Pathogens as the cause of infectious diseases
A disease is a particular kind of illness, with characteristic symptoms. For a long time, the causes of diseases were not understood, because the are not visible to the naked eye. Scientists have identified 3 types of causes for pathogens:
- Genetic
- Environmental (such as toxic chemicals or radiation)
- Infection with a pathogen
Pathogens are organisms that cause diseases. They are passed from one infected organism to another, either directly or indirectly. They enter the organism, multiply there and cause harm. Organisms that invade the body and can be seen with the naked eye, such as tapeworms, are usually considered to be parasites rather than pathogens. Pathogens are therefore microorganisms.
Pathogens must be able to multiply inside the body. Living organisms do this by reproduction. There are examples of pathogens in most of the principal groups of microorganisms:
- In bacteria —> Tuberculosis, gonorrhea, leprosy
- In fungi —> Athlete’s foot, thrush, aspergillosis
- In Protista —> malaria, toxoplasmosis, sleeping sickness
- Viruses —> influenza, measles, Ebola
Viruses are not usually considered to be alive because they cannot reproduce themselves. However, they are replicated by host cells and they certainly cause diseases so they are included among pathogens.
One group of infectious diseases have a surprising cause. These diseases affect the brain and are called spongiform encephalopathies. CJD (Creutzfeld-Jakob disease) is the human version. The causative agent has been found to be a prion, rather than a microorganism or virus. Prions are protein molecules. If they enter the body and reach brain tissue, they cause more proteins already there to turn into prion proteins. This causes brain cells to malfunction, leading to the development of the disease.
Skin and mucous membranes as a primary defence
Many different microbes can grow inside the human body and cause a diseases. Some microorganisms are opportunistic and although they can invade the body they also commonly live outside it. Others are specialized and can only survive inside a human body.
The primary defence of the body against entry of pathogens is the skin. It’s outermost layer provides a physical barrier. Much of the body is covered by a tough layer of dead cells containing large amounts of the protein keratin. This makes it very hard for pathogens to pass through.
Sebaceous glands are associated with hair follicles. They secrete a chemical called sebum, which maintains skin moisture and slightly lowers skin pH. Low pH (acidity) inhibits the growth of bacteria and fungi.
Mucous membranes are a thinner and softer type of skin. They are found in areas such as the valine, the foreskin and head of the penis, and the airways leading to the lungs. The skin in these areas secretes mucus, a sticky solution of glycoproteins. Mucus is a type of physical barrier: pathogens and harmful particles become trapped in it and are then swallowed or expelled. It also has antiseptic properties because of the presence of the anti-bacterial enzyme lysozyme.
Blood clotting to seal cuts
When the skin is cut, blood vessels in it are severed and start to bleed. Bleeding usually stops after a short time because of a process called clotting. Liquid blood emerging from a cut changes to a semi solid gel. This seals the wound and prevents further loss of blood, which would lead to a decrease in blood pressure. Clotting is also important because cuts breach the barrier to infection provided by the skin. Clots prevents entry of pathogens until new tissue has grown to heal the cut.
Blood clotting involves a cascade of reactions, each of which produces a catalyst for the next reaction. As a result, blood clots very rapidly. Clotting must be strictly controlled because blood clots inside blood vessels can cause blockages.
The clotting process is initiated by blood-cell fragments called platelets. When a cut of other injury causes damage to blood vessels, platelets aggregate at the site forming a temporary plug. They then release clotting factors that trigger the clotting process.
The cascade of reactions that occur after the release of clotting factors from platelets quickly results in the production of an enzyme called thrombin. Thrombin converts the protein fibrinogen, which is dissolved in blood plasma, into insoluble fibrin. The fibrin forms a mesh in cuts, trapping more platelets and also blood cells. The resulting clot is initially a gel, but if it is exposed to the air it dries to form a hard scab.
Differences between the immune system and the adaptive immune system
Differences between the innate immune system and the adaptive immune system
The immune system protects the body against infectious disease diseases and there are two parts:
- The innate immune system responds to broad categories of pathogens it does not change during an organism’s life. Phagocytes are part of the innate system.
- The adaptive immune system response in a specific way to particular pathogens. It builds up a memory of pathogens it has encountered, so it offers more effective protection against common infectious diseases. Antibody producing lymphocytes are part of the adaptive system.
phagocytes
If microorganisms get past the physical barriers of the skin and mucus membranes and enter the body, white blood cells provide the next line of defence. There are many different types of white blood cells. Some are phagocytes that squeeze out through the pores of the walls of capillaries and move to sites of infection. There they and engulf pathogens by endocytosis and digest them using enzymes from lysosomes. An infected wound will attract large numbers of phagocytes, resulting in the formation of a white liquid called pus.
Lymphocytes
About 25% of the white blood cells circulating in the blood are lymphocytes. These cells have a rounded nucleus and a small amount of cytoplasm. They are called lymphocytes because they also occur in the lymphatic system. This system consists of vessels that drain excess liquid from body tissues. At intervals along lymph vessels there are swollen structures called lymph nodes. There are large numbers of lymphocytes in the lymph nodes.
Lymphocytes produce antibodies which are large proteins that help to destroy pathogens. Antibodies have two functional parts: a hypervariable region that recognises and binds to a specific molecule on a pathogen and another region that helps the body fight the pathogen. This can be done by:
- Making a pathogen more recognisable to phagocytes so it is more readily engulfed
- Preventing viruses from docking to wholesale so they cannot enter the cells.
Our bodies can become infected by many different pathogens, including new strains that have only recently evolved. The immune system as a whole can produce a vast array of different antibodies but each individual lymphocyte can only produce one type of antibody. We have only a small number of lymphocytes for producing each type of antibody. As a result, when a new pathogen infects the body there are at first two few lymphocytes to produce enough antibodies to control the infection. When a pathogen affects the body for the first time the lymphocytes that can produce the appropriate antibodies work together to produce one or more large clones of cells. These cells can produce antibodies in large enough quantities to control the pathogen and clear the infection.
Antigens
Lymphocytes have to distinguish between body cells which are self cells and non-self cells such as invading pathogens. Lymphocytes recognise pathogens by differences between their molecules and those of body cells. The molecules used for recognition are called antigens. They are mostly glycoproteins or other proteins and also some large polysaccharides. They are usually located on the surface of a pathogen.
The immune response to an antigen is a production of specific antibodies any molecule that stimulates an immune response is referred to as an antigen. Search molecules are found on the surface of cancer cells, parasites and bacteria, on pollen grains and on the coats or on the surface of cells from another human. For example, antibodies are produced in response to molecules in transplanted organs that are of a different tissue type. If the wrong blood type is transferred into a patient then molecules on the surface of red blood cells act as antigens and trigger antibody production.
In response to an antigen, lymphocytes produce antibodies that bind to the antigen. This is similar to the binding of a ligand to a receptor, or the binding of a substrate to the active site of an enzyme: it is dependent on matching shapes and chemical properties. Protrusions on antigens match hollows on the corresponding antibody and positive charges match negative charges. However, unlike the binding of ligands to receptors, antibody to antigen binding is irreversible. Also it does not cause the antigen to change chemically, which is a property of enzymes.
The part of an antibody that binds to the antigen is the hyper variable region. As the name suggests, there is an immense variation in the hyper variable regions of antibodies. However, one lymphocyte only produces antibodies in one type of hyper variable region and this only binds to specific antigens. As with receptors and enzymes, there is specificity in binding.
Activation of B lymphocytes by helper T lymphocytes
Immune system produces a large amount of specific antibodies needed to fight an infection, without producing any of the hundreds of thousands of other types of antibodies that could be produced. This requires a series of interactions between different types of white blood cells.
Pathogens are ingested by macrophages a type of acidic white blood cell. Antigens from the pathogens are then displayed in the platinum membrane of the macrophage. Cells called helper T lymphocytes each have an antibody like receptor protein in their plasma membranes. This combined to antigens displayed by macrophages. There are many types of helper tea cells but only a few have receptor proteins that fit the antigen. These helper T cells bind with and are activated by the macrophage.
The activated helper T cells then bind to lymphocytes called B lymphocytes. Again only with B cells with a specific receptor protein to which the antigen binds are selected and undergo the binding process. The helper T cell activate the selected B cells, both by means of the binding and by release of a signalling protein.
Multiplication of activated B lymphocytes to form clones of antibody plasma cells.
When a pathogen first invades the body, development of immunity depends on the presence of B lymphocytes capable of producing an effective antibody. These be lymphocytes become activated but they do not immediately start to produce the antibody. This is because there are two few of them to make significant quantities of antibodies and in any case they do not yet have the organelles needed. Instead the activated B lymphocytes divide repeatedly by mitosis to form a clone of cells that all produce the same type of antibody. These be lymphocytes grow in size and develop an extensive endoplasmic reticulum with many ribosomes attached to it, along with a large Golgi apparatus. This allows rapid production of antibodies by protein synthesis. The cells that have grown and differentiated for antibody production are called plasma B cells.
Immunity
Immunity is the ability to eliminate an infectious disease from the body. Antibodies can give us immunity to a disease but they only persist in the body for a few weeks or months after being secreted by plasma B cells. The plasma cells that secrete antibodies are also gradually lost after an infection has been overcome, because the antigen associated with that infection are no longer present. However, immunity can last much longer in many cases for the rest of our lives.
Most B cells in a clone produced by mitosis become active plasma cells. These cells do not survive for long after fulfilling their role of rapid antibody production. A smaller number of cells in the clone do not immediately secrete antibodies but remain free a long time after the infection. These memory B cells remain inactive unless the same pathogen affects the body again, in which case they are activated and respond very quickly. Immunity to an infectious disease is due to having either antibodies against the pathogen or memory cells that rapidly allow production of the antibody.
Transmission of HIV
Human immuno deficiency virus is the cause of acquired immuno deficiency syndrome (AIDS)
The virus cannot usually survive for long outside the body and infection with HIV only occurs if blood or other body fluids pass from an infected to an uninfected person . In a person infected with HIV, there may be viruses in blood, semen, vagina fluids, rectal secretions and breastmilk. These viruses may pass to an uninfected person through these actions:
- Sex without a condom, during which abrasions to the mucous membranes can cause minor bleeding
- Sharing of hypodermic needles by intravenous drug users
- Transfusion of infected blood or blood products such as factor VIII
- Childbirth or breastfeeding
Infection of lymphocytes by HIV
Production of antibodies by the immune system requires inter interactions between different types of lymphocyte, including helper T cells. The human immuno deficiency virus invades and destroys helper cells. This leads to a progressive loss of the capacity to produce antibodies.
In the early stages of infection, the immune system makes antibodies against HIV. If these can be detected, a person is said to be HIV positive. The rate at which helper T cells are destroyed by HIV varies considerably. In most HIV positive patients who do not receive treatment, antibody production eventually becomes so effective that a group of opportunistic infections can strike. These are caused by pathogens which would be fought off easily by healthy immune system. Several of the infections are so rare that they can be used as marker disease for the later stages of HIV infections.
A collection of several diseases or conditions existing together is called a syndrome. When conditions caused by HIV are combined in a person, they have acquired immuno finn syndrome which is AIDS.
HIV is a retro virus so it has genes made of RNA. It uses transverse transcriptase to produce DNA copies of its gene after entering a wholesale. Anti-retroviral drugs inhibit reverse transcriptase. Other drugs to combat HIV target the enzymes that the virus uses to insert its DNA into host cells as chromosomes or to prepare coat proteins for assembly of only a few virus particles. People who become HIV positive cannot cannot be treated with a group of anti-retroviral drugs, which greatly slows down or prevents damage to the immune system
Antibiotics
An antibiotic is a chemical and inhibits the growth of microorganisms. Most antibiotics or antibacterial. They block processes that occur in prokaryotes but not in eukaryotes. They can therefore be used to kill bacteria inside the body without causing harm to human cells. Antibiotics target the processes of bacterial DNA replication, transcription, translation, ribose function and cell wall formation.
Many antibacterial antibiotics were discovered in saprotrophic fungi. These fungi compete with saprotrophic bacteria for the dead organic matter on which they both feed. By secreting antibacterial antibiotics, the fungi could inhibit the growth of their bacterial competitors. An example is penicillin. It is produced by some strains of the penicillium fungus at times when nutrients are scarce and competition with bacteria would be harmful.
Viruses are nonliving and can only reproduce when they are inside living cells. They use the chemical processes of a living wholesale, instead of having a metabolism of their own. They do not have their own means of transcription or protein synthesis and they rely on the whole cells enzymes for ATP synthesis and other metabolic pathways. These processes cannot be targeted by drugs as the host cell would be damaged.
All the commonly used antibiotics such as penicillin, streptomycin, chloramphenicol and tetracycline control bacterial infection infections but are not effective against viruses. It is inappropriate for doctors to prescribe antibiotics to treat viral infections and it contributes to the overuse of antibiotics and increases in antibiotic resistance and bacteria.
Antibiotic resistance
Strains of bacteria with resistance are usually discovered soon after the introduction of an antibiotic. This is not a huge concern unless the strain develops multiple resistance, but such strains are now widespread. Methicillin resistant staphylococcus can infect the blood or surgical wound of hospital patients and resist all commonly used antibiotics. Another example is a multi drug resistant tuberculosis. Which in 2020 the WHO reported caused about half 1 million cases annually with over 200,000 deaths
Evolution of multiple antibiotic resistance is made easier because genes can be passed from one species of bacteria to another. There is also evidence that antibiotic resistant genes are not lost from the genomes of pathogenic bacteria rapidly when an antibiotic is no longer used.
Multiple antibiotic resistance is an avoidable problem. These measures are required:
- Doctors must prescribe antibiotics only for serious bacterial infections and for the minimum period.
- Hospital staff must maintain high standards of hygiene to prevent cross infection.
- Farmers must avoid the use of antibiotics in animal feeds as growth stimulants
- Pharmaceutical companies must develop new classes of antibiotic no new types have been introduced since the 1980s
Zoonoses
pathogens are often highly specialised, with narrow range of hosts. For example, humans are the only known organism susceptible to the pathogens that cause syphilis, polio and measles. However, we are resistant to the virus that causes canine distemper, so that disease cannot be spread to us from dogs. Rats infected with the diphetheria toxin do not become ill , because their cells do not have the receptor that would bring the toxin into the cell. The bacterium Michael bacterium tuberculosis that causes most case of tuberculosis in humans does not cause disease and frogs because frogs rarely reached the 37° C Temperature necessary to support the proliferation of this bacterium.
However, some pathogens can use more than one species as a host. Michael bacterium bovis uses tuberculosis in cattle but can also infect a wide variety of other animals species such as badgers, which live in the same area as cattle herds in Europe. Milk infected by said cattle may contain Liv cells of Michael bacterium bovis which can cause tuberculosis in humans if the milk is drunk. This is an example of Zoonosis a disease that can be transmitted to humans from other animals and natural circumstances.
Some examples are as follows :
- With tuberculosis the microbacterium bovis infects cattle as the animal host from which the infection is transmitted to humans through drinking unpasteurised infected milk or inhaling droplets from coughing infected cows.
-With rabies the pathogen is lyssaviruses infecting many mammals but 99% of cases are from dogs, that animal host then transmit the infection to humans through a bite or a scratch by an infected animal or eye mouth or nose contact with saliva from an infected animal
- Lastly Japanese encephalitis is transmitted by the pathogen Japanese encephalitis virus to pigs or birds via mosquitoes and can be infected to humans by mosquito bites
Zoonoses are a current global health concern. A major factor contributing to the appearance of zoonotic diseases is in humans living in close contact with livestock. For example, pigs carry the Japanese encephalitis virus which can spread to humans and cause fatal disease. Another factor is displacement of wild animals when their habitats are disturbed by the spread of human population.
COVID-19 is thought to have originated in bats most likely via another animal species. Because of this, it is classified as a zoonotic disease even though since the initial transfer from animal to human, the spread of the disease has been human to human.
Vaccines and immunisation
Immunisation involves using a vaccine to trigger immunity. Most vaccines are given by intramuscular injection, they can also be given by subcutaneous injection or by mouth. Vaccines may contain one of the following active ingredients:
- A live but weekend version of the pathogen, examples of this are in measles, mumps and rubella vaccines
- A killed form of the pathogen, for example in rabies and influenza vaccines
- Subunits of the pathogen usually proteins that act as antigens, this is in hepatitis B and human papilloma virus vaccines
- MRNA coding for for a protein that acts as an antigen, which human cells can translate to make the protein, this is seen in some COVID-19 vaccines including that produced by Pfizer.
All vaccines contain either antigens that allow a pathogen to be recognised by the immune system or nucleic acids from which antigens can be made. The antigen stimulate a primary immune response, by activation of T lymphocytes and B lymphocytes and production of plasma cells and then specific antibodies. If memory cells are also produced, long lasting immunity develops. If a vaccine successfully triggered such immunity, the pathogenic microorganism will be destroyed by secondary immune response and if it ever enters a body.
Heard immunity and prevention of epidemics
Heard immunity is achieved when a significant portion of a population have already contracted a disease or have been vaccinated. As a result, the spread of a virus or other pathogen is impeded because it repeatedly encounters people who are already immune. When there is heard immunity, any new outbreak of the disease will decline and disappear.
Not everyone in the population has to be immune for her immunity to develop. For this reason etymologists prefer the term her protection. An important benefit of her immunity is that vulnerable individuals who have compromised immune systems and cannot be vaccinated or unlikely to contract the disease.
The following formula can be used to estimate the percentage of people who have to be immune for the population as a whole to be protected :
Where R is the average number of people that are infected person in fact —> (1 - 1/R) * 100%
