Defense Against Infectious DiseaseHow does my body fight disease?

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Pathogen •An organism or

virus that causes disease.

•This includes virus, bacteria, protozoa, fungi and worms of various types. Tripanosoma cruzi protozoans in blood – Chagas disease

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•Not every pathogen that enters our body results in disease

•This is because for most pathogens, our body has developed immunity.

•There are also chemicals called antibiotics that work against bacterial cells without affecting our body cells.

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How antibiotics work

• Saprotrophs, including bacteria and fungi, compete with each other for death organic matter on which they feed.

• To inhibit the growth of their competitors, some saprotrophs produce antibiotics.

• An antibiotic is a chemical that inhibits the growth of microorganisms.

• Antibiotics produced by fungi mostly inhibit bacteria. • There are smaller number of antibiotics produced by

bacteria that inhibit fungi.

How antibiotics work

•Antibiotics interfere with various metabolic processes

•Antibiotics are not effective against viruses.

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Organism that produces the antibiotic

Antibiotic produced Mechanism of action

Fungi:

Penicillium sp. Penicillin Inhibits cell wall synthesis

Cehalosporium sp. Cephalosporin Inhibits cell wall synthesis

Streptomyces sp. TetracyclineStreptomycin

Inhibits translation

Saccharopolyspora erytraea

Erytromycin Inhibits translation

Bacteria:

Bacillus sp. PolymyxingBacitracin

Disrupts cell membranesDisrupts cell wall synthesis

Preventing pathogens from entering our bodies.-

•The ability to resist infection by a disease is termed immunity.

•There are two types of immunity:▫Non specific immunity allows the body to

resist infection by a wide range of pathogens.

▫Specific immunity allows the body to resist a specific pathogen

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Non-specific immunityFirst line of defense:

SkinTough: provides a physical barrier against the entry of pathogens and protection against physical and chemical damage

Sebaceous glands and hair follicles

Secrete a chemical called sebum, which maintains skin moisture and slightly lowers skin pH to inhibit the growth of bacteria and fungi.

Mucous membranes

Line internal cavities that are exposed to the external environments such as nasal passages.Secrete mucus, which has antiseptic properties because of the antibacterial enzyme lysozyme.Mucus also acts as a physical barrier by trapping pathogens and harmful particles.Cilia in some mucus membranes (i.e. trachea) move trapped pathogens up and out.

Stomach acid The acidic environment of the stomach helps to kill most of the ingested pathogens.

Second line of defense

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Non specific immunity

Second line of defense:

Phagocytes

•If a microorganism get past the physical barriers and enter the body, leucocytes (white blood cells) provide the next line of defense.•Macrophages, are large phagocytes that change shape to surround an invader, taking it in through the process of phagocytosis and digesting it with enzymes from lysosomes.. •Macrophages can pass through pores in the walls of capillaries and move to the sites of infection. •When wounds become infected (septic), large numbers of phagocytes are attracted, resulting in the formation of pus.

Antibodies are produced in response to a specific pathogen.-

• If a microorganisms gets past the physical barriers and the phagocytes, a second level of immunity exists.

• Proteins and other molecules on the surface of pathogens are recognized as foreign by the body and they stimulate a specific immune response.Antigen A foreign substance that stimulates the

production of antibodies

Antibodies Proteins that recognize and bind to specific antigens

• The body is capable of responding in a highly targeted way to pathogens.

• A protein called antibody binds to antigens.

• Our body can produce a large variety of antibodies, each one specific to a certain antigen.

•The binding of an antibody to an antigen produces many benefits, including:

▫Makes the pathogen more recognizable to phagocytes

▫Prevents viruses from docking to host cells so that they cannot be taken up by host cells.

▫Cause agglutination (sticking together) of pathogens so they cannot enter cells and are easier for phagocytes to ingest.

•Immune response depends on different types of lymphocytes, each with a specific function. The process includes:

Macrophage Ingests the pathogen and displays antigens from it (proteins or other substances)

T-cells Identify specific pathogens produced by macrophages and activate B cells

B-cells Produce antibodies that specifically attach to the antigen, clone by mitosis to increase in number, eliminate the pathogen

Memory cells Some B-cells remain in the bloodstream to give immunity for a second infection.

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HIV and AIDS•The human

immunodeficiency virus (HIV) targets helper T-cells.

• In the early stages of infection, the immune system makes antibodies against HIV.

•If these can be detected in a person’s body, they are said to be HIV-positive.

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HIV•HIV invades and destroys lymphocytes

called helper T-cells resulting in the reduction of their number and the progressive loss of the capacity to produce antibodies.

•The rate at which this occurs varies considerably and it can be slowed down by using anti-retroviral drugs.

AIDS•  In most HIV-positive patients antibody

production eventually becomes so ineffective that a group of opportunistic infections strike.

• Several of these are normally so rare that they are “marker” diseases for the latter stages of HIV infection.

• When several infections affect someone due to HIV, the person is said to have acquired immune deficiency syndrome (AIDS).

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AIDS•AIDS spreads by HIV infection.

•The virus only survives outside the body for a short time.

•Infection normally only occurs if there is a blood to blood contact between infected and uninfected people.

Ways in which an infection of HIV can occur include:

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Social implications:• Sufferers are stigmatized in terms of

employment, insurance, education access, social acceptance and others

• families lose members• Those that die are both parents and

workers• Grandparents may be left to look

after orphaned children• Economic productivity is lost• The costs of treating the disease are

high and may overburden health services

• In countries where infection rates are high, there may be an undermining of social cohesion. http://archive.thedailystar.net/photo/2010/12/01/2010-

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Sources:• Allot, A., & Mindorff, D. (2010). IB Biology Course

Companion. New York: Oxford University Press.• Allot, A., & Mindorff, D. (s.f.). IB Biology Course

Companion 1st ed . Obtenido de Actual Book Pages: http://aura.edu.in/read/IB/BIOLOGY_COURSE_COMPANION_(OXFORD)/original/Page-226.html

• Damon, A., McGonegal, R., Tosto, P., & Ward, W. (2007). Biology Higher Level. London: Pearson Baccalaureate.