How does a white blood cell recognize a pathogen and how does it communicate?

Question

Also, how do white blood cells move outside of blood vessels or in a direction opposite of blood flow?

Answer 1

every white blood cell has certain recognizing areas where they fix the pathogen and recognize it... if the pathogen is of foreign complex, then the chemical reaction takes place and the white blood cell completely bombardes the pathogen and kills it

Answer 2

The B cell recognizes intact proteins at specific recognition sites called epitoopes and by signaling in its B cell receptor internalizes the antigen and under help from the T cell develops specific antibody in high titer. It can also "present " peptides from degradation of the antigen to T cells in the context of what is called MHC class II so it can receive help from CD4 T cells. T cells recognize peptides of particular length "presented " to them by various cell types. macrophage , B cell, dendritic cells  and respond by helping ( CD4 bearing T cells ) or by killing an infected cell ( CD8 bearing cytotoxic T cells )