HLA-DR4 and the flu, HLA-DP4 is very different (expressed on immune cells)

HLA-DR4

Flu viruses use dopamine receptors to enter cells and when it does the receptors are triggered

Influenza A is linked to HLA-DR4 infection of cells

Influenza A is likely using D2 dopamine receptors to infect cells
http://angelabiggs.blogspot.com/2016/08/5-types-of-dopamine-receptors-and-how.html

HLA-DR4 and type one diabetes
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC386650/
https://www.ncbi.nlm.nih.gov/pubmed/46642?dopt=Abstract

Further the groove in the HLA-4 seems to fit the flu proteins
https://www.ncbi.nlm.nih.gov/pubmed/16451200

Diabetes linked to flu
https://www.newscientist.com/article/dn22456-diabetes-linked-to-flu/

Dopamine D2-receptors when triggered inhibit insulin secretion
http://www.jbc.org/content/280/44/36824.long

(fast progression type one diabetes...is this the HLA-DR4 group? I think it is)

HLA-DP4

HLA-DP4 is the most common HLA group II...60% of the world's population
https://www.ncbi.nlm.nih.gov/pubmed/12471126

HLA-DP is the antigen presenting mailbox of immune system cells. As the most common it might have a wide range of binding epitopes.

The heart disease group of T. cruzi is connected to DPB1.04
https://www.ncbi.nlm.nih.gov/pubmed/10689123

HLA-DP4 and RSV
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC369497/

Hepatitis B and HLA-A2 with HLA-DP4
https://books.google.com/books?id=piaGObdInuIC&pg=PA136&lpg=PA136&dq=hla-dp4&source=bl&ots=BMv4ZYm1TN&sig=ocVFBWxHp651MbaqCpVtomTgpdo&hl=en&sa=X&ved=0ahUKEwjrkduU6KDQAhVCImMKHWlCBfQQ6AEIZDAJ#v=onepage&q=hla-dp4&f=false