Topic 6: Human physiology : 6.3 Defense against infectious disease

First line defence – physical barrier

Skin and mucous membranes are physical barriers against infection from pathogens, including bacteria, virus, fungi and parasite
Skin is continuous, so it is very hard to find an opening for pathogen to invade.
Skin is constantly replacing its outermost epidermal layer of skin. These dead cells provide effective protection against foreign pathogens.
Skin has about a pH about 5, which is not favorable to pathogens.
There are lysozyme on the skin, which can break down pathogens
Natural organism presents on the skin will compete living environment with pathogens.
Mucous is sticky and traps foreign particles and pathogens contained in the air before they reach the lungs.
Mucous contains lysozymes (enzymes) that can damage and kill pathogens.
Trapped pathogens can also be expelled through the mouth or nose, or swallowed and destroyed by the high acidity of the stomach.
There is also unfavorable pH and natural organisms in mucous

Blood Clotting

Blood clotting is the process in which cuts or broken blood vessels are repaired and sealed to prevent excessive blood loss.
When a blood vessel is broken or cut, blood platelets collect at the site of the damaged blood vessel forming a platelet plug.
The platelets and the damaged tissue release chemical factors called clotting factors.
The clotting factors convert the clotting protein prothrombin to its active form thrombin (enzyme).
The enzyme thrombin converts clotting protein fibrinogen (which is soluble) into the insoluble fibrous protein fibrin.
Fibrin forms a mesh at the point of the broken vessel further trapping other blood cells sealing up the damaged vessel and forming a stable clot.
Once the damaged vessel has fully healed, the blood clot dissolves in the blood

Atherosclerosis

Atherosclerosis is a disease of the arteries characterized by the deposition of plaques of fatty material on their inner walls. (blood clots in artery)
Damaged coronary arteries wall leads to a growth of fibrous tissue, which will trap cholesterol.
Plague of cholesterol will form on the artery
High pressure of blood opens up the artery.
Red blood cells bind to the plague.
It will reduce blood flow, causing death of heart cells.
Myocardial infarction (heart attack) occurs if a coronary artery becomes completely blocked.
Coronary muscle tissue dies as a result of lack of oxygen.
Risk factors: genetics/age/smoking/diet/exercise/obesity/stress
Bypass surgery can avoid the block and supply blood to the heart

Phagocytes

Phagocytes use chemotaxis to find pathogens.
Chemotaxis: movement in response to chemicals
Protein produced by pathogen and phospholipids released by damaged cells will attract phagocytes to come
Phagocytes attach to the pathogen’s cell surface protein and then engulf it.
Once the pathogen is engulfed, lysosomes within the phagocyte contain hydrolytic enzymes that will digest and destroy the foreign pathogens.
Using endocytosis to engulf pathogens to form phagosome.
Lysosome attaches to phagosome to form phagolysosome, which destroy the pathogens inside
The corpse is expelled by exocytosis

Lymphocytes

When a pathogen enters the blood, the specific antigen on the surface of the membrane is identified as being foreign or non-self
This stimulates a specific immune response in which antibodies are produced that are specific for that particular antigen
Antibodies: globular protein that recognizes a specific antigen and binds to it as a part of an immune system
Antigen: substances found on a cell or virus causes antibody formation.
B-lymphocytes are white blood cells that produce antibodies that bind to the antigen on the invading pathogen
Once an antigen has been encountered the B-lymphocytes are stimulated to divide to produce a large amounts of clones of themselves (clonal replication)
The active B-lymphocytes that are produced are called plasma cells which will begin to produce antibodies.
The plasma cells created, produce and release mass amounts of antibodies into the bloodstream.
Plasma cells fire antibodies to antigens and signal phagocytes to destroy them
Some of these divisions also produce B-cells called memory cells, which stay in the blood in case of a second infection to provide a quick response to the new infection.
The primary response is the production of antibodies to the initial challenge by the invading antigen.
The secondary response which is much quicker because memory cells are still in the blood occurs after a subsequent challenge by the same antigen.
Memory cells create immunity

Antibiotics

Antibiotics are a type of drug or chemical that inhibits the growth of microorganisms; mainly bacteria
Antibiotics block cellular processes such as DNA replication, transcription, translation, and cell wall formation
The first antibiotic discovered by Alexander Fleming was identified as penicillin
Later on, two scientists named Florey and Chain were able to develop a method of growing the Penicillin in liquid cultures and purifying the Penicillin in these cultures.
Since viruses lack their own metabolism, they have to use the chemical processes of a cell from a host that they infect
They are unable to reproduce on their own and cannot perform protein synthesis, transcription and other metabolic functions
Antibiotics work by blocking these vital processes in bacteria, killing the bacteria, or stopping them from multiplying
Virus lacks metabolic pathways so antibiotics won’t work on them
Antibiotic resistance may develop due to evolution by natural selection, mutation, fast division

Vaccination

Active immunity can be acquired through vaccination.
A vaccine is a weakened version of a pathogen.
It is introduced to the body through an injection, which causes a primary immune response to the pathogen.
This will create the plasma B-cells necessary to fight off the initial infection from the vaccine and the memory B-cells necessary for a secondary immune response if the person is exposed to the real pathogen.
This secondary response is much quicker and more intense producing more antibodies in less time
Sometimes “booster shots” are given which is a second round of vaccination that causes a secondary immune response.

HIV

HIV (human immunodeficiency virus) is a retrovirus that causes AIDS, which is a condition in humans where the immune system fails and is susceptible to life-threatening opportunistic infections.
HIV targets helper-T cells
Helper-T cells play an important role in the production of clonal B lymphocyte cells, which produce antibodies for immune response.
Therefore the reduction of T cells will reduce the amount of antibodies produced needed to fight off infection from invading pathogens.
This inability to fight off disease is what eventually causes the person to die.
HIV is a retrovirus, which means it inserts its own DNA into the host cells and use their cellular machine to reproduce