Is annatto a nut? Or a seed? Or a fruit? Well, for one thing, it's not on the FDA list of nuts, which does include coconut, which can be classified as a nut, seed, or fruit. However, some people with peanut or nut sensitivities report having reactions to annatto.
Annatto is a dye, spice, or food additive derived from the seeds of the Lipstick Tree (Bixa Orellana). It is also called achiote, and can be found in Mexican food. The USDA classifies Bixa Orellana as a "tree or shrub." It is a woody plant that grows in tropical areas and produces seed pods full of seeds covered in bright red pulp. The pulp and/or seeds are used for making natural dyes for foods and cosmetics, such as cheese and Mexican rice.
There has not been any large-scale study done of annatto allergy, although there are a few case studies of anaphylaxis due to annatto in the medical literature.
Some tree nuts that are closely related to each other have high levels of cross-reactivity with other nuts. For example, if you are allergic to walnuts, you are likely to also have or develop an allergy to pecans and hazelnuts. Walnuts and pecans are strongly cross-reactive tree nuts, and are both members of the same family (Juglandaceae).
There are other nuts that are not related, and yet still can be cross reactive. One group includes cashews, Brazil nuts, pistachios, and almonds. Cashews and pistachios are both members of the family Anacardiaceae, Brazil nuts and almonds are unrelated biologically, yet are still cross-reactive.
There is also some evidence of cross-reactivity between coconut, walnut, and hazelnuts, which are not related.
Lipstick Tree stands alone -- there are no other plants in its biological family. A sensitivity to annatto could be a cross-reactivity with another nut allergy, or simply a sensitivity to the annatto itself.
Because the status of annatto as a tree nut is uncertain, food manufacturers are not required to put a nut warning label on foods that contain annatto. They are, however, required to list annatto in the ingredient list.
If you think you are allergic to annatto, ask your doctor about allergy testing. Testing can help narrow down the list of possible causes for your allergic reactions.
Ebo, D.G., et al. Allergy for cheese: evidence for an IgE-mediated reaction from the natural dye annatto. Allergy; Oct 2009, Vol. 64 Issue 10, p1558-1560.
Goetz DW, Whisman BA, Goetz AD. Cross-reactivity among edible nuts: double immunodiffusion, crossed immunoelectrophoresis, and human specific igE serologic surveys. Ann Allergy Asthma Immunol. 2005 Jul;95(1):45-52.
Lucas CD, Hallagan JB, Taylor SL. The role of natural color additives in food allergy. Adv Food Nutr Res. 2001;43:195-216.