What are so-called cross allergies?

Cross reactive?: I think what you are asking is what are so-called cross-reactive allergies. Plants in similar families can show cross reactivtiy. This is true of trees such as cottonwood, aspen, and poplar. Many of the so-called "northern pasture grasses" (grasses found in north america) also show cross reactivity - that is, if you are allergic to one species, you are probably allergic to other related species.

Cross-reactivity: Typically this refers to cross-reactivity between various allergens and pollens. If you are allergic to say, bermuda grass, the likelihood you are also allergic to johnson grass is very high. There is significant pollen cross-reactivity.