Glycans are said to be the face of cells, and the glycosylation on the cell surface changes depending on the tissue and desease state.
As a result, the glycosylation of exosomes released from cells drags the glycosylation of the cell surface, but for some reason, expression of sialic acid tends to be very strong.
For example, there is a paper written by Shimoda and Akiyoshi at. al., Kyoto University (see below).
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6687741/
Why is this?
There is a paper that says it may be aimed at masking the immune system (in Japanese), but is that true?
for example,
https://www.jstage.jst.go.jp/article/dds/38/4/38_270/_pdf
In contrast, the authors cited above suggest that it is involved in the uptake of exosomes via Siglecs on the cell surface.
https://www.sciencedirect.com/science/article/abs/pii/S0006291X17314845?via%3Dihub
（cited from the above listed paper）