What attracts cells to: pathogens, professional antigen-presenting cells, and cells with an antigen on its MHC-1 protein?
What attracts leukocytes and antibodies to pathogens in the first
  place?


They just bump into them. If secreted antibody is already present in the blood then complement system can be activated. 


  What attracts CD4+ cells to professional antigen-presenting cells
  (APC)?


APC release chemokines. All that is known is that this is a CC-type chemokine and signals via CCR4/CCR8 receptors. See this list of articles (from Google Scholar). This study reports a peptide called Chemrin that promotes chemotaxis via ChemR23 receptor.


  What attracts CD8+ cells to cells with an antigen on a MHC-1 protein?


CD8+/NK cells are activated by CD4+ cells before they kill their target. The MHC-1+antigenic peptide is just a mark for the cells that are to be attacked, as far as I understand.


  What attracts macrophages to pathogens that have been tagged with
  antibodies?


Complement factors.

NOTE: Many of these concepts are old but still relevant. You should check out recent articles in this area for novel/atypical mechanisms.