaction. Do not consider your Epi-pen sufficient if you do have an anaphylactic reaction. You must seek hospital care.
As you might expect, the first reaction to shellfish usually occurs in adulthood. Most children are not exposed regularly or significantly to shellfish, so the allergy may be delayed in development. Most people are allergic to shellfish proteins. However, you can also be allergic to shellfish "gelatin" which is made from the skin and bones of the shellfish. As a result, allergy sufferers may have to be careful when taking a calcium supplement. Some calcium products use calcium from shells, and this could cause a reaction for those who are very sensitive.
If you are allergic to shellfish, you are usually allergic to all types of shellfish. Most species of shellfish are closely enough related to pose a significant problem. So, if you've been diagnosed with this allergy, be sure to avoid crab, lobster, shrimp, prawn and all types of mollusks like clams, mussels and scallops.
Since shellfish is not a common "hidden" ingredient in food, most sufferers will find it easy to avoid. The bigger issue would be the potential for cross contamination in a food processing plant that also handles shellfish. One easy way to know that your processed food is shellfish free is to buy food that is labeled Kosher. In Kosher dietary law, shellfish is considered a food to be avoided completely, and therefore Kosher products will be shellfish free (and will be produced in plants that do not have any shellfish products produced in them).