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AnonTechie writes:

When foreign pathogens, such as bacteria or a virus, enter our body, our immune system responds in a concerted effort to eliminate them. B cells produce antibodies that recognize markers (called antigens) on the surface of the invaders; these antibodies are then used to tag foreign pathogens for destruction.

B cells typically require interaction with T cells for full activation and antibody production, which is critical to overcoming an infection. But there are some cases where the T cells are not required. Now, researchers have figured out how this works—and discovered that it relies on the remains of long-dead viruses that litter our genomes.

Large, repetitive sugar structures that are often found on the surface of bacteria and viruses are the key to activating antibody production without the help of a T cell. These sugary structures engage proteins called B cell receptors, which activate the B cells. B cells then grow, forming short-lived cells that produce antibodies and long-lived memory cells that will recognize the same invader upon subsequent infection

http://arstechnica.com/science/2014/12/remains-of-long-dead-viruses-in-our-genomes-aid-our-immune-response/

[Abstract]: http://www.sciencemag.org/content/192/4238/467