Fig. 2
From: Clinical validation of controlled exposure to birch pollen in the Environmental Exposure Unit (EEU)
Topographic features of birch and ragweed pollen on microscopic examination (not to scale). Birch pollen (left) have smoother surfaces with three pores compared to ragweed pollen which have spiky and barbed surfaces that causes the pollen grain to clump together. Both pollen are of comparable size, 20–22 microns in the case of birch pollen and 18–20 microns for ragweed pollen