The Immune Response

The human body has several different lines of

defense against infections and disease. The immune

system is made up of different cells, tissues, and

organs working together to fight infection.

Image: Electron microscope image of an HIV1 virus budding from a white blood cell.
Credit: CDC/C. Goldsmith, P. Feorino, E.L. Palmer, W.R. McManus

Learning Objectives

By the time you finish this lesson, you should be able to:

Explain the natural defenses your body has to fight disease.
Describe how your body produces immunity.
Describe the immune response to pathogens.
Explain the differences between active and passive immunity.

Vocabulary

active immunity— resistance to disease acquired by either exposure to a pathogen or by receiving a vaccine against a pathogen.

antibody— a substance made by the body in response to an antigen.

antigen— any type of substance that the immune system recognizes as foreign to the body and that can trigger an immune response.

B cells—a lymphocyte that produces antibodies that bind to an antigen.

T cells—lymphocytes that produce chemicals that kill infected cells or tumor cells.

helper T cell—a lymphocyte that stimulates cell divisions that result in increased numbers of T cells after exposure to an antigen.

immunity—resistance to disease.

lymphocyte—a type of white blood cell.

macrophage—a large cell that engulfs cells foreign to the body.

passive immunity— immunity that is produced from the body’s response to antibodies that have not been produced by the body’s own immune system.

pathogen—an organism that causes disease.

vaccine—a solution made from a killed or weakened virus; produces immunity to a disease.

Lines of Defense

Your immune system is a complex line of defenses that helps your body fight disease. It helps defend you against attack from viruses, bacteria, harmful chemicals and even cancer cells.

Your immune system helps you fight off bacterial infections, but your first line of defense is actually helpful bacteria that live on your skin. Millions of these bacteria kill harmful bacteria on your skin. Disease-causing bacteria can enter your body through breaks in the skin. If harmful bacteria enter the body, your immune system’s white blood cells start to attack the bacteria to kill them.

Your circulatory system carries white blood cells throughout the body to engulf pathogens (disease-causing organisms) and foreign chemicals wherever they find them. If they cannot destroy the pathogens fast enough, a fever might develop. The fever helps fight the infection.

Your respiratory system contains cilia (tiny hair-like structures) and mucus that traps pathogens that enter the body through your nose. Your digestive system produces enzymes in the stomach, liver, and pancreas that destroy pathogens. Hydrochloric acid in the stomach kills bacteria that may be on the food you eat.

As you can see, your body has quite a few lines of defense that protect you against disease and foreign materials. But, if pathogens enter the body and you cannot fight them all off, your body also has both active and passive immunity defenses.

When a pathogen or foreign material enters the

body, several types of white blood cells go into

action. White blood cells (lymphocytes) produce

chemicals that kill infected cells or tumor cells.

B lymphocytes produce antibodies. Antibodies

are proteins that bind to the pathogen and make it inactive. Large cells called macrophages fight off the pathogens by engulfing and destroying them. Image: Electron microscope image of a red blood cell (left), platelet (center), and lymphocyte (right). Image: The National Cancer Institute at Frederick (NCI-Frederick)

The antibodies made by the B lymphocytes can bind to the antigen so it cannot invade body cells. Antibodies stay in blood and when you are exposed to the same antigen again, the antibodies can fight off the pathogen before you ever get sick. For many diseases, you only get sick from that disease once; the antibodies you made in response to the pathogen fights off all other exposures. This type of immunity is active immunity.

Sometimes, antibodies made in another organism are given to people to fight off a disease. These antibodies were not made by the person in active response to a disease. This type of immunity is called passive immunity.

Vaccines can produce immunity by causing the body to produce antibodies in response to dead or weakened virus particles. You get immunized with the vaccine and you do not get sick with that disease.

For more information about vaccines and immunity, read How Vaccines Prevent Disease.

Electron microscope image of an HIV particle.

Electron microscope image of a red blood cell (left), platelet (middle) and a white blood cell (right).