Defense against infectious disease

Topic 6.3

6.3.1: What is a pathogen?

Antibiotics and bacteria

6.3.2: 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

6.3.3: 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

6.3.4: 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

6.3.5 & 6.3.6: 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

6.3.7 & 6.3.8: 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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