Introduction 
Pathogens Humoral immunity 
Figure 14. When a pathogen invades the body, the immune system responds with three types of reaction. The cells of the 
humoral immune system ( B cells ) secrete antibodies that can bind to the pathogen. Cells of the cellular system (T cells ) carry 
out two major types of functions. One type of T cell (the cytotoxic T cells, CTL ) develops the ability to kill pathogen-infected 
cells. Helper T cells, on the other hand, secrete factors (lymphokines) that stimulate the body's overall response. 
From Molecular Cell Biology, 2nd edition. James Darnell, Harvey Lodish, and David Baltimore, p. 1005. Copyright ® 1990 
by Scientific American Books, Inc. Reprinted with permission by W.H. Freeman and Company. 
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