Click4Biology

6.3.1 Define Pathogen

A pathogen is an organism that can cause disease.
Pathogens include bacteria, viruses, protista, fungi and other parasitic multicellular organisms.

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6.3.2 Antibiotic action

Antibiotics block specific metabolic pathways found in bacteria, but not in eukaryotic cells.

These graphs show how the two kinds of drug (see below) affect bacterial growth curves.

Some antibiotics are cidal (bactericidal, fungicidal, etc.), which means they kill the microbes.
Others are antibiotics are static (bacteriostatic, fungistatic, etc.), which means they stop further growth, but don't kill existing cells.
Both are useful medically, because if the growth of an infective pathogen is stopped, the body's immune system will be able to kill it.
Bacteria, as prokaryotes, have many unique features not present in eukaryotes, so antibiotics can be selectively toxic by targeting such features as the bacterial cell wall, 70S ribosomes, and enzymes that are specific to bacteria. In this way the human eukaryotic cells are unaffected.

Viruses reproduce using the host cell metabolic pathways that are not affected by antibiotics.

Viruses do not have metabolic pathways like bacteria and therefore antibiotics do not work on viruses.

Viruses can only be treated by their specific anti-microbial agent and antibiotics should never be prescribed for viral infections (such as flu).

6.3.3 Barriers to infection

As a first line of defence the body has many mechanism to try to stop microbes entering the body, particularly the blood-stream. These are :

The skin is a tough, impenetrable barrier (which is why we use it to make leather shoes). The outer layer, the epidermis, is 20-30 cells thick (about as thick as a sheet of paper) and its cells are toughened by the protein keratin. The next layer, the dermis, is 20-40 times thicker and provides the main structure for the skin as well as all the receptor cells, blood vessels and hairs. Cells are constantly being lost from the surface of the skin (to form dust) and are replaced by new cells from further down.

The respiratory tract is another potential entry route, but it is protected by sticky mucus secreted by glands in the bronchi and bronchioles, which traps microbes and other particles in inhaled air before they can reach the delicate alveoli. Mucus contains lysozymes, and cilia constantly sweep the mucus upwards to the throat, where it is swallowed so that the microbes are killed by the stomach acid.

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6.3.4 Cellular defence

The second line of defence is the non-specific immune system, a host of quick, non-specific methods of killing microbes that have passed the first line of defence and entered the body. Phagocytic leucocytes are an example of this type of defence from infection.

Phagocytes are large, irregularly-shaped leukocyte cells that remove bacteria, vi-ruses, cellular debris and dust particles.

The phagocytes are constantly changing shape, and they flow over microbes, surrounding and ingesting them through the process of phagocytosis to form a phagosome.

The phagosome then fuses with lysosomes - small vesicle containing lysozymes, which are released into the phagosome, killing and digesting the microbe.

Different phagocyte cells work in different locations: neutrophils circulate in the blood, while macrophages are found in lymph, tissue fluid, lungs and other spaces, where they kill microbes before they enter the blood.

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6.3.5 Antigen and antibodies

Antigen is often used to describe something that has infected the body. However it is more accurate to describe them as follows:

An antigen is a large molecule (protein, glycoprotein, lipoprotein or polysaccharide) on the outer surface of a cell.

All living cells have these antigens as part of their cell membrane or cell wall.

The capsid proteins of viruses and even individual protein molecules can also be classed as antigens.

Their purpose is for cell communication, and cells from different individuals have different antigens, while all the cells of the same individual have the same antigens.

Antigens are genetically controlled, so close relative have more similar antigens than unrelated individuals.

Blood groups are an example of antigens on red blood cells, but all cells have them.

The link with infection is that when a pathogen or toxin enters the body it this that the immune system reacts against.

Antibodies are proteins secreted from lymphocytes that destroy pathogen and antigen infections

B-cells make antibodies.

An antibody (also called an immunoglobulin) is a protein molecule that can bind specifically to an antigen.

Antibodies all have a similar structure composed of 4 polypeptide chains (2 heavy chains and 2 light chains) joined together by strong disulphide bonds to form a Y-shaped structure.

The stem of the Y is called the constant region because in all immunoglobulin's it has the same amino acid sequence, and therefore same structure.

The ends of the arms of the Y are called the variable regions of the molecule because different immunoglobulin molecules have different amino acid structure and therefore different structures.

These variable regions are where the antigens bind to form a highly specific antigen-antibody complex, much like an enzyme-substrate complex.

Each B-cell has around 10 ⁵ membrane-bound antibody molecules on its surface and can also secrete soluble antibodies into its surroundings.

Every human has around 10⁸ different types of B cell, each making antibodies with slightly different variable regions.

Between them, these antibodies can therefore bind specifically to 10⁸ different antigens, so there will be an antibody to match almost every conceivable antigen that might enter the body.

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6.3.6 Antibody production

(a) There are many different lymphocytes.

(b) The antigen infects and is presented to the lymphocytes

(d) The Lymphocyte clones to produce many plasma cells. This occurs in the lymph nodes.

(e) The clone of plasma cells

(f) The gene for the antibody is expressed and secreted into the plasma and tissue fluid.

(g) The antibody circulated in body fluids destroying the infectious antigen

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6.3.7 HIV and the immune system

HIV is a virus that selectively infects Lymphocytes

(a) Different lymphocytes

(b) HIV virus

(d) The infected lymphocyte is 'disabled' by the virus

(e) When an antigen infection is presented the lymphocyte cannot produce antibodies.

(f) The antigen is not challenged by the immune system and is able to freely proliferate

The consequence is that the infected individual will have no immune and develop that disease.

Therefore an individual who is HIV +ve (infected ) will eventually develop a disease which will go unchecked. The consequence is that that disease will severely damage the infected person and will eventually bring about their death.

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6.3.8 Transmission and social implications of AIDS

Source: Aids takes an economic and social toll by Belinda Beresford Africa Recovery June 2001

AIDS: Acquired Immuno deficiency syndrome.

Acquired relates the infectious nature of AIDS through the transmission of the HIV virus.
Immuno deficient relates to the way diseases cannot be resisted.
Syndrome relates to the variation in the way the disease manifest itself. People who develop AIDS can be a affected by quite different set of diseases.

Cause: is the HIV retro-virus that selectively infects cells of the immune system effectively disabling primary and secondary response to infection.

Transmission: Through contact with the body fluids of an infected person. In particular the fluids are blood and semen, vaginal mucus. There is a very low risk ( almost zero) associated with salivary mucus.

Defence from infectious disease.

An infectious disease is one that can be transmitted from one person to another. Transmission requires that the disease causing organism (pathogen) is transported by a vector from one person to another.

Disease prevention can focus on the prevention of transmission, inoculation against the disease or treatment of the infection.