unit 4lesson 1 part 3: BARRIERS

• Lesson 1 focuses on barriers in the immune system.

Objectives: Lesson 1

• Three key functions of the immune system will be named.
• Key pathogens that cause disease, and their transmission routes will be described.
• Layers of immune defense, including the first and second lines of defense will be described.

Immune System Functions

• Immune defense is a key function.
• Immune homeostasis is a key function.
• Immune surveillance is a key function.
• Abnormal immune functions can lead to hypersensitivity, immunodeficiency, autoimmune disease etc.
• Normal immune functions include the clearance of infection, elimination of injured cells, and tolerance of self components.

Immune Defence

• The immune system protects the body from disease-causing pathogens and includes a series of defenses.
• The immune system includes three lines of defense against foreign invaders: physical and chemical barriers, nonspecific resistance, and specific resistance.
• The first line of defense consists of physical and chemical barriers and is considered a function of innate immunity.
• The second line of defense is nonspecific resistance, also considered a function of innate immunity.
• The third line of defense is specific resistance, considered a function of acquired immunity.

Innate vs Adaptive Immunity

• The first and second lines of defense are part of the innate immune system, while the third line of defense is part of the adaptive immune system.
• The innate immune system includes physical barriers like the skin.
• Additional barriers include the gastrointestinal and respiratory tracts, nasopharynx, cilia, eyelashes, and other body hair.
• Defense mechanisms of innate immunity involved secretions, mucus, bile, gastric acid, saliva, tears, and sweat.
• General immune responses of the innate system are inflammation, complement, and non-specific cellular responses.
• Leukocytes in the innate immune system include phagocytes, macrophages, mast cells, neutrophils, eosinophils, basophils, natural killer cells, and dendritic cells.
• The complement system involves opsonization, chemotaxis, cell lysis, and agglutination.
• Innate immunity is non-specific and defends against anything identified as foreign; it is also fast, acting within minutes or hours.
• The adaptive immune system includes B cells that mature in bone marrow, contributing to antibodies that bind directly with specific antigens and humoral immunity.
• T cells mature in the thymus, express T cell receptors and CD4 or CD8, and contribute to cell-mediated immunity.
• T cell receptors recognize antigens bound to major histocompatibility complex (MHC) class I or II molecules.
• CD4 and CD8 contribute to T cell recognition and activation by binding to either MHC I or MHC II.
• Adaptive immunity is highly specific, identifying pathogens and differences in molecular structures, and is slow, taking days to respond.

Pathogens

• Bacteria are single-celled organisms without a nucleus, such as Escherichia coli, that can cause strep throat, staph infections, tuberculosis, food poisoning, tetanus, pneumonia, or syphilis.
• Viruses are non-living particles that reproduce by taking over living cells, such as the herpes simplex virus, that can cause the common cold, flu, genital herpes, cold sores, measles, AIDS, genital warts, chicken pox, or smallpox.
• Fungi are simple organisms, including mushrooms and yeasts, such as the death cap mushroom, that grow as single cells or thread-like filaments and can cause ringworm, athlete's foot, tineas, candidiasis, histoplasmosis, or mushroom poisoning.
• Protozoa are single-celled organisms with a nucleus, such as Giardia lamblia, that can cause malaria, traveller's diarrhea, giardiasis, or trypanosomiasis (sleeping sickness).

Pathogen Transmission

• Infection can occur via droplets or particles in the air, contact, blood/tissue, or contaminated water/food.

Layers of Immune Defence

• Layers from bottom upwards: Barriers > Soluble components > Specialist immune cells > Learn (adaptive).

The First Line of Innate Defence

• Consists of different types of barriers that keep most pathogens out of the body and is always the same regardless of the pathogen.

Mechanical Barriers

• Skin is a tough barrier that is difficult for pathogens to penetrate.
• Mucous membranes secrete mucus to trap pathogens and have hair-like cilia that remove pathogens.
• Sneezing or coughing removes pathogens from the nose, throat, or upper airways.
• Tears wash pathogens from the eyes.

Chemical Barriers

• Body openings or inner body linings prevent the spread of pathogens
• Sweat, mucus, tears, and saliva contain enzymes that kill pathogens.
• Urine and stomach acid are too acidic for many pathogens.

Molecular Response

• Complement System in the blood consists of blood proteins/proteolytic cascade leading to opsonisation (phagocytosis) or formation of a lytic pore on cells.
• Pattern recognition receptors (PRRs) in the innate immune cell recognizes conserved molecular patterns common to various microbes
• Lysozymes in tears, nasal secretions, saliva, and gastric secretions are lytic enzymes that attack the protective cell walls of bacteria.
• Interferons (IFN) are signalling proteins made and released by host cells in response to pathogens like viruses, bacteria, parasites, and tumour cells.
• C-reactive protein (CRP) in the blood binds to lysophosphatidylcholine expressed on the surface of dead/dying cells.

Biological Barriers

• Organisms help to protect the body.
• Harmless bacteria can be found on the skin or in the gastrointestinal tract.
• The microbiome provides a protective barrier.

The Second Line of Defence

• After a pathogen enters the body the inflammatory response is triggered as the first reaction to tissue damage or infection.
• Chemical mediators, like cytokines and histamines, trigger it.
• Chemical mediators communicate with other cells and coordinate the inflammatory response.
• Symptoms at the site of infection/wound include redness, warmth, and swelling.

Leukocytes in the Second Line Of Defense

• The chemicals that trigger the inflammatory response attract leukocytes to the site of injury or infection.
• Leukocytes are white blood cells that fight infections and remove debris.
• Leukocytes may respond with either a nonspecific or specific defense.
• A nonspecific defense remains the same regardless of the type of pathogen involved.
• An example is phagocytosis, where leukocytes engulf and break down pathogens and debris.

Case Study: Wound Infection

• Bronwyn, 38, cut her finger and the next day the skin around the cut was red and warm.
• Signs that a wound may be infected include contamination with dirt, pus, or bodily fluids, presence of gravel or glass before cleaning, and a jagged edge.
• Signs of an infected wound include swelling, redness, increasing pain, pus formation, feeling unwell, high temperature (38C or above), and swollen glands under the chin, neck, armpits, or groin.
• A bacteria-infected wound can usually be successfully treated with antibiotics.