Immunity in Living Organisms and Plants

Immunity in Living Organisms

• Living organisms are continuously threatened by biological and non-biological sources.
• Organisms have developed defense mechanisms such as camouflage and toxin secretion to survive.
• Immunity is the body's ability to resist pathogens through the immune system.
• This involves preventing pathogen entry or attacking/destroying them.
• The immune system has two systems: innate and acquired (adaptive) immunity, which work together.

Immunity in Plants

• Plant diseases and death mainly stem from dangerous enemies, unsuitable conditions, and toxic substances.
• Plants defend against pathogens via structural immunity (physical structures) and biochemical immunity (secreting chemicals.
• Humans protect plants by using herbicides and stimulating disease resistance, plant breeding, and genetic engineering.
• Activation compounds for protection and resistance transfer between plant cells through a transport system, similar to blood vessels in animals.

Structural Immunity

• Serving as the first line of defense, preventing pathogen entry and spread
• Natural barriers include pre-existing and induced structural defenses.

Pre-existing Structural Defenses

• Epidermal cells act as the first resistance bulwark.
• They may have a waxy, water-repellent surface, preventing fungal and bacterial growth.
• Epidermis may have hairs and thorns to deter grazing animals and water accumulation, decreasing infection risk.
• The cell wall provides outer protection, especially the epidermal layer, made of cellulose and lignin.

Induced Structural Defenses

• Cork formation isolates cut or torn areas due to plant growth, fruit collection, leaf fall, and animal/human encroachment, preventing pathogen entry.
• Tyloses are protoplast overgrowths from adjacent cells that protrude into xylem vessels and tracheids via pits.
• They form when the vascular system is cut or invaded by pathogens, obstructing pathogen movement.
• Gums are secreted by infected plants via wounds or cuttings, surrounding the infection site to prevent microbe entry.
• Morphological changes occur due to invasion, such as cell wall swelling in epidermal layers, inhibiting pathogen penetration.
• Mycelium is surrounded with an insulator to prevent fungus transmission from cell to cell.
• Plants undergo Hypersensitive Response to kill infected tissue, preventing pathogen spread.

Biochemical Immunity (Biochemical Defenses)

• Receptors recognize pathogens and activate plant defenses.
• Concentrations of receptors increase post-infection, stimulating the plant's innate immune system.
• Plants secrete antimicrobial chemicals to resist pathogens.
• Phenols and Glycosides are toxic compounds that kill or inhibit pathogenic organisms.
• Non-protein amino acids act as plant protectants, including toxic compounds like Canavanine and Cephalosporin.
• Plants produce antimicrobial proteins after infection, reacting with pathogen toxins and changing them into non-toxic compounds.
• Detoxifying enzymes interact with and invalidate pathogen toxins.
• Plants promote and strengthen defenses post-infection for future protection.

Human Immune System

• Immune organs are positioned throughout the body and functionally act as one unit called the lymphoid organ.
• Lymphoid organs house lymphocytes, key components of the lymphatic system.

Lymphoid Organs

• Contain large numbers of lymphocytes for maturation and differentiation.
  • Bone marrow: Tissue inside flat bones responsible for producing red blood cells, white blood cells, and platelets.
  • Thymus gland: Located on the trachea, it secretes Thymosin to stimulate T-cell maturity and differentiation.
  • Spleen: A small, dark red organ in the abdominal cavity playing an important role in immunity with many macrophages and lymphocytes.
    • Macrophages pick up foreign material and disintegrate senescent cells.
    • Lymphocytes release antibodies to defend against germs and viruses.
  • Tonsils: Two lymphoid glands on both sides of the mouth that trap microbes and foreign bodies from food or air.
  • Peyer's patches: Small lymphoid cells in the small intestine's mucous membrane that play a role in immune response against pathogens.
  • Lymph nodes: Purify lymph, store lymphocytes, and range in size from a pinhead to a bean.
    • Contain B-lymphocytes, T-lymphocytes, macrophages, and other white blood cells.
    • Connected by lymph vessels that transfer lymph for filtration.

Lymphocytes

• Forming 20-30% of white blood cells and produced in red bone marrow.
• They mature and differentiate in lymphoid organs to gain immune capabilities.
• They circulate in the blood to identify and eliminate pathogenic microbes.
• Types:
  • B-cells: 10-15% of lymphatic cells, identify foreign materials, adhere to them, and produce antibodies.
  • T-cells: 80% of lymphocytes, mature in the thymus gland, and further differentiate.
    • Helper T cells (TH): Activate other T cells and stimulate B cells to produce antibodies.
    • Cytotoxic T-cells (Tc): Attack foreign cells, including carcinogenic and virus-infected cells.
    • Suppressor T-cells (Ts): Regulate the immune response by limiting T and B cell action after pathogen elimination.
  • Natural killer cells (NK): 5-10% of lymphocytes, produced and matured in the bone marrow.

Other White Blood Cells

• Basophils, Eosinophils, and Neutrophils are distinguishable by size, nucleus shape, and granule color.
• Granules disintegrate pathogen cells, ingest/digest pathogens during phagocytosis, and combat bacterial infections/inflammations.
• Single-core monocytes destroy foreign bodies and become phagocytes when needed.

Macrophages

• Fixed macrophages: Large phagocytes found in most body tissues, engulfing foreign particles and microorganisms.
• Mobile macrophages: Collect information about microbes and foreign particles in lymph nodes.
• Immune cells prepare defense mechanisms like antibodies and killer cells to deal with these microbes.

Assisting Chemical Substances

• Chemokines: Recruit phagocytes to sites with microbes/foreign particles, preventing reproduction and spread.
• Interleukins: Mediate communication between immune cells and other body cells, aiding immune defense.
• Complements: Proteins and enzymes that destroy microbes in blood after binding to antibodies, facilitating phagocytosis.
• Interferons: Proteins produced by virus-infected cells, binding to healthy cells to induce enzyme production that inhibits viral replication.

Antibodies

• Surface of bacterial cells that invade body tissues have compounds called antigens
• Receptors on the surface of B- lymphocytes recognize and join with antigens on the surface of bacterial cells or foreign bodies and produce antibodies.
• Specific proteins, also known as immunoglobulins (Ig), with five types (IgG, IgM, IgD, IgE, IgA) circulating in blood and lymph.
• They consist of two pairs of polypeptide chains (heavy and light) joined by disulphide bonds.
• Each antibody has two identical antigen-binding sites with unique shapes.
• The antigen-binding site's shape varies (variable region), while the rest of the antibody has a constant shape (constant region).
• Specificity is determined by the conformation of amino acids at the antigen-binding site, creating a mirror image of a specific antigen.

Mechanisms of Antibodies

• Antibodies have two antigen-binding sites, and antigens have many, ensuring confirmative binding.
• Antibodies stop antigen action through:
  • Neutralization: Antibodies bind to viruses, preventing them from adhering to host cells.
  • Agglutination (clumping): Antibodies like IgM bind to multiple microbes, making them weaker and easier to engulf.
  • Precipitation: Antibodies bind to soluble antigens, forming insoluble complexes for easier phagocytosis.
  • Lysis: Antibodies activate proteins/enzymes, lysing antigen coats and dissolving contents.
  • Antitoxins: Antibodies bind to toxins, activating complements for detoxification.

Human Body Defense

• Two systems of immunity: natural (innate) and acquired (adaptive), are working together.

Natural (non-specific or innate) immunity

• A rapid response to resist foreign bodies, classified into two lines of defense:
  • First line of defense: Physical barriers like skin, mucus, tears, sweat, and stomach acid prevent pathogen entry.
    • The skin's tough layer and sweat gland secretions can kill microbes.
    • Cerumen (ear wax) kills microbes.
    • Tears contain enzymes which lysis microbes.
    • Mucus in respiratory tracts traps microbes and is expelled by cilia.
    • Saliva kills/dissolves microbes.
    • The stomach's acidic gastric juice kills microbes.
  • Second line of defense: Internal mechanisms to prevent microbe spread, starting with inflammation.
    • Inflammatory response: Nonspecific defense mechanism triggered by tissue damage.
    • Blood vessels dilate due to inflammation-generating substances like histamines, increasing permeability for white blood cells (neutrophils, monocytes, macrophages) to fight infection.
    • Interferons and natural killer cells (NK) are components.

Acquired (specific or adaptive) immunity

• Third line of defense using lymphocytes to resist pathogens through specific defense mechanisms, or immune response.
• Humoral or antibody-mediated immunity: Defends against antigens and pathogens by producing antibodies.
  • B-lymphocytes recognize and attach to specific antigens using immune receptors
  • The antigen binds with a protein in the B-lymphocytes.
  • Macrophages engulf and digest the antigen into fragments; then the complex transfers to the plasma membrane of the macrophage to be presented on its outer surface.
  • T helper lymphocytes (TH) recognize the antigen and activates B cells to produce large amounts of antibodies.
  • Antibodies bind to pathogens, activating macrophages to engulf them.
  • Memory cells are produced for future infections.
  • Cellular or cell-mediated immunity:
    • Done by T lymphocytes.
      • Macrophages engulf and decompose pathogens and presenting the antigen to T helper lymphocytes (TH).
        • This will activate the (TH) cells. -The activated T helper cells also secrete different types of the proteins cytokines that do the following:
      • Attract the macrophages to the site of infection in large amounts.
      • Stimulate the macrophages and other types of T lymphocytes ( Tc ) and B lymphocytes.
      • Activating the natural killer cells to attack the abnormal body cells like cancer cells or cells infected by pathogens.
  • Cytotoxic T cells recognize and destroy foreign bodies like transplanted tissues or cancer cells.

Inhibition of Immune System

• T suppressor cells (Ts) bind to plasma cells, T helper cells, and T cytotoxic cells after destroying antigens.
• Lymphokines are secreted to suppress the immune response and produce memory cells in lymphatic organs.

Stages of acquired immunity

• Primary immune response: Occurs during the first encounter with a pathogen, requiring time for cell multiplication (5-10 days).
• Secondary immune response: A rapid response upon subsequent infection due to memory cells, destroying the pathogen before symptoms appear. Memory cells: Store information about past antigens and enable a quick response upon re-infection.