THE HUMAN BODY Part C: Immunity and Disease

Disease Disease is best described as a condition that impairs the normal function of the organism. Infectious – these are diseases caused by disease causing agents called pathogens. Non-infectious – these are diseases that may be caused by poor diet, accident, environmental stress or inherited from one’s parents.

Pathogens Pathogens are both cellular based organisms and non- cellular agents capable of causing disease. Cellular pathogens include: bacteria, protozoans, oomycetes, fungi, worms, arthropods. Non-cellular pathogens include: viruses, viriods and prions.

Bacteria Bacteria are prokaryotic cells that do not contain membrane- bound organelles. Bacteria are classified on their shape, organisation, capsule, mobility, oxygen use, nutrition requirements and response to the Gram-stain. This information allows pathologists to recommend the most appropriate treatment.

Bacteria It is usually the toxins produced by bacteria which cause the disease. This occurs when the bacteria is in large numbers in one’s body. Examples include; Tetanus, Typhoid, and Tuberculosis. Bacteria are usually controlled using antibiotics.

Protozoans Protozoans are single celled organisms. Three classes are pathogenic: Flagellates (those which use flagella for mobility) such as Trypanosoma which causes sleeping sickness. Sporozoans such as Plasmodiums which cause malaria. Sarcodinians such as Entamoeba histolytica which causes amoebic dysentery.

Protozoans Protozoans are usually transmitted via a host, such as mosquitos and fleas, or ingested by drinking infected water. The best way to treat protozoans is usually to break their life cycle by removing a host or protecting one’s self from the host. For example kill all the mosquitos, or use insect repellants, clothing which covers the skin and mosquito nets.

Oomycetes Oomycetes where once considered fungi, but have been reclassified as Protists due to the presence of flagella and cellulose walls. They are usually plant pathogens causing untold damage in agricultural regions (blights and downy mildew) and natural systems (Phytophthora cinammoni – cinnamon fungus, which destroy native forests)

Fungi A number of Fungi species are pathogens. Many are plant pathogens causing diseases such as leaf spot, rust, ergot and Dutch elm disease. Few are animal pathogens, but there are some such as tinea, ringworm and thrush. Fungi have also been of use in the production of food, drinks and medicines.

Worms There are a number of worms that infect animals and plants causing disease: Flatworms – the tapeworm is the best known of these. It is a parasite which attaches to the gut of the host and it grows using the food consumed by the host. Roundworms – include hookworms, thread worms and pinworms – act as parasites causing a wide range of diseases.

Arthropods Insects, such as; ticks, mites, lice are examples of arthropods that infect humans causing skin irritations and fevers. Arthropods also attack plants damaging leaves, stems and roots.

Viruses While viruses contain DNA (double or single stranded) or RNA they are unable to reproduce by themselves and lack other features to be considered living. They act as parasites injecting their DNA or RNA into the host cell, causing the host cell to reproduce many copies of the virus before it dies. Viruses attack both animals and plants.

Viroids Viroids are single-stranded RNA molecules that lack a protein coat or membrane. Hence they are often referred to as ‘naked’ They infect the host in the same way as viruses. So far, they have only be identified in plants.

Prions A prion is a naked piece of protein which infects a host cell in the same way as a virus and reproduces in the cell. The infected proteins tend to accumulate in the nervous system of the host causing a slow degeneration. The most famous example is Creutzfedlt-Jakob disease (or mad-cow disease).

Controlling Pathogens When controlling pathogens it is important to assess the effectiveness of the drug to kill or control the pathogen, while causing little damage to the host. Antiseptics – applied to the skin to kill pathogens. Disinfectants – applied to objects to kill pathogens. Antibiotics – naturally occuring substances which control bacteria.

Controlling Pathogens Antimicobials – chemicals that are used to control microbes without damaging the hosts cells. Fungicides – chemicals used to kill fungal pathogens. Antivirials – a number of drugs have been produced in recent years (eg. AZT and Interferon) to control viruses. It is very difficult to control viruses with drugs as they are not as susceptible as living organisms. Antiprotozoan and Antihelminthics – there are drugs available to control many of these pathogens.

Non-specific Defence As we discussed in the previous chapter, we are surrounded by and constantly under attack from pathogens – yet we survive. This is due to our immune system. The first level of defence is called non-specific as it occurs no matter what pathogen is attacking us.

Specific immunity Specific immunity has evolved in the immune systems of higher vertebrates. The immune system essentially learns how to recognise and deal with a pathogen after contact with the pathogen.

Recognising Self Our cells contain chemical markers on the surface of the membrane that indicate we belong to the organism. That is, our cells have ‘self’ markers. These markers are recognised by the cells of the immune system and are left alone. Cells that lack ‘self’ markers are treated as hostile and dealt with.

Plant Defences Plants lack immune systems, but have developed a number of defence systems against disease: Cell Wall Secondary metabolites – such as antibiotics High levels of toxins stored in cells

Mammalian Lymphatic System The immune system uses both the blood circulatory system and the lymphatic system to move cells about the body. White blood cells leave the circulatory system, move through the interstitial fluid and re-enter the lymph system which drains back into the circulatory system The Lymph system contains fine tubes and a series of nodes and organs where the immune response occurs.

Physical and chemical barriers The first layer of defence is the skin. Pathogens must find a way to penetrate the skin to be able to infect the host. There are also a number of chemicals secreted onto the skin surface, especially at the orifices (eye, ear, mouth, nose, urethral opening, vagina and anus) that assist by killing or suppressing pathogens.

Defensive Molecules Should pathogens find their way across the skin, then they are met by a number of defensive molecules: Cytokines – signalling molecules that indicate the presence of damage or of an invader triggering a variety of responses from the immune system. Interferons – a cytokine that are produced by virus infected cells signalling to the immune system the presence of the virus. Complement – an array of blood proteins which can kill cells recognised as non-self.

Killer Cells The immune system contains a number of cells that, put simply, kill other cells: Phagocytes – these are white blood cells that engulf and digest foreign cells. They are constantly moving about the body. Natural Killer Cells – these cells detect infected or diseased body cells and destroy them.

Protective processes When the external barrier in mammals is breached then other responses offer protection: Inflammation – increased blood flow to damaged area, increased permeability of blood vessels and increased movement of white blood cells to the damaged area. Preventing Blood Loss – small arteries constrict, blood platelets become sticky and fragile, blood coagulates, and new tissue grows over the wound. Fever – this increased body temperature causes pathogen growth to slow and improves the inflammatory response.

Specific adaptive immunity Antigens – are molecules, usually found on the surfaces of cells and outer coats of viruses or released as toxins by bacteria, that bind with antibodies. Antibodies – are molecules produced by lymphocytes such as T and B cells. They are able to bind with specific antigens on pathogens.

Specific adaptive immunity T-cells – these are produced in the thymus gland. They have antibody-like molecules on their surface which bind with foreign antigens. Two types: Helper T-cells – produce and release cytokines in response to antigens. Cytotoxic T-cells – when stimulated they target foreign and infected cells and kill them (lyse)

Specific adaptive immunity B-cells – these are formed in the bone marrow and spleen. They are usually small, non-dividing, inactive cells. When they meet specific antigens they become very active and produce plasma cells and memory cells. Plasma Cells – specialise in producing antibodies to fight the infection. Memory Cells – remember the antigens of pathogens and respond to later infections.

Clonal Selection Theory Sir Frank Macfarlane Burnett, a prominent Australian scientist and Nobel laureate, proposed the clonal selection theory to explain how the immune system could deal with the infinite number of antigens it may meet. The variable portion of the antibody contains relatively few genes that are rearranged freely and randomly to produce millions of difference combinations.