Action of Ebola Viruses
Viruses can infect any cell:


  a virus must have a host cell (bacteria, plant or animal) in which to live and make more viruses. Outside of a host cell, viruses cannot function. 


Source

This includes the dendritic cells:


  Dendritic cells are antigen-presenting cells of the immune system. Their main function is to process antigen material and present it on the cell surface to the T cells of the immune system. They act as messengers. They don't actually release any antibodies.


Source 

Quite simply, at the beginning of an infection, the viruses / bacteria will be destroyed by macrophages. 



The dendritic cell will "pick up" some of the leftover pieces.



When it finds a T-Cell that has the same configuration, it orders lots of identical T-Cells to be made. These T cells have multiple jobs, one of which is activating the B cells. It is the B cells that release the antibodies.





So Ebola.

If the dendritic cells are "invaded", then the only line of defence is the macrophages. This is often not enough to stop an attack, and it multiplies out of control.

The reason the Dendritic cells can be invaded is because they are just as vulnerable as other cells. They haven't activated the T cells, and they haven't activated the B cells, so there is nothing protecting them.

Images Source (YouTube video)