Body DefencePrepared by Ms W.S.Kwan

PathogensPathogens

Microorganisms causing diseases

eg. bacteria viruses fungi protozoa

Pathogens cause diseases

• By direct attack, destroying the cell &/or• By releasing toxins, which upset our internal

environment• Diseases caused pathogens are called infectious

diseases

Body Defense Body Defense SystemSystem

prevent pathogens from entering the body

kill or inactivate any pathogens that gain entry into the body

Ways of Pathogen Ways of Pathogen spreadingspreading

In Droplets / saliva & sneeze e.g. influenza, cold

By Touch e.g. boils, athlete’s foot, AID

By Dust e.g. diphtheria, scarlet fever

In Faeces e.g. cholera, hepatitis A

In Food e.g. salmonella

By Insect e.g. malaria

By Vertebrates e.g. rabies

Ways of Pathogen Ways of Pathogen spreadingspreading

Pathogens can be spread:      By ingestion of contaminated food or water (e.g.

salmonella, cholera & Hepatitis A)

Pathogens are common in faeces & water.

Pathogens can be spread: • By inhalation of dust with pathogens / airborne pathogens

Pathogens can be spread: • By body fluid contact: Saliva contact / droplets

(e.g. influenza)  Blood contact

(e.g. Hepatitis B)

Sexual contact(e.g. Hepatitis B)

Pathogens can be spread: By direct contact with an infected area (e.g. athlete’s foot)

Pathogens can be spread:      By vectors ‘insects & other vertebrates like dogs’ (e.g.

malaria by mosquito, rabies by dogs)

Body defence•Nonspecific DefencesNonspecific Defences•Specific DefencesSpecific Defences

Nonspecific DefenceNonspecific Defencess

11stst line of defence line of defence• Barrier

Physical Barrier

Chemical Barrier

• Blood clotting

• Phagocytes

Prevent entry

Prevent entry

Kill pathogens

Kill pathogens

(1)Skin Tough outermost layer

‘epidermis’ acts as a mechanical barrier to prevent entry of pathogen

Physical /mechanical Barrier

Hair at entrance/nostril: filter air Secrete Mucus: trap bacteria Beating cilia: waft/carry the

trapped bacteria towards the throat

Physical /mechanical Barrier

(2) Ciliated epithelium of respiratory tract

Sebaceous glands of skinProduce oily secretion (sebum)

which has antiseptic properties Tears & salivaContain lysozymes (enzyme) wh

ich destroy bacteria

Chemical Barrier

Gastric juice in stomach Contain acid which can

destroy most bacteriaAcid secretions in vagina of

womenreduce growth of pathogens

Chemical Barrier

When exposed to air in wound, blood platelets release a substance to turn soluble fibrinogen into insoluble fibrin which catches blood cells & seal off the cut.

Blood clotting

BLOOD CLOTTINGBLOOD CLOTTING is is important because this important because this can …can … Prevent entry of pathogens

Stop bleeding (prevent blood loss)

WBC WBC (Phagocytes)(Phagocytes)

• Nonspecific defence (after infection)Nonspecific defence (after infection)• WBC engulf & destroy the pathogens

by phagocytosis

macrophage

• inflammatory response

– (before inflammation : skin arterioles constrict to prevent excessive bleeding)

– skin arterioles in the infected area dilates so that more blood flows to the area

– the permeability of skin capillaries increases so that more WBC & fluid come into the infected tissues

– the skin becomes red & swell up with pain (because of high pressure)

InflammatiInflammationon

(infected area becomes red, swollen & painful)

Specific DefenSpecific Defencesces

What are Specific Defences ?What are Specific Defences ?• When a pathogen is able to get pa

st the nonspecific defences,• immune system will produce a ser

ies of immune responses to attack the pathogen.

• These immune responses are the specific defences of our body.

• After pathogens get into the blood & lymph,

• Antigens on the surface of pathogen stimulate WBC (lymphocytes) to produce specific antibodies

Note: Antibodies are specific & protein in nature.

What stimulates immune systWhat stimulates immune system ?em ?

How antibodies kill pathogens?

lysis - burst the pathogen

clump the pathogen together

stick to pathogen (enhanced phagocytosis)

neutralize the toxins from pathogens

Primary Primary (immune) (immune) ResponsesResponses

The action in response to the 1st exposure to a certain antigen which stimulates the white blood cell to produce antibodies.

or

The action in response to the 1st invasion of

a certain pathogen which stimulates the whit

e blood cell to produce antibodies.

Secondary Secondary (immune) (immune) ResponsesResponses

The action in response to the 2nd invasion

of the same type of pathogen which

stimulates the white blood cell to produce

much more quickly & much larger amount

of antibodies specific the antigen.

Time (days)

antibodyconc.

primaryresponse

secondaryresponse

second exposureto antigen X

first exposureto antigen X

In the 1st exposure to an antigen, certain WBCs in the body will memorize the antigen.

In subsequent exposures to the same antigen, the memory WBCs will multiply immediately to produce large amount of phagocytic WBC & antibodies specific to the antigen.

Immunity

• Immunity means

Body is able to resist the disease

• Once the body has the threshold amount of antibodies, the body can resist the disease (i.e. Immunity)

Time (days)

antibodyconc.

Have immunity

no immunity

primaryresponse

secondaryresponse

second exposureto antigen X

first exposureto antigen X

Thresholdvalve

Disease symptom shown in 1st exposure, but not in 2nd

Comparison between primary & secondary immune response

Primary response Secondary response

Production Antibodies, phagocytic WBC &

memory WBC

Antibodies, phagocytic WBC &

memory WBC

Production level & rate

Low & slow High & fast

Immunity No, disease symptom shown.

Yes, no disease.

How can we acquire immunity?Apart from actual contact,

Vaccine story

• In 1771 in England, smallpox widespread.

• Have cowpox, no smallpox.

• Experiment on a boy.

VaccinatioVaccinationn

Vaccine = dead / weakened pathogens

To stimulate the WBC to produce antibodies

Vaccine taken:

By injection, skin scratching or mouth (orally)

Times of taken:

First→ second → (booster) to get full immunity

Vaccination is a form of artificial immunity.

Principles of vaccination:

• A previous exposure of the body to an antigen will be memorized by certain type of white blood cell in the body.

• Subsequent exposure to the same type of antigen will initiate a rapid production of a large amount of phagocytic white blood cells & antibodies specific for that antigen.

Injection of a SerumInjection of a Serum

antibodyconc.

Time (days)injection of antibody

(in serum)

Have immunity immediate after injection

No disease symptom

Thresholdvalve

Protection does not last long. Antibodies will be broken down & rejected.

Pupils should be able to:

1. Explain the function of the skin as mechanical barrier against bacteria.

2. Describe the action of the ciliated epithelium of the respiratory tract in filtering dust particles & bacteria from air.

3. State that hydrochloric acid in the gastric juice kills most bacteria in the food reaching the stomach.

4. State that the blood clot covers the wound to stop bleeding & to prevent entry of pathogens.

5. State that white blood cells can protect our body by engulfing bacteria & producing antibodies. 

Pupils should be able to:

6. Explain the principles of vaccination, a form of artificial immunity, including:

(a) A previous exposure of the body to an antigen will be memorized by certain type of white blood cell in the body.

(b) Subsequent exposure to the same type of antigen will initiate a rapid production of a large amount of phagocytic white blood cells & antibodies specific for that antigen.

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