In this task, you are given a question and answer options for that question. Using this information, you have to classify each text into different topics: medicine, nursery, psychology, chemistry, pharmacology, biology.
Q: Question: The so-called poly-Ig receptor:
Options: <0> Facilitates the transport of IgA through the mucosal epithelium. <1> Mediates the transport of IgG through the placenta. <2> Stabilizes the polymer structure of an IgM. <3> It is expressed on the surface of B lymphocytes. <4> It is expressed on the surface of tissue macrophages.
A:
biology