Diagrams are considered to be invaluable teaching and learning tools in biochemistry, because they help learners build mental models of phenomena, which allows for comprehension and integration of scientific concepts. Sometimes, however, students experience difficulties with the interpretation of diagrams, which may have a negative effect on their learning of science. This paper reports on three categories of difficulties encountered by students with the interpretation of a stylized textbook diagram of the structure of immunoglobulin G (IgG). The difficulties were identified and classified using the four-level framework of Grayson et al. [1]. Possible factors affecting the ability of students to interpret the diagram, and various teaching and learning strategies that might remediate the difficulties are also discussed. (Contains 1 figure.)