There is no obvious place where bacteria and viruses cannot infect once they breach the physical epithelial barriers. The cells of the immune system need to have their bio-destructive effector mechanisms available throughout the body. As a rule cells tend to stay in defined areas, which is obvious when we think about the liver, lungs, brain, etc. The cells of the immune system are largely confined to an area we call the lymphatics, which consists of an extensive plumbing network with nodes and a reticulation system that is tightly linked to the plumbing of the blood system. Although not well documented there is some evidence that T cells can actually enter a liver parenchyma cell and exit the other side without killing the liver cell.
T cell do, however, readily squeeze through the vascular endothelium without any loss of integrity.
The lymphatics are like the veins that all lead back to a common puddle before being recycled. There is no heart-like pump running the lymphatic drainage which depends to some extent on the overall pressure differences. This puddle that collects lymph from all over the body starts as a tangle of lymph nodes that are attached to the small intestines. These are called the mesenteric lymph nodes. From here they drain into the thoracic duct which empties the lymph directly back into blood via the subclavian artery, and around they go again.