Antibodies against pathogens:

Antibodies are like keys and will only be
Able to destroy pathogens with the matching
Antigens.

Blood stream:
When a pathogen enters the blood stream the
Immune system sends B cells. There antibodies
Connect to the pathogens antigens. The B cell then
Divides.

Zoomed in blood stream:

After B cells divide they make plasma cells
Which only make antibodies straight into the
blood stream.

http://www.youtube.com/watch?v=Ys_V6FcYD5I&feature=player_embedded