Specific and Non-Specific Defences
Specific and non-specific defences
Non specific defences: They are body’s first line of defence against all pathogens
Specific defences: are directed at a particular pathogen. Eg. If you become infected (or vaccinated) with chickenpox virus, the body will make specific antibodies to combat the virus. These antibodies are only effective against chickenpox virus
Non-specific defences
External defences
Skin
Skin: effective barrier covering outside of the body. Provided it is not cut or broken, it is very effective at stopping entry of micro-organisms.
Huge number of bacteria live on skin all the time. Normal bacteria make it difficult for potential pathogens to become established
Sebum: Oily secretion produced by oil glands of skin & contains substance that kill some pathogenic bacteria
Sweat: secreted onto skin contains salts & fatty acids preventing growth of many micro-organisms
Mucous membranes
Mucous membranes: line body cavities that open to exterior. They secrete mucus, which inhibits entry of micro-organisms
Whole of digestive, urinary & reproductive tracts protected in this way
Hairs
Hairs: are found in nose cavity & ears
In nose, hairs & layer of mucus enable nose to trap up to 90% of particles inhaled when breathing
Cilia
Cilia: tiny, 'hair like' projections from cells that are capable of a beating motion. Mucous membranes lining nose cavity, trachea & air passages have cilia
Beating of cilia moves mucus, containing trapped particles & micro-organisms, toward throat, where it is coughed up or swallowed
Acids
Acids: Acid kills many of bacteria taken in with food or those contained in mucus swallowed from nose & windpipe
Stomach juices are strongly acidic
Vagina has acid secretions reducing growth of micro-organisms
Sweat on skin also slightly acidic
Lysozyme
Lysozyme: enzyme that kills bacteria
Eyes are protected by flushing agent of tears which contain lysozyme
Saliva, sweat, secretions of nose & tissue fluid all contain this enzyme
Cerumen
Cerumen (ear wax): protects outer ear against infection by some bacteria
Only slightly acidic & contains lysozyme
Movement of body fluids
Flushing agent of body fluids: helps to keep some areas relatively free of pathogens
Urine flowing through the urethra has a cleansing action. Prevents bacterial growth & helps stop bacteria reaching bladder & kidneys. Women have shorter urethra than men so tend to suffer more bladder infections
Tears, sweat & saliva also involved in flushing & cleansing
Protective reflexes
Reflex: automatic, involuntary response to stimulus. Protective reflexes help to protect body from injury, such as blink reflex or vomiting.
Four reflexes help to protect against infection:
Sneezing: stimulus for sneezing is irritation of the walls of nasal cavity. Irritation can be caused by noxious fumes or dust particles, which are all likely to carry micro-organisms. Forceful expulsion of air from lungs carries mucus, foreign particles & irritating gases out through nose & mouth
Coughing: stimulus is irritation in lower respiratory tract (bronchi & bronchioles). Air forced from lungs to try to remove irritant. Air drives mucus & foreign matter up trachea toward throat & mouth
Vomiting: psychological stimuli, excessive stretching of stomach & bacterial toxins can all induce vomiting. Contraction of muscles of abdomen & diaphragm, not contraction of the stomach, that expel stomach contents
Diarrhoea: Irritation of small & large intestines by bacteria, viruses, protozoans can cause diarrhoea. Irritation causes increased contractions of muscles of wall of intestines so irritant removed as quickly as possible. Faeces is very watery due to quick movement through large intestine & water not being absorbed
Internal non-specific defences
Phagocytosis
Phagocytes: Specialised white blood cells (leucocytes) that can engulf & digest micro-organisms & cell debris. Eliminating many pathogens before they have a chance to take hold. A number of different types of cells are phagocytic:
Monocytes & macrophages
Monocytes leave the bloodstream and enter the tissue when it becomes infected or inflamed.
Here, they differentiate into macrophages, which are particularly important in removing microbes & dying cells through phagocytosis.
Some macrophages move through tissue looking for & destroying pathogen, while others fixed in one place & deal with pathogens that come to them
Neutrophils
Described as granulated leucocyte, due to granules in their cytoplasm, and have a lobed nucleus
Most abundant leucocyte (55-70%)
During an infection, they are the first cells to move into the tissue to destroy the pathogen by phagocytosis
Important in killing pathogen inside the **cell
Short lifespan and die after a few days. Dead cells make up the pus that forms after an infection
Dendritic cells
Characterised by projections from the cytoplasm
Their function goes beyond phagocytosis
Ability to detect, engulf & process foreign particles. They then use this information about the ingested particles to assist with specific immunity
Inflammatory response
Inflammation: response to any damage to the tissues. Purpose is to:
Reduce spread of any pathogens, destroy them, prevent entry of additional pathogens
Remove damaged tissue & cell debris
Begin repair of damaged tissue
‘itis’ indicates inflammation of specific organs or tissues. Eg. Laryngitis, meningitis
4 signs of inflammation:
Redness
Swelling
Heat
Pain
Steps to Inflammation
Damage to tissues stimulates integrated series of steps in inflammatory response:
Mast cells: specialised cells that are present in most tissues.
They stimulate and coordinate inflammation by releasing chemicals.
When stimulated by mechanical damage or by local chemical changes, mast cells release histamine, heparin & other substances into tissue fluid
Histamine: increases blood flow through the area due to vasodilation & causes walls of blood capillaries to become more permeable so fluid is filtered from blood.
Increased blood flow causes heat & redness, and escape of fluid from blood causes swelling
Heparin: prevents clotting in immediate area of injury.
A clot of fluid around area does form & this slows spread of pathogen into healthy tissues
Phagocytes are attracted by the chemicals released from mast cells, and complement system proteins.
Phagocytes, particularly neutrophils, actively consume micro-organisms & debris by phagocytosis
Pain is felt due to stimulation of pain receptors by the abnormal conditions in the tissue
Pus forms from dead phagocytes (filled with bacteria, debris & dead cells) & tissue fluid
New cells produced by mitosis & repair of damaged tissue takes place
Fever
Fever: elevation of body temperature due to an infection. Eg. Flu & common cold
This change in body temp. is due to a resetting of body’s thermostat, controlled by hypothalamus, to a higher level than normal
When a person has a fever, the body temperature is still regulated in response to heat or cold, but the set point is at a higher level
This reaction is thought to be due to chemicals called pyrogens that are released by WBCs during the inflammatory response & act on hypothalamus
Interleukin-1: type of pyrogen that is predominantly produced by activated macrophages, but is also produced by other cells such as dendritic and epithelial cells
Steps for a Fever
Onset of fever is frequently gradual, but can occur rapidly
In case of rapid onset:
Hypothalamus responds to pyrogens by increasing the thermoregulatory ‘set point’ above the normal value
Person’s thermoreceptors detect the body temperature, and hypothalamus recognises that as lower than the new, higher set point
Person feels cold and, as a consequence, homeostatic mechanisms such as vasoconstriction, shivering etc occur
These responses conserve heat & increase heat production, driving the body temperature up rapidly
Crisis (the point where the fever breaks), the hypothalamus changes the ‘set point’ back to normal
In this situation person feels hot & appears flushed, as skin vasodilation & profuse sweating occurs
Reasons for a Fever
Fever is beneficial, up to a point
High body temp. is believed to inhibit growth of some bacteria & viruses
In addition, heat speeds up rate of chemical reactions, which may in turn help body cells repair themselves more quickly during a disease
Fever may also inhibit viral replication by allowing chemicals called interferons to operate more quickly
Lymphatic System (non-specific)
Lymphatic system consists of:
Network of lymph capillaries - joined to larger lymph vessels
Lymph nodes - which are located along length of some lymph vessels
Spleen, tonsils and thymus are also lymphatic organs
Main function: to collect some of the fluid that escapes from blood capillaries & return it to circulatory system. In addition, is an important part of body’s internal defence against pathogenic organisms
Lymph entering lymph nodes contains cell debris, foreign particles & micro-organisms that have made it through the body’s external defences
These micro-organisms may be pathogenic, and if not destroyed could cause disease
Lymph nodes occur at intervals along lymphatic vessels; containing masses of lymphoid tissue, cells of which are criss-crossed by network of fibres
Lymph entering the lymph nodes contain cell debris, foreign particles & micro-organisms that have penetrated the body’s external defences
Some of these micro-organisms may be pathogenic and, if not destroyed, could cause disease
Larger particles such as bacteria, are trapped in meshwork of fibres as lymph flows through spaces in the nodes
Macrophages destroy these particles. They ingest particles by phagocytosis
Phagocytosis (during infection)
Phagocytosis: Projections from macrophage surround particle & take it into the cell where it’s destroyed by enzymes
Most bacteria are destroyed within 10 – 30 minutes
Lymph flows in one direction away from tissues, stimulates immune response in nodes
During infection, nodes can swell and become sore. Formation of lymphocytes increases during this time
Negative: Lymph nodes can over-swell & act as home for bacterial infections, as well as spread bacteria or cancerous cells
Good hygiene and mechanical barriers
Good hygiene
Many positive things can be done to reduce risk of infection with pathogens. If we are infected, such as with cold or flu, some practices will reduce possibility of passing infection to others:
Thorough hand washing with soap & water (most effective way to reduce spread of micro-organisms)
Cover your mouth when you cough or sneeze
Wear gloves when cleaning up blood or bodily fluids
Wipe surfaces with disinfectant if they have been contaminated with blood or body fluids
Use tongs, pliers or tweezers (or heavy gloves) when picking up discarded syringes or condoms. Dispose of needles & syringes appropriately
Never share personal articles such as toothbrushes, razors, towels & syringes
Mechanical barriers
Mechanical barriers provide an obstacle to invading pathogens & therefore reduce risk of getting disease
Surgical mask: surgeons wear them, as well as people who feel there is a high risk of inhaling disease-causing pathogens.
Reduces risk of wearer breathing out pathogenic organisms & therefore reduce spread of disease
Surgeons wear masks because most operations involve cutting through skin (first line of defence), pathogens can therefore more easily enter tissues
Surgeons ‘scrub up’ before an operation
Protective clothing: can be important in areas where individual may be bitten by insects, ticks or mites
Gloves: worn by surgeons, nurses, medical staff
Safety glasses: if there is a chance that eyes may be affected
Mechanical barriers (such as condoms): Used during sexual activity
Australian Regulations
Australia has many regulations in place to help keep transmissible diseases to a minimum:
Immunisation
Quarantine - people from other countries must show evidence of vaccination against certain diseases and can be subject to medical examination on arrival.
Notification of diseases – Doctors must notify the state Health Department when they diagnose certain diseases. Eg. Measles, typhoid.
Health regulations - disposal of garbage and sewage, handling of food, purity of water supply, cleanliness of abattoirs and restaurants
Education schools and public campaigns
Specific defences
Cells of the immune system
The immune system is composed of cells and proteins that protect the body against pathogens, alien chemicals and cancerous or abnormal cells.
Non-Specific = Phagocytes
Specific = B-Lymphocytes and T-Lymphocytes
When B cells and T cells react, it is called an immune response
Immune response
Homeostatic mechanism
When micro-organisms or foreign substances enter the body, the immune response helps deal with the invasion and restore the internal environment to its normal condition
T-cells and B-cells (type of WBCs called lymphocytes) are the key cells involved in the immune response. These cells are produced in bone marrow & end up in lymphoid tissue
T-cells are the cells that mature in the thymus before going into lymphoid tissue
B-cells are the cells that mature in the bone marrow before going into lymphoid tissue
Most of the lymphoid tissue is in the lymph nodes; however, it also occurs in other parts of the body, such as the spleen, thymus gland and tonsils
Recognising foreign compounds
Antigens
Both (humoral and cell mediated) immune responses are triggered by antigens.
An antigen is any substance capable of triggering a specific immune response. They are large molecules such as proteins, carbohydrates, lipids of nucleic acids.
Virus particles
Whole micro-organisms (bacterial cells)
Part of a bacterium (flagella, cell wall or capsule)
Toxins
Molecules on cells such as blood cells (think back to year 11!)
Pollen grains
Egg whites
Self antigens = Large molecules that are produced by the person's own body do not cause an immune response.
Non-Self antigens = Foreign compounds that trigger an immune response when they enter the body.
Antigen-presenting cells
Antibodies
Antibodies are a Y-shaped specialised protein that is produced by plasma cells in response to a non-self/foreign antigen
They belong to a group of proteins known as immunoglobulins (Ig)
The antibody combines with its specific antigen and forms an antigen-antibody complex
Antigen molecules have specific active sites that the antibody can combine with using the lock and key principle.
Antibodies are antigen-specific!
How do Antibodies work to protect the body?
Inactivate foreign enzymes or toxins by combining with them or inhibiting their reaction with other cells or compounds
Bind to the surface of viruses and prevent them from entering cells (and replicating)
Coat bacteria so that they are more easily consumed by phagocytes (during non-specific defences)
Cause particles to clump together so they can be easily captured and consumed in a process called agglutination
Dissolve organisms
React with soluble substances to make them insoluble and thus more easily consumed by phagocytosis
Parts of the Immune Response
There are two parts of the immune response:
Humoral/Antibody Mediated
Involves the production of special proteins called antibodies by the B-cells, which circulate the body and attach invading agents.
Antibody-mediated (humoral) response
Involves the production and release of antibodies into the blood and lymph. The antibodies work to provide resistance to viruses, bacteria and bacterial substances before they enter the cells.
Process of Antibody-mediated Immunity
Responses (i.e. primary + secondary)
Cell Mediated
Occurs due to T-cells and involves the formation of special lymphocytes that destroy invading agents.
Cell Mediated Immunity
Cell-mediated immunity provides resistance to the intracellular phase of bacterial or viral infections.
It is also important in fighting whole cells, such as fungi and parasites, and in rejecting foreign-tissue transplants. It also appears important in fighting cancer cells.
During this process we discuss the role of T-cells. They occur in the same lymphoid tissue as B-cells, but occupy different areas.
Process of cell-mediated immunity