Pathogens, Disease and the Immune System

• Oluwakemi Yusuf

Page 1 Pathogens And Diseases

Pathogens are biological agents that cause or can cause infection when they get inside a living system. They are mainly micro-organisms with the ability to replicate at a very high rate when they gain access to the substrate in a living thing: plants and animals. Access to cells, tissues and organs of a living thing may alter the homeostasis that may lead to illness or fever. Such homoeostatic imbalance may be caused by one or more of their high proliferation rate, the chemical substances they produce- toxins, the cosmetic marks or discolouration of their invasion site or location of the mass they produce in the process of replications.

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Guton A.C Medical Physiology 11^th Edition: Elsevier Saunders. Pp 439- 513

CHOLERA

Description: Cholera is an infection by bacterial pathogens called Vibrio cholerae.

Infection: Consumption of Vibrio chilerae infected water.

Life cycle of Vibrio cholerae (credit: Textbook of Parasitology:Prof. A. J.Smith)

Risk factors/Transmission method:

• Poor sanitary conditions in areas where person per land mass ratio is very high, such as in slums. This is a high rate of fast spread and infection spread due to contact from the aquatic reservoir. When many people share the same source of water, such as a river, there is a high chance of a quick spread of Cholera by oral ingestion of water from such source. Since the transmission of Vibrio cholera is by contamination of a source from the colonisation and multiplication of Small Intestine in the GIT, defecating in the river that serves the community spreads the bacterium to other victims.
• In areas where there is contamination of pipe-borne water due to leakage in the distribution network, there are good chances of contamination of the pipe-borne water by water from the sewage system. This is a huge risk to people living in the periphery of slums and urban centres where the pipe water had traversed the slum. The contaminated water would be consumed and infect people living far away from the epi-centre of the cholera epidemic.

Athlete’s foot (Tricophyton) or Ringworm

Description: A fungal infection of the feet and the spaces between toes. The affected area of skin may be scaly, inflamed and dry.

Infection: Contact with an infected skin or fluids, such as shared water in the swimming pool and infected clothes and socks.

Risk factors/Transmission method:

• Having contact with infected person: There is a possibility that siblings that share play and work materials or students in the same Physical Education class are at risk of sharing the infection. Contacts allow the spores from the infected site to be transferred from contact.
• Sharing toiletries and intimate materials, such as towels and socks physically transfer the spore of Athlete’s foot to other locations. The spores are then left to thrive on another substrate and the infection keeps spreading.
• Swimming pool, body spa instruments, pedicure scissors, etc. are other common instruments that transfer the spores of the fungus. The spores move on the medium of water or instruments from infected sites to infect a new contact. The growth rate and phase depends on the presence of nutrients in the new environment. In one study, adherence and germination was observed within 6 hours of incubation and small colonies forming by 24 hours.

Malaria (Plasmodium)

Description: A mosquito-borne infectious disease caused by parasitic protozoans. The Plasmodium is the causative agen but Mosquito is the vector.

Infection: Bite of an infected mosquito. The Plasmodium is released from the glands of mosquitoes into the blood stream of humans.

Risk factor/Transmission:

• Presence of the habitat for the vector of malaria (Mosquito) to thrive and complete metamorphosis. Such environment as stagnant water provides the ground for adult mosquitoes to lay their eggs and grow to imago. The adult mosquito is the carrier of the protozoan that causes malaria. Disrupting the metamorphosis of mosquito is a good intervention in the prevention of malaria. Pouring oil in the stagnant water around residence is a good way to deny oxygen to the larvae of mosquitoes.
• Allowing human contact with the adult mosquitoes by the use of mosquito net is very desirable.
• Taking medication that can kill gametocytes of the protozoan in the human system is a good treatment option.
• preventing malaria by taking medication that can kill sporozoites immediately they enter the body system. This nips the possibility of malaria attack since the incubation period of plasmodium in the human system is about 10 days.

Life cycle of Plasmodium specie (credit: Textbook of Parasitology:Prof. A. J.Smith)

Influenza

Description: A viral infection of the epithelial cells in the upper and lower respiratory tract of human.

Infection: Contact with body fluid of infected person, such as sweat, mucus , sputum and sneeze fluids. It also spread through the air from infected persons over a reasonably short distance.

Life cycle of influenza virus: (Adapted with kind permission of Linda Stannard. )

There are three stages in the life cycle of Influenza virus. They are as follows:

• Entry into the host cell. (stages 1 and 2 in the diagram)
• Entry of vRNP into the nucleus of host cell. (stages 3and 4 in the diagram)
• Replication and transcription of the genome of virus. (stage 5 in the diagram)
• Exportation of vRPN from the nucleus (stage 6 in the diagram)
• Budding and assembly on the cell membrane of the host cell (stage 7 in the diagram)

Risk factor/Transmission:

• Influenza attack is mainly seasonal. The outbreak of influenza is reasonably predictable to occur in the Northern and Southern hemisphere in the winter months and may occur anytime in the equatorial region. It is easy to prepare for the season of attack by having a yearly vaccination against the flu before the season.

• Since the virus is very contagious, Isolation of infected person is a good way to contain the spread of flu.

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Textbook of Parasitology Smith A. J. 4^th Edition Heinemann UK.

Immunity

The body of human is under a constant attack of foreign invaders of diseases, infections and toxins. Pathogens, the current point of study is one of the sources of infection. The human body is imbued with numerous safeguards, both internally and externally.

The human body has a strong protective barriers at the openings of the body to the external environment. The physical barriers that prevent invaders from entering the body systems provides the first line of defence.

Skin provides a a protective layer to the entire body and prevents external agents from gaining access to the body. Skin, being the largest organ of the body also provides a layer of defence to protect blood vessels, muscles and all internal organs. The skin is also a chemical covering of the the body. In the cases of skin tears or bruises, the blood vessels under the skin reacts to prevent external agents from entering the body. There is a mesh of blood clotting created by platelets and white blood vessels aggregate around the spot to form protection around the spot of bruise or cut.

There are few areas of the body anatomy that is not completely covered by the skin, such as nostrils, mouth, anus, private part in both men and women. Such areas are covered by ciliated squamous epithelium that produces sticky mucus that can trap invaders. After trapping foreign invaders, the epithelium of covering the entrance to the body system is de-squamated and rapidly replaced by another layer of squamous epithelium. For this reason, the epithelium of the lining the nose, anus, pudendum of humans have a high rate of replication. Recent research has proved that the cells of human nose grows forever and retain the unique ability to replicate until death. For this reason, cells from the nose has been used as stem cells to grow nervous tissues to treat paraplegia.

From insects to dusts, the most-outward lining of the nose and other parts of the Upper Respiratory Tracts traps, catch and destroy invaders. The product of this trapping is the phlegm that is produced after exposure to dusts and throat infections. The stomach acid, alkaline saliva and the colony of bacteria in the gastro-intestinal tract protects the body from foreign invaders.

A very important line of barrier is the blood-brain-barrier. These are specialised phagocytic cells that filters every drop of blood flow to the brain and spinal cord because of the sensitivity of the Nervous system. The blood-brain-barrier however does not stop the flow of nutrients, such as glucose to the brain.

All these means of protection from the invasion of the body system are called Non-specific (innate) immunity.

Other innate immunities are found in the activities of lysosomes that is found in tears that breaks down the bacteria in the eyes, commensals and harmless bacteria that cover every inch of the GIT and does not allow space for new invasion of bacteria and fever that deploys the mechanism of allowing the rise of the body temperature to be able to kill invading agents.

Specific Immunity

The immune system in infancy is reasonably complete at the time of birth. The ImmunoglobinG received by infants from mother at birth is a useful defence mechanism during the first days and weeks of birth. In the period of time after birth, the infant begins to develop his own long inventory of immunity. Breast milk provides protection against many common illnesses for passive protection from ImmunoglobinA. By this, breast milk gives protection from microbes. Breast milk is also believed to stimulate the infant in the development of his own immune system. The most significant influence of breast milk is the stimulation of B and T lymphocytic cells (T cells matures in the Thymus-in the neck region and B cells mature in the bone marrow) to produce immunity by exposure to microbes which colonise the gut during birth. The lymphocytes run within the lymphatic system which is a network of tubes that permeates tissues in the body. There are lymph nodes periodically in the conduit of pipes around the body.

The lymphatic system carry infection-fighting lymphocytes. When lymph nodes are inflamed, producing some discomfort, it is a good sign that the body is fighting some infection. This is because the lymphocytes proliferates and produce swellings very close to the site of infection.

An organ located very close to the Pancreas called spleen plays a major role in the fight iof infection and it is a part of the lymphatic system. The spleen contains lymphocytes that sieves the vascular system. Thus, the spleen has importance in fighting infections that have invaded the blood. There are also accessory lymphoid tissues that provides a strong barrier along the point of entry of such as the lungs and the gut.

The responses of the immune system are coordinated by a number of cells and soluble molecules they secrete to indicate danger. The central cells to all the immune system are the leukocytes (the white blood cells.) They also provide the signals that activates chemical messengers to be released from the T cells and macrophages.

The B-cells are involved in the production of antibodies. B cells are the progenitors to plasma cells. There is a significant rate of replication of B cells during infection and they then metamorphose to plasma cells. The plasma cells produce antibodies against a particular antigens in a microbe.

T-cells are a group of lymphocytes. They are so named because theyy can be traced to the Thymus gland in the neck region. They are involved in the coordination of the immune system by secreting lymhokines.

Phagocyte are unicellular phagocytes that engulfs foreign organisms or particles. Phagocytes provide the contact between the specific and non-specific immunity.

Antibodies are protein compounds deployed by the immune system to designate and neutralise invaders like bacteria and viruses. Each antibody is specific to an antigen.

There are five classes of antibody – IgG, IgA, IgD, IgE and IgM.

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Guton A.C Medical Physiology 11^th Edition: Elsevier Saunders. Pp 439- 449

Difference between natural and acquired immunity

Difference between active and passive immunity.

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Guton A.C Medical Physiology 11^th Edition: Elsevier Saunders. Pp 439- 449