Defense Against Infectious Disease Biology Essay


Antibiotics, produced by microorganisms, substances which can inhibit growth of or kill bacteria, slows or kill off bacterial diseases by blocking specific metabolic pathways within cell.

Found in nature, mainly fungi or bacteria, slow down growth of microorganisms. Enter bacterial cells during their growth & division.

Bacteria different to human cells, antibiotics taken by humans to kill bacteria without harming human cells.

 Virus, different as they don't carry out many metabolic processes themselves.

Instead they rely on host cell (human) to carry out these processes for them.

Virus cannot be treated with antibiotics as, impossible to harm virus without harming human cells.

Virus don't have metabolic pathways like bacteria, therefore antibiotics doesn't work on viruses

Virus can only be treated by specific anti-microbial agent & antibiotics should never be prescribed for viral infections (e.g. flu)


Effectiveness of antibiotic depends on two factors:

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Spectrum: diversity of bacteria against which antibiotic acts.

Selective toxicity:measure of degree which antibiotic is harmful to bacteria but not bacteria host

Antibiotics with high selective toxicity disrupt enzymes or structures unique to bacteria; low selective toxicity inhibit same process in bacteria as host cells, or damage host cells in some other way.



Selective toxicity

Mechanism of action:


Methicillin & penicillin



Inhibit peptidoglycan formation by binding to enzyme transpeptidase

Inflammation of lungs, strep throat, pathogenic toxins in blood, skin, infections, gonorrhea




Bind to small ribosomal subunit, interferes with tRNA binding

Acute diarrhea, vomiting or cramps, inflammation of lungs, muscular pains combined w/ skin eruptions

Trimethoprim- sulfamethoxazole



Inhibit folic acid synthesis (bacteria must make, mammals acquire in diet)

Urinary tract infections, bronchitis, ear infections


6.3.3 Outline role of skin & mucous membranes in defense against pathogen

Several physical & biochemical barriers to infection outside immune system with vital role in maintaining health. Intact skin, most important physical barrier with mucus in respiratory tract, gut & genitals, trap pathogens & irritant particles

Mucus, saliva & tears, contain enzyme lysosome, hydrolyzes bonds in polysaccharide wall of bacteria, damaging bacterium's outer protection.


Made from closely packed epithelial cells

Sebaceous glands secret lactic acids & fatty acids

Outer layer contains dead cells mainly composed of keratin, forms tough impenetrable barrier

Very arid (dry), most microorganisms cluster around sweat pores

Sweat contains lactic acid, slows down growth of microorganisms

Vast number of non-pathogenic bacteria cover our skin, known as natural microflora. Do not harm body, aid in preventing harmful bacteria gaining place on our skin by 'crowding' them out.

Skin perforated (pierced) blood clot forms rapidly, prevent bacterial entry

Produces chemical secretions, prevent growth of fungi & bacteria

Low pH would hinder colonization of bacteria

Mucus Membranes:

Sticky polysaccharide produced by goblet cells, line many of body's passage

Works to trap microorganisms; mucus then removed from body

Respiratory tracts cilia line passages & waft mucus to back of throat, where swallowed into acid environment of stomach or ejected by coughing

6.3.4- Outline how phagocytic leucocytes ingest pathogens in the blood and in body tissue


form of leukocytes, amoeba-like cells. Has the ability to extent parts of cell in different directions.

It is part of body's second line of defense against disease.

Ingests bacteria, viruses & dust particles, then destroys them

Can ingest pathogens in blood but also within body tissue

Can pass through pore of capillaries & into tissues

Able to change shape to ingest microbes.

Pseudopodia, extensions of phagocytes, used to engulf microbes



1)     Detection- Phagocyte detects microbe by chemotaxis- chemicals they give off & sticks microbe to surface

2)    Ingestion- Microbe engulfed by phagocyte wrapping pseudopodia  around it to form vesicle/phagozome. [Membrane of phagocyte changes shape to surround microbe]

3)    Phagosome Forms- Phagosome (Phagocytic Vesicle) formed, enclose microbes in membrane

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4)    Fusion with Lysosome-  Phagosome fuses with Lysosomes releasing hydrolytic enzymes to kill/digest microbes

5)    Digestion- Microbes broken down by enzymes into chemical constituents

6)    Discharge- Indigestible material discharged from phagocyte cell


6.3.5- Distinguish between antigens & antibodies

Antigen, foreign substance stimulates production of antibodies. Can bind with any of wide range of substances including cell walls of pathogenic bacteria or fungi & protein coats of pathogenic viruses. Recognized by immune system, triggering immune response.

Antibodies, proteins that recognize & bind to specific antigens. Defend body against pathogens by binding to antigens on surface of pathogen & stimulating its destruction. Immunoglobin. These have T or Y shape made from polypeptide chains


6.3.6 Explain antibody production

Antigen Processing:

Macrophage engulfs pathogens, destroys it- Phagocytosis

Phagocytes digests pathogen and lysosomes, broken down into short peptide chains, peptide chains "displayed" on surface of phagocytes in body attached to special molecule, MHC II (Major histocompatibility complex class II)

Macrophage, "Antigen presenting cell"

Helper T Cell Stimulating B Cell

Antigen-Presenting Cell travels to lymph nodes  & finds specific Helper T-cells with correct complementary antigens

Helper T cell & antigen bind together, helper T cell becomes activated

T-cells finds B-cells to produce antibodies and binds with it (clonal selection), activates B-cell

Stimulates B Cell, divide and form clones of B-cells (clonal expansion)

Plasma B-cells, producing antibodies released to blood vessels/plasma

B-cells not producing antibodies > Memory B-cells (Giving immunity)

T-cells clones up > Memory T-cells

After infection, if same pathogen enters body, immune response more rapid due to presence of Memory B & T cells


Pathogen foreign because didn't have antigens found on same cells.

Antibodies clump bacteria and helps phagocytes engulf


6.3.7 Outline the effects HIV on immune system

HIV- Human Immunodeficiency Virus. Retrovirus, uses RNA to make DNA. Virus contains 2 single strands of DNA, together with some enzymes surrounded by protein coat.

Retrovirus reverses normal flow of genetic information, from DNA of genes to mRNA in cytoplasm.

Infect host cell, information in RNA in cytoplasm translated into DNA, then inserts itself into host cell's chromosome

HIV Infection:

Attachment to T cell membrane:

On cell surface membrane of all cells, receptors. At least 2 receptors for HIV virus on surface of T lymphocytes. HIV infection of T lymphocyte requires virus to attach to both receptor sites

Entry of viral RNA:

Tight attachment of viral particle allows it to fuse with cell surface membrane, contents enter cytoplasm of T lymphocyte

Reverse Transcriptase: Converting viral RNA into DNA

Enzyme found in HIV virus, called reverse transcriptase transcribes viral RNA into DNA double helix

Integration of viral DNA

Virus contains another enzyme, integrase, helps splice host cell's DNA & integrate viral DNA. Integrated DNA, provirus

Replicated with host cell's genes every time cell divides. Viral genes remain latent, giving no sign of presence in host cell, person said to be HIV+


6.3.8 Discuss cause, transmission & social implications of AIDS

AIDS, Acquired Immune Deficiency Syndrome