AAAACHHOOO!! Your friend has a terrible cold, and he sneezes right next to you without covering his mouth. Not only is this gross, it sprays millions of cold viruses into the air around you. Luckily, you don’t get sick. Why not?

Your body has an amazing internal defense mechanism called the immune system which protects you from bacteria and viruses that can lead to illness. A healthy immune system produces a variety of different cells to attack the invading bacteria and viruses.


Your immune system patrols and
protects your body from harmful invaders


If your immune system finds an invader,
it takes care of the problem


Your blood contains many different types of cells. Red blood cells carry oxygen to all the parts of your body. Platelets stop bleeding by helping with blood clotting. White blood cells, known as leukocytes (LUKE-oh-sites), make up the immune system portion of the blood.

Leukocytes are divided into three main groups:

  • Granulocytes – cells which contain granules which contain chemicals that are used to kill bacteria and viruses
  • Lymphocytes- cells which attack most of the bacterial and viral infections in our bodies
  • Monocytes - cells which become macrophages, large cells that engulf harmful particles in our bodies

These three types of leukocytes are even further divided into more specialized cells, each with their own unique task in the immune system. To learn more details about the immune system, see Additional Resources.

How does your immune system know which cells to attack and which cells are part of your own body?

Your immune system can recognize cells based on the proteins present on the surface of cells. Viruses, bacteria, and other foreign cells are recognized as being different from your own cells and are attacked by your immune system. Sometimes, one of your own cells changes, or mutates, giving the cell the ability to multiply continuously. Such mutations often are the cause of cancer. Your immune system has the ability to recognize mutated cells and attack them before they can grow into a tumor.

Not an invincible defense…

Despite the amazing ability to protect your body, the immune system is not foolproof.Not only can certain viruses outwit your immune system's defenses, but genetic malfunctions can result in an ineffective immune system.

The Human Immunodeficiency Virus (HIV), infects the CD4+ T-cells, a type of lymphocyte, causing the cells to die. If enough cells are killed, the immune system no longer functions and the person becomes susceptible to many different diseases.

An example of a genetic disease of the immune system is lupus which causes your immune system to mistakenly attack your body’s healthy cells.

Damage to the immune system can
cause problems to go undetected.

Exposure to certain toxic chemicals can also affect your immune system. The study of substances that harm the immune system is called immunotoxicology (immuno: related to the immune system, toxicology: the study of harmful substances).

After exposure to an immunotoxicant, a chemical that harms the immune system, your body may not be able to produce the variety or number of defense cells that it needs to protect itself.

If the immune system is damaged, it cannot attack foreign cells such as viruses, bacteria or tumor cells that can cause health problems.

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