Defense against infectious disease

Topic 6.3

Essential idea: The human body has structures and processes that resist the continuous threat of invasion by pathogens.


What is a pathogen?

Antibiotics and bacteria

6.3.U1 The skin and mucous membranes form a primary defense against pathogens that cause infectious disease

Skin, mucous membranes and defense against pathogens

The first line of defense against pathogens is the skin and mucous membranes. The skin provides a protective barrier against the entrance of pathogens while the mucous membranes produce substances that either stop the growth of pathogens or kill the pathogens. The figure below outlines how the skin and mucous membranes defend the body against infection by pathogens.

Skin and mucous membranes

6.3.U2 Cuts in the skin are sealed by blood clotting

Sealing a cut

6.3.U3 Clotting factors are released from platelets

Platelet release of clotting factors

6.3.U4 The cascade results in the rapid conversion of fibrinogen to fibrin by thrombin

Blood clotting process

6.3.U5 Ingestion of pathogens by phagocytic white blood cells gives non-specific immunity to diseases

Phagocytosis is the next line of defense

When pathogens managed to get by skin and mucous membranes a second line of defense is available to remove them, phagocytic leukocytes ingest any pathogens in the blood and body tissues. The importance of phagocytosis is outlined in the figure below.

phagocytosis link phagocytosis animation

6.3.U6 Production of antibodies by lymphocytes in response to particular pathogens gives specific immunity

Antibody production

The immune system must encounter a foreign substance, called an antigen, before it can produce the necessary response to effectively remove the pathogen. An antigen is any foreign molecule found on the surfaces of viruses, bacteria, cancer cells that elicits an immune response. The response by the immune system to the antigen is the production of antibodies. An antibody is a protein found in blood plasma that attaches to a particular antigen to help control its effects. These antibodies are specific to that antigen.

immune response link immune response animation
Production of antibodies

6.3.U7 Antibiotics block processes that occur in prokaryotic cells but not in eukaryotic cells

Why are antibiotics effective against bacteria?

Antibiotics are effective against bacteria but not against viruses. Antibiotics block specific pathways found in bacteria. Viruses do not have their own metabolic pathways, therefore, in order to reproduce us to use the metabolic pathways of the cells they infect. Antibiotics are not effective against viruses that have infected human cells because the metabolic pathways are not affected by the antibiotics.

Antibiotics and bacteria

Processes in bacteria affected by antibiotics

Processes affected by antibiotics in bacteria

6.3.U8 Viruses lack a metabolism and cannot therefore be treated with antibotics. Some strains of bacteria have evolved genes                   that confer resistance to antibiotics and some strains of bacteria have multiple resistance.

Antibiotic resistance mechanisms in bacteria


6.3.A1 Causes and consequences of blood clot in coronary arteries.

Formation of an occlusion in a coronary artery

6.3.A2 Florey and Chain's experiments to test penicillin on bacterial infections on mice

Timeline to Florey and Chain's penicillin experiment

6.3.A3 Effects of HIV on the immune system and methods of transmission

Effects of HIV on the immune system

HIV is a virus that attacks the immune system, specifically lymphocytes. This results in a reduction of the number of lymphocytes and a loss of the ability to produce antibodies. The figure below outlines how HIV leads to AIDS and the social implications as a result of the disease.

Effects of HIV on the immune system

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