How does exocytosis contribute to immune responses?

When a microorganism invades the body it will be recognized by white blood cells as "foreign." They will then use a process called "phagocytosis" to engulf the foreign agent. Once in the cell the organism becomes trapped in a membrane bound vesicle called an endosome. This will then fuse with the lysosome (think of it as the "stomach" of the cell) that contains enzymes and free radicles that will break down the trapped microbe. Some of it will be released into the cytoplasm as "food" for the cell and the waste of it will be "exocytosed" to the surrounding. 

Exocytosis is also important because almost all of these immune cells release different signaling chemicals such as cytokines and immunoglobulins, that play a role in activating other immune cells, etc. So exocytosed chemicals are important in killing tumor and virus infected cells. 

Lysosomes also use their hydrolytic enzymes to destroy pathogens (disease-causing organisms) that might enter the cell. A good example of this occurs in a group of white blood cells called macrophages, which are part of your body's immune system.

Lysosomes have digestive enzymes that can break down materials and can destroy pathogens that are harmful to our immune system.