Thank you for your inquiry.
As you alluded to in your question, latex, banana, and avocado share antigens. There is quite often cross-sensitization and clinical crossreactivity as well. And, as you point out, the clinical reactivity can be systemic or simply limited to mucosal surfaces (pollen-fruit syndrome). For example, in one study looking at the cross-reactivity between latex and banana, banana prick skin tests were positive in 35% of patients with latex allergy, and oral symptoms after eating banana were reported in 52% of 31 patients (1). Similar such crossreactivity has been demonstrated for avocado (2).
Unfortunately, even though such crossreactivity has been well documented, to my knowledge, and according to a literature search, there has been no longitudinal studies to delineate the percent of children allergic to banana or avocado who will develop a latex allergy. So, your question actually cannot be answered. The only thing that one can say is that the child you described is at increased risk of developing latex allergy with exposure. It is not possible to quantitate that risk in my opinion.
It is encouraging that he does not have symptoms upon exposure to latex except for latex Band-Aids, and that could well be due to the adhesive rather than the latex itself since other forms (e.g., balloons) do not produce symptoms. In addition, since the risk of the development of allergy to latex is exposure-dependent, if you are concerned about the development of latex allergy in this child, you could consider suggesting latex avoidance.
Finally, there does not appear to be commercially available component testing for avocado or banana. For your interest, here is a link to the allergens available for component testing (by ImmunoCAP).
Thank you again for your inquiry and we hope this response is helpful to you.
1. J Allergy Clin Immunol 1994; 93(6):990-996.
2. J Allergy Clin Immunol 1998; 102(3):476-481.
Phil Lieberman, M.D.