The Immune System

Introduction

• Definition of Antigen:

  • An antigen is any substance that induces an immune response in the body, specifically the production of antibodies.

  • Antigens can include:

    • Pathogens such as bacteria, viruses, fungi, and parasites.

    • Non-infectious substances like pollen, food proteins, and even some chemicals that trigger allergic responses.

• Dual Nature of the Immune System:

  • The immune system consists of two primary components:

    1. Innate Immunity (Non-Specific):

       • Immediate response that does not recognize specific pathogens.

       • Acts against all pathogens with physical barriers like skin, chemical barriers such as mucous membranes, and various immune responses including inflammatory processes.

    2. Adaptive Immunity (Specific):

       • Slower response that is tailored to specific pathogens.

       • Involves the activation of lymphocytes (T and B cells), which provide memory for future infections, leading to a stronger response upon re-exposure to the same pathogen.

• Key Terms in Immune Response:

  • Antigen-Specific:

    • Refers to the ability of the immune response to specifically recognize and respond to distinct antigens.

  • Systemic:

    • Indicates that the immune response is not limited to the site of infection; it can affect the entire body, often leading to systemic symptoms.

  • Memory:

    • Refers to the capability of the immune system to remember past infections, allowing for a faster and more efficient response upon re-exposure to the same pathogens.

• Self/Non-Self Recognition:

  • Achieved through the presentation of antigens by Major Histocompatibility Complex (MHC) molecules.

  • This mechanism enables immune cells to distinguish between the body's own cells and foreign invaders, thereby preventing autoimmune responses.

• Types of Leukocytes:

  1. Neutrophils:

     • The most abundant type of white blood cells that respond quickly to infections, primarily bacterial.

  2. Lymphocytes:

     • Include B cells (antibody-producing) and T cells (cell-mediated immunity).

  3. Monocytes:

     • Differentiate into macrophages and dendritic cells that present antigens.

  4. Eosinophils:

     • Combat multicellular parasites and allergic reactions.

  5. Basophils:

     • Involved in inflammatory responses and release histamines.

The Lymphatic System

• Definition of Lymph:

  • Lymph is a clear fluid that circulates within the lymphatic system, originating from interstitial fluid in tissues and containing white blood cells, especially lymphocytes.

• Functions of Lymph:

  • Transports excess interstitial fluid back to the bloodstream to maintain fluid balance.

  • Absorbs dietary fats and fat-soluble vitamins from the intestine through specialized lymphatic vessels called lacteals.

  • Provides a conduit for immune cells to travel throughout the body, allowing for effective immune surveillance.

• Role of Lymph Nodes:

  • Filter lymph and trap pathogens, foreign particles, and cancer cells, allowing immune cells to mount a response, which can lead to swollen lymph nodes during infections.

• Types of Lymphoid Organs:

  • Primary Lymphoid Organs:

    • Examples include bone marrow and thymus, where lymphocytes are produced and mature.

  • Secondary Lymphoid Organs:

    • Examples include lymph nodes, spleen, and mucosal-associated lymphoid tissue (MALT) where immune responses are initiated and further refined.

Innate Immunity

• Definition and Role:

  • The innate immunity system serves as the body's first line of defense, providing immediate but non-specific protection against a wide range of pathogens.

  • This includes physical barriers (skin, mucous membranes), chemical barriers (acids, enzymes), and immune responses such as inflammation and fever.

• Most Important Barrier:

  • The skin acts as a robust physical barrier preventing pathogen entry, supplemented by antimicrobial peptides found on skin surfaces.

• Normal Flora:

  • Refers to the microorganisms residing on and within the human body, crucial in maintaining health by:

    • Outcompeting harmful pathogens for nutrients and space.

    • Supporting immune function by training the immune system to recognize self from non-self.

• Phagocytes:

  • Immune cells such as neutrophils and macrophages that encapsulate and digest pathogens and debris through a process known as phagocytosis, which is critical for clearing infections.

• Macrophages:

  • Large phagocytic cells derived from monocytes, found in tissues, vital for:

    • Detecting, engulfing, and destroying pathogens.

    • Initiating immune responses by secreting cytokines that activate other immune cells.

• Function of Natural Killer Cells:

  • Identify and destroy infected or cancerous cells by releasing cytotoxic substances that induce apoptosis (programmed cell death), thereby maintaining host integrity.

Adaptive or Acquired Immunity

• Types of Lymphocytes:

  • Two major types play distinct roles:

    1. B Cells:

       • Produced in the stem cells of the bone marrow, they are crucial for producing antibodies in response to antigens and are involved in humoral immunity (mediated by antibodies).

    2. T Cells:

       • Non-antibody-producing lymphocytes produced in the bone marrow, maturing in the thymus. They play a central role in cell-mediated immunity by directly attacking infected cells or coordinating other immune responses.

• General Mechanism of Cell-Mediated Immunity:

  • Involves the activation of T cells that directly attack infected cells or help other immune cells, enhancing the overall immune response and ensuring targeted actions against pathogens.

• Cytotoxic or Killer T Cells:

  • These cells release perforin and granzymes, which induce apoptosis in infected or dysfunctional cells, an essential mechanism for eliminating threats to the body.

• Helper T Cells:

  • Serve as regulatory units by activating B cells, cytotoxic T cells, and macrophages through the secretion of cytokines, orchestrating and amplifying the immune response.

• Function of Helper T Cells:

  • Recognize antigens presented by antigen-presenting cells (APCs), stimulating other immune cells for a comprehensive immune response.

• Artificial Immunity:

  • Obtained through medical interventions such as vaccines or immunotherapy that mimic actual infections to develop immunity without causing diseases.

• Role of Lymphokines:

  • Act as signaling molecules mediating communication between T cells and other immune cells, crucial for coordinating the immune response effectively.

• Role of Suppressor T Cells (Regulatory T Cells):

  • Downregulate the immune response post-infection clearance to prevent excessive tissue damage and maintain immune homeostasis.

• Role of Memory T Cells:

  • Persist in the body post-infection and respond rapidly upon re-exposure to the same antigen, ensuring a quicker and more effective immune response during reinfection.

Humoral Immunity

• Immunocompetency:

  • Defined as the ability of the immune system to recognize and respond to specific antigens effectively, leading to the formation of immune memory and long-term protection.

• Sensitization of B-Lymphocytes:

  • Occurs when B-lymphocytes interact with their specific antigens with the assistance of Helper T cells, resulting in clonal selection and differentiation into plasma cells that produce antigen-specific antibodies, a fundamental aspect of the humoral immune response.

• Antibodies:

  • Also known as immunoglobulins, these are glycoproteins produced by B cells that specifically bind to antigens to neutralize them or mark them for destruction by other immune cells, playing a critical role in pathogen elimination.

• Types of Antibodies:

  • IgG: The most abundant antibody in blood, crucial for long-term immunity.

  • IgA: Found in mucosal areas, providing protection in the respiratory and gastrointestinal tracts.

  • IgM: The first antibody produced in response to an infection.

  • IgE: Involved in allergic reactions and responses to parasitic infections.

  • IgD: Functions mainly in the activation of B cells.

• T Cells and B Cells Interaction with Antigens:

  • Helper T cells assist in B cell activation by recognizing antigens presented by B cells and releasing cytokines, which stimulate B cells to proliferate and differentiate into antibody-secreting plasma cells, ensuring a robust adaptive immune response.

• Goal of Vaccines:

  • To stimulate the immune system to develop long-lasting immunity against specified pathogens by invoking a primary immune response without causing the disease, leading to the creation of memory T and B cells.

• Secondary Immune Response:

  • A more rapid and robust immune response occurring upon re-exposure to previously encountered antigens due to memory cells, leading to quicker antibody production and heightened immunity.

Summary of Types of Immunity

• Categories of Immunity:

  • Immunity can be categorized into several types, essential for understanding immune responses and developing effective treatments:

    1. Natural Immunity:

       • Acquired through natural exposure to pathogens or from maternal antibodies received during pregnancy or breastfeeding.

    2. Innate Immunity:

       • Defense mechanisms present at birth providing immediate, non-specific protection against diverse pathogens.

    3. Acquired/Adaptive Immunity:

       • Develops over time through exposure to specific pathogens, leading to memory for faster and more effective future responses.

    4. Active Immunity:

       • Developed through engagement with an antigen, either from an infection or vaccination, leading to antibody production and longer-lasting defense.

    5. Passive Immunity:

       • Provi des immediate but temporary protection through the transfer of antibodies from one individual to another, such as maternal antibodies to infants, which is crucial in early life before the infant's immune system develops fully.