Understanding the Immune System

What is the Immune System?

• The immune system is a biological defense system.
• It protects organisms against harmful infections, bacteria, viruses, and fungi.
• In humans and animals, it includes physical barriers like skin and mucous membranes.
• It also consists of immune cells like antibodies and proteins.
• In plants, it comprises physical barriers like the cell wall.
• Also chemical barriers like the production of antimicrobial compounds.
• The immune system plays a critical role in survival by controlling and preventing diseases in all organisms.

What Does the Human Immune System Do?

• The human immune system consists of organs, cells, and proteins throughout the body.
• It safeguards against infections caused by germs, viruses, fungi, parasites, and cancer cells.
• It also shields the body's own cells.
• It operates to protect the body from infections for most individuals.
• Some individuals have immune system disorders that disrupt this protective mechanism.

Main Parts of the Human Immune System

• White blood cells travel through blood and tissues, searching for foreign invaders and initiating an immune response.
• White blood cells consist of lymphocytes, including B-cells, T-cells, and natural killer cells.
• Antibodies fight microbes and toxins by recognizing antigens on the surfaces or chemicals of foreign invaders.
• They mark them for destruction.
• Cytokines are chemical messengers guiding immune cells, performing tasks like regulating inflammation during immune responses or tissue healing.
• The complement system consists of proteins that enhance the functions performed by antibodies.
• The lymphatic system manages body fluid levels, reacts to bacteria, deals with cancer cells, prevents disease, and absorbs fats from the intestine.
• Lymph nodes (lymph glands) trap microbes.
• Lymph vessels carry the colorless fluid that bathes body tissues and contains white blood cells that fight infections.
• Lymphocytes (white blood cells) produce antibodies, regulate immunity, and destroy virus-infected cells.
• The spleen keeps white blood cells that protect the body against intruders.
• It purifies blood and reuses old and harmed cells.
• Tonsils and adenoids, in the throat and nasal passage, capture intruders like bacteria or viruses when they enter the body.
• The thymus assists T-cells in maturing before they move to different parts of the body to provide protection.
• Bone marrow produces blood cells necessary for survival, including white blood cells that support the immune system.
• Skin prevents germs from entering the body.
• Mucosa, a triple-layer membrane, coats cavities and organs across your body.
• It produces mucus that traps invaders, allowing the body to eliminate them.

How the Immune System Works

• Macrophages engulf infected cells displaying antigens.
• T cells with matching receptors bind to the antigen-MHC complex and release cytokines.
• Cytokines trigger cell death in infected cells and activate mitosis in T cells.

Proper Function of the Immune System

• Distinguishes between cells that are yours and those that are foreign to your body.
• Activates and mobilizes to kill germs that may harm you.
• Halts the attack once the danger has disappeared.
• Learns about germs once you've encountered them and develop antibodies to fight them.
• Releases antibodies to eliminate germs attempting to invade your body later on.
• If the immune system doesn't function correctly, it may be insufficient to repel invaders, or it could initiate an excessively strong reaction.
• Various conditions can impair the immune system, increasing vulnerability to infection.
• An overactive immune system can mistakenly attack harmless substances or continue attacking after an invader is gone.
• This can lead to issues like autoimmune diseases and allergies.

Innate Immunity vs. Acquired Immunity

• Innate immunity is the defense you have from birth.
• It doesn't require previous training to distinguish between cells that are part of your body and those that are not.
• White blood cells that participate in innate immunity do not acquire the ability to identify specific intruders.
• They lack a memory of assaulting invaders and do not provide defense against particular germs.
• Acquired immunity, known as adaptive or specific immunity, is the defense your body develops (gains) over time through exposure to pathogens.
• Some white blood cells, known as lymphocytes, can recognize specific invaders and identify when they are foreign to your body.
• Vaccines enhance your acquired immunity by teaching cells to eliminate intruders before they cause illness.

Conditions and Disorders

• Allergic diseases are triggered when the body excessively responds to a substance that is usually safe.
• Asthma makes breathing difficult due to inflammation and tightness in the airway muscles.
• Autoimmune diseases develop when the immune system mistakenly targets its own healthy cells.
• Rheumatoid arthritis is a long-lasting autoimmune condition that leads to inflammation and pain in the joints.
• Primary immunodeficiency diseases are hereditary disorders that impact the immune system and result from mistakes in the genetic material of the immune cells.
• Wiskott-Aldrich Syndrome is a genetic disorder caused by a mutation in the WAS gene on the X chromosome.
• Infectious diseases occur when pathogens infiltrate the body, multiply, and cause damage.
• Infectious mononucleosis (mono) leads to symptoms like swollen lymph glands, fever, sore throat, and significant fatigue and spreads mainly through contact with saliva.
• Cancer cells in your bone marrow disrupt the regular creation of blood cells necessary for combating infection.
• Lymphoma affects lymphocytes, which are crucial white blood cells.
• Sepsis is a severe immune reaction to infection where your immune system begins to harm healthy tissues and organs, leading to dangerous inflammation.

What Does the Plant Immune System Do?

• The immune system in plants consists of receptors and biochemical responses that activate defense genes to resist potential diseases.
• Plants rely on their innate immunity in combating pathogens due to lacking a circulatory system.

Main Parts of the Plant Immune System

• The physical barriers in the immune system of plants include their various structural characteristics.
• The plant cuticle is a waxy, waterproof layer that prevents pathogens from easily penetrating the plant's stem or leaves.
• The epidermal cell wall (ECW) thickens in response to a pathogen attack to make it harder for them to enter the plant cell.
• Cork can form in response to a pathogen attack and repels water while protecting tissue.
• Bark, present in woody plants, serves as an extra protective layer that blocks pathogens.
• Trichomes (hairs) on the leaves and stems of plants have the ability to physically trap pathogens.
• Plants have innate chemical barriers that can activate chemical responses upon detecting danger.
• Pathogen Recognition: Plants use pattern recognition receptors (PRRs) that trigger immune responses when they detect pathogen-associated molecular patterns (PAMPs) released by invaders.
• Example: Flagellin-sensitive 2, a receptor kinase gene that has the ability to aid plants in detecting and recognizing bacterial flagellin
• Antimicrobial compounds are chemicals produced by plants that prevent the growth of pathogens, such as phytoalexins and defensins.
• Reactive Oxygen Species (ROS) can act as secondary messengers that signal immune responses against pathogenic infection.
• Resistance (R) Proteins are specialized protein molecules that act as receptors, recognizing pathogen effectors (pathogen-derived molecules) that triggers a defense response within the plant.
• Hormonal Signals include salicylate (SA), jasmonate (JA), ethylene (ET), abscisic acid (ABA), auxins, cytokinins, gibberellins and brassinosteroids that play a key role in coordinating defense responses to combat invaders.
• They allow the plant to mount specific immune responses against the specific type of pathogen it faces

How the Plant Immune System Works

• The plant's innate immune system carries out specific immune responses in order to combat microbial pathogens.
• These responses can be broken down into 3 essential steps: microbial recognition by immune receptors, signal transduction within plant cells, and immune execution directly suppressing pathogens.

1. Microbial recognition by immune receptors:

• Plants respond to infection by recognizing pathogen-associated molecular patterns (PAMPs) through cell surface-localized pattern recognition receptors (PRRs) and intracellular nucleotide-binding domain leucine-rich repeat receptors (NLRs).

2. Signal transduction within plant cells:

• Signal transduction pathways within the plant cells work together in order to relay that information and coordinate an effective defense response to the given circumstance.
• This can be done through hormonal regulation, reactive oxygen species (ROS), calcium influx, or mitogen-activated protein kinase (MAPK) cascades.

3. Immune execution directly suppressing pathogens

• The plant then carries out the direct defense responses in order to suppress the pathogen invasion.
• It reinforces the plant's physical barriers, such as strengthening the cell wall with callose and lignin or stomatal closure that prevents pathogen entry.
• These steps equip plants with the tools required for protection and survival.

What Does the Animals Immune System Do?

• The immune system in animals is a defense system that consists of specialized cells, tissues and organs that detect and fight infections.
• It recognizes pathogens like bacteria and viruses and helps animals develop immunity against future diseases.

Main Parts of the Animals Immune System

• Physical and Chemical Barriers are the first line of defense and consist of physical structures that block harmful microorganisms before they can enter the body.
• Skin contains antimicrobial proteins like defensins and lysozyme that help destroy pathogens.
• Mucous Membrane traps dust, bacteria and viruses.
• Tears and Saliva contains lysozyme that breaks down bacterial cell walls.
• Stomach Acid (Hydrochloric Acid, HCI) kills most ingested microbes before they reach the intestines.
• Non-specific innate Response consists of immune cells and proteins to nonspecifically recognize and eliminate any pathogen that enters the body.
• White blood cells (Leukocytes) travel through the bloodstream and attack invaders like bacteria, viruses and parasites.
  • Macrophages and Neutrophils are two types of phagocytes and special types of white blood cells that digest and destroy harmful microbes in a process called phagocytosis.
• Inflammatory Response increases blood flow to the affected area when the body detects an infection, causing swelling and redness to help immune system to fight off the invaders
• Specific Adaptive Response aims to eliminate specific pathogens that have been encountered by the immune system.
• Lymphocytes (B Cells & T Cells) produce antibodies that fight infections (B cells), T cells help destroy infected cells and coordinate immune responses.
• Antibodies are the proteins made by B cells and neutralize specific pathogens to prevent them from spreading
• Memory Cells “remember” the pathogen so the body can respond faster if it encounters it again in the future.

Comparison of Immune Defenses in Invertebrates and Vertebrates

• Invertebrates (non-specific/innate immunity):
  • Exoskeleton blocks pathogen entry.
  • Low PH inhibits microbial growth.
  • Enzymes break down pathogens.
  • Phagocytes digest and destroy harmful microbes.
• Vertebrates (both innate/non-specific and adaptive/acquired immunity):
  • external barriers: skin, mucous membrane, microbida, hair, cilia, secretions
  • internal barriers (if pathogens cross external barriers)
  • Specific cells: phagocytes, natural killer cells, chemicals (interferons, complement system)
  • Inflammatory Response

The Immune System of Vertebrate Animals

• FISH - have both innate and adaptive immunity, but their immune system is less developed that mammals.
  • Innate Immunity:
  • primary defense, trapping and inhibiting their growth
  • macrophages, natural killer (NK) cells and cytotoxic T cells (phagocytosis and cell - mediated cytotoxicity)
  • Adaptive Immunity:
  • organs like thymus, spleen and kidney serve that process, but lack bone marrow and lymph nodes
  • possess B and T lymphocytes
  • immunoglobulin M plays crucial role in humoral immunity
  • other immune functions may be suppressed in colder conditions.
• Reptiles - bridges the evolutionary gap between amphibians and birds/mammals.
  • Innate Immunity:
  • scaly skin acts as barrier, possessed nonspecific leukocytes
  • Adaptive Immunity
  • lymphocyte development and maturation occur. Functional T and B cells, and immunoglobulins
  • ectothermic
• Amphibians - dual life cycle (aquatic and terrestrial) influences their immune system, but can be weaker than responses of higher vertebrates.
  • Innate immunity skin and mucus secretions
  • Adaptive Immunity bone marrow present, produce IgM, IgY and IgX,weaker immune memory than mammals
• Birds - highly developed immune system, similar to mammals.
  • Innate Immunity: feathers, skin and mucus secretions help block infections, heterophils, macrophages and natural killer (NK) cells.
  • Adaptive Immunity: Birds produce IgM, IgY and IgA, T and B cells recognizes and attack pathogens
• Mammals - most advanced immune systems, capable of strong responses and long term immune memory.
  • Innate Immunity: skin, mucus, tears and saliva protect microbes, Has macrophages, neutrophils, dendritic cells and Natural Killer (NK) cells
  • Adaptive Immunity: T cells attacks infected cells, placental immunity

The Immune System of Invertebrate Animals

• Arthropods
  • strong innate immune systems to fight infections but lack adaptive immunity. -exoskeleton protects, hemocytes help destroy, antimicrobial peptides
• Mollusks - rely only on innate immunity for protection.
  • shell and mucus, hemocytes in the blood.
• Cnidarians - a basic innate immune system with no specialized immune cells
  • mucus and body wall protect against pathogens, toxins to kill, antimicrobial peptides
• Echinoderms - it uses coelomocytes to fight infections
  • spiny outer skin protects, releases reactive oxygen species to kill pathogens.
• Sponges - a basic immune system that relies only on innate immunity.
  • amoebocytes help detect and destroy, antimicrobial peptides (AMPs) that kill bacteria and fungi.