3.  Lymphocytes: white blood cells that help the body fight pathogens  B cell: white blood cell that produces antibodies  Helper T cell: WBC that signals B cells to produce antibodies  Antibody: special protein that helps fight infection  Macrophage: WBC that surrounds and destroys pathogens

4.  Pathogen invades body  helper T cells send signals to B cells  produces antibodies  B cells enter the lymph nodes and lymph tissues  antibodies travel through the blood to destroy pathogen  destroyed pathogens enter lymph, filtered in lymph nodes, and removed by spleen

5.  The body’s resistance to disease-causing agents  Active immunity: resistance to disease due to the presence of antibodies  Vaccine: substance containing dead or weakened pathogens  Passive immunity: results from introducing antibodies into a person’s bloodstream; short-term and used when risk of developing disease in immediate

6.  A germ that causes disease  Communicable disease or infectious disease: illness caused by pathogens that can be spread from one living thing to another  Bacteria: single celled microorganism; can be beneficial; those that are harmful cause disease by releasing toxins into the body  Fungi: single and multi celled parasitic organisms; obtain their food from organic materials

7.  Viruses: one of smallest known pathogens; takes over the cell and causes it tomake more viruses; newly produced viruses are released and take over other cells  Protozoa: tiny, single-celled organisms that produce toxins that cause disease

8.  Direct contact  Through the air  Contact with contaminated objects  Handling or being bitten by an insect or animal carrying the pathogen  Contact with contaminated food and water