Apart from the nutrients from breastfeeding, I recall that it is part of an immune system feedback loop. If the child is fighting off something, it is transferred to the nipple during breastfeeding. The mother's immune system develops a response and provides help to the child in the breast milk next feeding.
The main factor at play here is maternal passive immunity: https://en.wikipedia.org/wiki/Passive_immunity#Maternal_pass...
As far as I know, there's no (direct, retrograde) transfer of pathogens from the child to the mother through the breast. But if the child has an infection, there is often a higher chance for the mother to get ill as well (due to close body contact, aerosol particles, etc.) and to develop a "mature immune response", which could lead to a (delayed) positive secondary effect on the child through passive immunity.
As a side note, the German associations of oncology publish their guidelines here (HTML and SVG graphs): https://www.onkopedia.com/de/onkopedia/guidelines