2020 Biology HSC Responses-to-Pathogens-Notes PDF

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2020 Biology HSC Responses-to-Pathogens-Notes...

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Responses to Pathogens Plant responses to pathogens Passive defences Plants have two major types of passive defences against pathogen invasion: physical barriers and chemical barriers. Physical Barriers - Physical barriers, such as a thick cuticle, cell walls and small stomata, all inhibit pathogen entry and are the first line of defense. - Thick cuticles are better able to withstand pathogens as some pathogens secrete cuticle dissolving enzymes - Bark offers plants extra protection against pathogens that attempt to reach the food source (sap) in the phloem beneath the tree bark - Vertical hanging leaves, which do not accumulate a water film, reduce the likelihood of pathogen reservoirs building up on the outside of leaves. Chemical Barriers - The presence of chemical compounds in the tissues of plants, can reduce fungal and bacterial growth, and ward off vectors of viruses. - Examples of chemicals are glucosides and saponins - Plants may also produce enzymes that break down pathogen-derived toxins. - Chemical receptors on plant cells can detect the presence of a pathogen - for examples, by detecting pathogen-associated molecular patterns (PAMPs) secreted by bacteria - Stomata may close in response to the presence of bacteria through chemical signalling methods involving detection of PAMPs - Such receptors can activate the next stage of defence Active defenses When the passive barriers are breached, the plant is now at grave risk of harm. The next line of defence involves more targeted responses by the plant. Pathogen recognition - Plants are able to recognise pathogens by detecting certain physical and chemical signals. - Refer to PAMPs (above) Rapid active response (minutes to hours) - Recognition of a pathogen by proteins on the surface of cells in plants causes changes in the permeability of the plant cell membrane. This allows the movement of certain ions

(calcium in particular) into the cell and triggers defence responses by activating the expression of certain genes. Examples include: - The release of hydrogen peroxide (H2 O2) in an oxidative burst (the rapid release of reactive forms of oxygen from certain cells) to kill microbes directly. - Reinforcement of the cell wall with aggregates of material in the cytoplasm near the defect in the wall, known as cell wall apposition - Programmed cell death (apoptosis) causes a cluster of dead plant cells to accumulate around the pathogen to isolate it, followed by the secretion of antimicrobial compounds Delayed active response (days) - It limits the spread of the pathogen - One important response is to repair wounds in the bark, through cork cell production and gum secretion - Lysozyme-like chemicals are also released and have an antimicrobial action. - Salicylic acid may act as a signalling agent of subsequent infections and play a role in the plant’s ‘memory’ of a particular pathogen - This is known as systemic acquired resistance (a whole-plant response following an earlier exposure to a pathogen) and limits the severity of subsequent infections with that pathogen

Animal response to pathogens Lines of defence Innate immunity - Present at birth and is genetically determined - Responses to pathogens are non-specific - First line - Consists of barriers to entry - Such barriers may be physical (skin), chemical (tears) or biological (sphincters) - Second line - When a barrier is breached the second line of defence is activated. - It involves a nonspecific chemical and cellular attack on the pathogen, characterised by the process of inflammation (a reaction to the presence of an antigen, in which tissues become hot, red, swollen and painful). An antigen is a molecule capable of inducing an immune response Adaptive immunity - Third line - Specific defense mechanism consisting of specialised cells that act if the pathogen persists in its invasion

The role of the lymphatic system The lymphatic system consists of lymph (a milky fluid GACHIBASS), lymph nodes, lymph vessels, thymus, spleen, tonsils, and adenoids. - Lymph vessels form a one-way drainage system from all parts of the body back to a point near the heart, where the cleansed lymph fluid is drained back into the blood. - At different points along the lymph vessels, structures called lymph nodes filter out microbes, cellular debris and cancer cells from the lymph fluid - Lymph nodes are a good indicator of the body’s response to infections, as changes in size, shape or texture help pinpoint the site of an infection. - The lymphatic system has multiple interrelated functions: - It is responsible for the removal of interstitial fluid from tissues - It absorbs and transports fatty acids and fats as chyle from the digestive system - It transports white blood cells to and from the lymph nodes into the bones - The lymph transports antigen-presenting cells, such as dendritic cells, to the lymph nodes where an immune response is stimulated. White blood cells (leukocytes) - Plays pivotal role in innate and adaptive responses to pathogen - There are many types of white blood cells, which differ in: - Size - Presence of granules in their cytoplasm (granular or agranular) - Colour of granules and cytoplasm (pink - eosinophilic, blue - basophilic) - Shape of nucleus (lobed or mononuclear)

The microbiome: a natural barrier to infection Microbiomes are beneficial bacteria that inhabti the human body, living in mainly on kin, in the intestines, the colon, the mouth and the vagina in women. The body supplies these microbes with nutrients, while the presence of the microbiome inhibits the growth and multiplication of many pathogens, as they are out-competed by the natural microbes. - An example of a disease where the balance of the microbiome is changed is candidiasis (thrush) - Caused by the fungus Candida albicans,  which is usually kept low by competition from other microorganisms Physical barriers against infection Physical barriers are structures that the body uses to restrict entry to pathogens, by making it difficult for the pathogen to adhere to cells of to penetrate tissues.

Skin Classified as Epithelial tissue (thins tissues that forms the outer surface of the body and the inner lining of the digestive system and other hollow structures), and consists of three layers: the outer epidermis, dermis, and hypodermis (subcutaneous tissue) - Well supplied by blood, contributing to its effectiveness as a barrier by providing early access for white blood cells, red blood cells and platelets to any wound. - When there is a loss of integrity of the skin barrier due to wounds or burns, the body has special processes to seal the site asap: inflammation, proliferation and maturation. - The epidermis consists of sheets of cells covered by keratin, a waterproof protein separates the organism from its environment, being mechanically tough and resistant to degradation by bacterial enzymes - The epidermis itself consists of a layer of flattened and dead skin cells, forming a physical barrier. As dead cells exfoliate (die and flake off) (look its me), they take pathogens with them Mucous membranes Many internal cavities are lined with mucous membrane, a special type of epithelial tissue, that forms a barrier. - Typically pink, moist lining tissues, such as the lining of our mouth and nose. Aldo found in digestive, respiratory and genitourinary systems The following features help restrict pathogenic entry: - Cell junctions (intercellular bridges formed by the plasma membranes of adjacent cells, enabling contact between the cells) between epithelial cells are designed to anchor them together, increasing cohesion and restricting access to pathogens - Many epithelial cells are lined with tiny hair-like structures called cilia, which beat in a coordinated way to remove particles from them respiratory system - Secrete a number of protective substances such as mucous, lysozyme and immunoglobulins (antibodies) Tight junctions Tight junctions occur in places between endothelial cells (Single layer of cells that line organs and cavities e.g. blood vessels), forming a physical barrier between the extracellular tissues and the bloodstream. - E.g. Blood brain barrier - It is formed by brain endothelial cells connected by tight junctions, and restricts the diffusion of microscopic objects such as bacteria into the brain Mucus

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Slippery substance secreted by cells that line the mucous membranes Protects the linings of the body by trapping foreign substances such as pathogens, dust and pollen Increased mucus production is often a sign of hill health Some intestinal mucus have been shown to contain substances that inhibit the replication of viruses, such as rotaviruses.

Peristalsis - The coordinated muscle contractions in the alimentary canal. That move food in one direction only - Stasis (lack of movement) of the intestines can lead to intestinal bacterial overgrowth, and therefore constant movement prevents bacteria from proliferating. Sphincters - Circular muscle that maintains constriction of a natural body passage or orifice - Examples include - Lower esophageal sphincter - between the oesophagus and stomach - Pyloric sphincter between the stomach and the duodenum - Urethral sphincter - prevent the release of urine from the bladder unless done so voluntarily - Sphincter of Oddi - ensures the one-way flow of digestive juices from common bile duct to the duodenum Physical responses to infection Granuloma formation - Sometimes cells die to seal off an area of tissue that is infected and not being successfully defended, preventing the infection from spreading. - This wall of dead cells forms a capsule known as a granuloma. Vomiting (emesis) and Diarrhoea - Vomiting is a reflex action coordinated by the vomiting centre (chemoreceptor trigger zone) of the brain, due to the presence of pathogens in the gut (gastroenteritis) - Both processes allow the body to quickly expel microorganisms quickly from the gastrointestinal system Increased Urination - Occurs when the bladder lining is attacked by a pathogen. Is is thought to be a response by the body to help flush out pathogens.

Wound healing - When there is a breach in the body’s barriers, the tissues are exposed to environmental pathogens and the microbiome of the skin. The priorities of wound healing are to: - Stop the bleeding (haemostasis) to maintain normal blood pressure. - To do this, the blood vessels contract and a platelet (thrombocyte, a cell that helps in the formation of a clot to stop bleeding) plug is formed. - A protein called fibrin forms a mesh, to trap more platelets and form a clot (thrombus, a thick mass of coagulated liquid, often blood) to seal the wound - Confront the pathogens, to prevent infection - Heal and repair the wound to re-establish the barrier Chemical defenses against infection Urine - The flushing activity that takes place during micturition (urination) assists in keeping pathogens, which can attempt to ascend the lower urinary tract, away from the bladder. - Antimicrobial peptides (AMPs) secreted by cells lining the urinary tract prevent the binding of bacteria to epithelial cells and lyse(break down) bacterial cells - pH of normal human urine is within the range of 4.5 to 8, with average of 5-6. Phagocytes (special type of white blood cell) such as neutrophils work best when urine is alkaline. Sebum and sweat - Sebum is an oily material secreted by sebaceous glands. It’s purpose is to waterproof and lubricate the skin - The acidic pH (around 5.5) of the skin is typically because of acids in sweat and sebum - Lysozyme is secreted in perspiration and lyses or breaks down bacterial cell walls Saliva - Produced by salivary glands - Is a mixture of water, mucus, electrolytes, enzymes such as amylase, and antimicrobial substances such as lysozyme and immunoglobulin A. - Has a flushing action against microbes as well as chemical activity against them, due to antimicrobial molecules such as AMPs Tears - Lacrimal glands produce tears, production of tears called lacrimation - Glands along eyelid edges secrete sebum like substance with antimicrobial properties - Also crying has kinda a flushing effect

Gastric (stomach) acid - Parietal cells lining the stomach wall will secrete hydrochloric acid, creating a highly acidic environment (pH1-2). Hence, growth and survival of microbes are discourage. - However, eating food may raise the stomach’s pH to above the threshold needed to destroy these bacteria - As food moves to the duodenum, there is a rapid change in pH to around 6, another limiting factor to pathogen growth

Summary of first line of defence Inflammation: a chemical response - Inflammation is a chemical response that helps wound repair and leads to pathogen destruction. The five cardinal signs of the inflammatory response are (in latin) - Dolar - pain due to release of chemical mediators of inflammation - Colar - heat due to the increase in microcirculation - Rubor - redness associated with increased microcirculation - Tumor - swelling (oedema) as fluids move from the intravascular space to extracellular space - Functio laesa  - loss of function due to pain and swelling - Inflammatory response is a non specific defence mechanism, and occurs at the site of infection

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When cells are challenged by pathogens or damaged, they release chemical ‘alarm’ signals to increase blood flow to the site of infection/injury - Histamines - trigger vasodilation and increase vascular permeability - Bradykinin - Serotonin - Prostaglandins - These chemicals also increase permeability of blood vessels, allowing certain white blood cells to move from the blood into the tissues to attack the invading pathogens Endogenous pyrogens (molecules produced by leukocytes in response to pathogens that increases that set point for body temperature, inducing a fever), are released to inhibit the growth of pathogens, inactivates some enzymes and toxins made by the pathogens, and increases biochemical rate of reaction in the body. When damage occurs to tissues but the wound is sterile, damage-associated molecular patterns (DAMPs) are released by damaged cells. This sends a chemical signal to initiate an inflammatory response. Afterwards, cellular replacement to restore damaged tissue occurs.

Phagocytosis - The process by which phagocytes change their shape so they can surround a foreign particle, such as a bacterium and completely enclose it, and destroy it with released enzymes after fully enclosing - Main types of phagocytes are: - Neutrophils - monocytes/macrophages - Dendritic cells - Natural killer cells - Phagocytosis is not always successful, as pathogens can sometimes repel the phagocytes and escape before being completely destroyed.

Neutrophil - Originate in blood marrow, like all phagocytic cells

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Capable of deforming and squeezing between the endothelial cells First to move to site of the infection, are short acting then self-destruct after a few days Used to fight acute (short,severe) infections

Monocytes - Circulate the blood until attracted to inflamed tissue, and migrate through capillary walls to the tissue, where they undergo transformation into macrophages and dendritic cells - Used to fight chronic (long lasting just like my crippling depression) infections - After the foreign particle is destroyed, the macrophage displays part of the antigen on its surface (antigen presentation) - Remove microbes, lipids and dying cells through the process of phagocytosis - Dendritic cells and macrophages act as ‘bridges’ between the innate and adaptive immune systems The complement system - A group of around twenty soluble proteins that assist other defence mechanisms. They can: - Stimulate phagocytes to become more active - Attract phagocytes to the site of the infection - Destroy the membranes of the invading pathogen - Manufactured in liver cells and macrophages - Example of response to infection - During third life defence, some pathogens bind to proteins called antibodies - Complement proteins are attracted to pathogen-antibody complex and bind to the as well, creating a coating layer - This acts as a signal for phagocytes and other lymphocytes called B cells third line defence) to destroy the pathogen. - Process called opsonisation (an immune process whereby an antigen is marked for destruction by immune cells such as phagocytes)

Fever: refer to pyrogens (just ctrl f) Cytokines - Chemical messengers that are produced during an infection and promote the development and differentiation of T and B lymphocytes for the third line of defence - E.g. Interleukin (IL): - These chemicals form a link between innate and adaptive immune systems - E.g. Interferons - Act as chemical signal to viruses to stop replicating - Done by signalling uninfected cells to destroy RNA and reduce protein synthesis - Infected cells also signalled apoptosis (programmed cell death)...