Defenses Against Disease

If pathogens are everywhere, why aren’t you sick all of the time?

When you do get sick, what keeps pathogens from multiplying until they take over your body?

The Answer• Your body has a number of defenses against infection.

• Your body’s first line of defense against infectious disease includes both physical and chemical defenses that prevent pathogens from entering your body.

*see figure 21-3: Physical and Chemical Defenses

Skin

• Your skin serves as both a physical and chemical barrier against pathogens.

• The surface cells are hard and have no gaps between them.

• Sweat acts as a chemical barrier because it contains acids that kill many bacteria.

• Old skin cells are shed constantly, and the pathogens on these cells are shed too.

• In fact, microorganisms usually cannot get through your skin unless you have a cut, scrape, burn, or other injury.

Mucous Membranes

Openings to your body, such as your

are covered by protective linings call mucous membranes. Mucous membranes secrete a fluid called mucous that traps pathogens and washes them away. Mucous also contains chemicals and specialized cells that attack pathogens.

Cilia• Some of your body’s

mucous membranes are lined with tiny hair-like structures called cilia.

• Cilia and mucous help trap and remove pathogens.

• When you cough, sneeze, or blow your nose, the pathogens are removed along with the mucous.

Saliva & Tears• Your saliva and tears can trap pathogens and

wash them away. Like mucous, saliva and tears also contain chemicals that attack pathogens.

Digestive SystemChemicals in your digestive system, including your stomach, kill many pathogens.

In addition, the normal motions of the digestive system not only move food through your system but also move pathogens out.

Inflammation

• If pathogens are able to get past the physical and chemical defenses and to injure cells, your body is ready with a second line of defense- Inflammation.

• Inflammation is your body’s general response to all kinds of injury, from cuts and scrapes to internal damage.

• Inflammation fights infection and promotes the healing process.

Phagocytes• Within seconds after your body is injured, the damaged cells

release chemicals that cause blood vessels in the injured area to enlarge.

• Blood, other fluids, and white blood cells called phagocytes

leak out of the enlarged vessels.

The phagocytes engulf and destroy pathogens.

Meanwhile the infected area becomes red, swollen, and sore-in other words, inflamed.

Healing• Phagocytes give off substances that cause

healing to begin. The fluids, phagocytes, and dead cells that accumulate at the injury cite often result in the formation of a thick, white liquid called pus.

• Your body’s third line of defense against pathogens is your immune system.

• The immune system fights disease by producing a separate set of weapons for each kind of pathogen it encounters.

When a pathogen enters your body for the first time, it often causes disease.

If your immune system is working, why does this happen?

The Answer• Your immune system must build up its arsenal

of weapons against the newly encountered pathogen.

• When a pathogen enters your body it takes time for it to multiply and cause disease.

• Once the immune system’s arsenal is built up, however, the immune system kills the pathogen, and your body gradually recovers.

White blood cells called lymphocytes carry out most of the immune system’s functions.

Immunity

• If your body as previously been attacked by a pathogen it will recognize it when it is exposed to it again.

• This time, your immune system will quickly recognize the pathogen and launch an immediate attack!

• When this happens you are said to be immune.

• Immunity is your body’s ability to destroy pathogens that it has previously encountered before the pathogens are able to cause disease.

Killer T Cells

• Killer T Cells destroy any body cell that has been infected by a pathogen.

Helper T Cell

• Helper T Cells produce chemicals that stimulate other T cells and B cells to fight off infection.

Suppressor T Cells

• Suppressor T cells produce chemicals that “turn off” other immune system cells when an infection has been brought under control.

B Cells• B Cells produce antibodies. Antibodies are

proteins that attach to the surface of pathogens or to the toxins produced by pathogens.

This binding action keeps the pathogen or toxin from harming the body.

Once the infection is overcome, your B cells

stop producing antibodies, but they do not “forget”

how to produce

them.

See Figure 5 for The Immune Response

Passive vs Active Immunity

• There are two types of immunity-• 1. Passive• 2. Active

• Both types are important in protecting your body against defenses.

Passive Immunity

• Immunity that is acquired by receiving antibodies from a source other than one’s own body.

• This type of immunity is temporary and not lifelong.

Examples• It occurs naturally in babies who receive

antibodies from their mothers before birth.• After birth antibodies may also be passed to

an infant through the mother’s breast milk.

Or

• When a doctor gives someone injections of rabies antibodies if they were bitten by a dog with rabies.

Active Immunity• Active immunity is immunity that your own

immune system creates.• Active immunity results from either having a

disease or from receiving a vaccine.

Vaccines

• Vaccines contain small amounts of dead or modified pathogens or their toxins.

• A vaccine causes your immune system to produce antibodies against a pathogen, as if you had actually been infected.

• You develop immunity without having to experience the disease.

Booster Shots

• After a few years, you may receive a booster dose of some vaccines to “remind” your immune system to maintain your immunity.