Which one of the following statements best describes the role of B cells and T cells in the human body?

Option 4 : They protect the body from the diseases caused by pathogens.

The correct answer is option 4.

• B cells and T cells are the main types of lymphocytes.
• B cells work chiefly by secreting substances called antibodies into the body’s fluids. 
• Antibodies ambush antigens circulating the bloodstream. 
• They are powerless, however, to penetrate cells. The job of attacking target cells—either cells that have been infected by viruses or cells that have been distorted by cancer—is left to T cells or other immune cells.
• Each B cell is programmed to make one specific antibody. For example, one B cell will make an antibody that blocks a virus that causes the common cold, while another produces an antibody that attacks a bacterium that causes pneumonia.
• When a B cell encounters its triggering antigen, it gives rise to many large cells known as plasma cells. 
• Every plasma cell is essentially a factory for producing an antibody. 
• Each of the plasma cells descended from a given B cell manufactures millions of identical antibody molecules and pours them into the bloodstream. 
• An antigen matches an antibody much as a key matches a lock. Some match exactly; others fit more like a skeleton key. 
• But whenever antigen and antibody interlock, the antibody marks the antigen for destruction.