Immunology

Overview of the Immune System

• Functions of the Immune System:

  • Defends the body against infectious agents, including bacteria, viruses, fungi, and parasites.

  • Eliminates damaged, infected, and precancerous cells through various mechanisms.

  • Orchestrates wound healing and tissue repair processes.

  • Regulates bodily functions and maintains homeostasis.

• Main Components:

  • Immune Cells:

  • Diverse types, including phagocytes and lymphocytes that carry out immune responses.

  • Non-cellular Molecules:

  • Include antibodies, cytokines, and complement proteins that facilitate communication between immune cells and enhance pathogen destruction.

• Recognition of Non-self vs Self:

  • The immune system utilizes specific receptors to differentiate between foreign (non-self) and the body's own (self) molecules.

  • This recognition involves complex signaling pathways to initiate appropriate immune responses without attacking healthy cells.

• Types of Immunity:

  • Mucosal Immunity:

  • Provides defense at mucosal surfaces, crucial for preventing infections at respiratory, gastrointestinal, and urogenital tracts.

  • Systemic Immunity:

  • Involves immune responses that occur throughout the body, significant for systemic infections.

Innate Immunity

• Characteristics:

  • Generalized, non-specific defense mechanism that acts quickly.

  • Immediate response to pathogens with no immunological memory to previously encountered pathogens.

• Main Types of Defense Mechanisms:

  1. Physical Barriers:

  • Skin acts as the first line of defense, while eyelashes prevent foreign particles from entering the eyes.

  1. Biochemical Defenses:

  • Enzymes found in mucus and saliva can break down bacterial cell walls; the complement system enhances opsonization and lysis of pathogens.

  1. Mechanical Processes:

  • Actions like peristalsis in the gut help expel pathogens.

  1. Cellular Defenses:

  • Immune cells like macrophages and neutrophils engulf pathogens, attack infected cells, and produce signaling molecules (cytokines) that recruit other immune cells.

Adaptive Immunity

• Characteristics:

  • Highly specific to particular antigens, mobilizing B cells (humoral immunity) and T cells (cellular immunity).

  • Has a delayed response upon first exposure to an antigen but develops immunological memory for faster responses in subsequent exposures.

  • Primary vs Secondary Response:

  • The secondary response is quicker, often stronger due to memory cells formed during the primary response.

  • Vaccinations can induce this memory effectively, enhancing long-term immunity.

• B Cells:

  • Differentiate into plasma cells that produce specific antibodies tailored to neutralize particular pathogens, and memory B cells that persist long-term for rapid responses in future infections.

• T Cells:

  • Include various subsets distinguished by surface markers, like CD4+ helper T cells that assist other immune cells and CD8+ cytotoxic T cells that directly kill infected cells.

Complement System

• Components:

  • Comprised of several serine proteases (enzymes) labeled from C1 to C9 that act in a cascade to enhance immune reactions.

• Functions:

  • Assist antibodies and phagocytic cells in clearing pathogens through various mechanisms, including the formation of the membrane attack complex (MAC: C5b-9) which disrupts pathogen membranes.

• Activation Pathways:

  1. Classic Pathway:

  • Triggered by antigen-antibody complexes that activate C1.

  1. Alternative Pathway:

  • Activated spontaneously by C3 hydrolysis, often initiated by microbial surfaces or aggregated immunoglobulins.

  1. Lectin Pathway:

  • Initiated by the binding of mannose-binding lectin to specific sugars on pathogen surfaces, leading to complement activation.

Overview of Immune Cells

• Innate Immune Cells:

  • Dendritic Cells:

  • Capture antigens and present them to T cells, acting as vital links between the innate and adaptive immunity.

  • Basophils, Eosinophils, Neutrophils:

  • Have varying life spans and functions: basophils release histamine in allergic responses, eosinophils target parasitic infections, and neutrophils are key players in phagocytosis.

  • Mast Cells:

  • Play important roles in allergic reactions and inflammation through histamine release.

  • Macrophages/Monocytes:

  • Involved in engulfing pathogens, producing cytokines, and presenting antigens to T cells.

  • Natural Killer Cells (NK Cells):

  • Part of innate immunity, recognized for their ability to kill virus-infected or cancerous cells without prior sensitization.

• Adaptive Immune Cells:

  • Plasma Cells:

  • Specialized B cells that produce large quantities of specific antibodies.

  • Memory B Cells:

  • Enable quick antibody response upon re-exposure to the same pathogen.

  • Helper T Cells (CD4+), Cytotoxic T Cells (CD8+):

  • Helper T cells coordinate the immune response, whereas cytotoxic T cells target and destroy infected or aberrant cells.

Production of Immune Cells

• Origin:

  • All immune cells are derived from pluripotent hematopoietic stem cells located in the bone marrow, where they differentiate into either lymphoid (producing T cells, B cells, NK cells) or myeloid lineage cells (including monocytes and macrophages).

• Maturing Locations:

  • Primary Organs:

  • Bone marrow is vital for B cell development, while the thymus is essential for T cell maturation.

  • Secondary Organs:

  • Include lymph nodes, spleen, and Peyer's patches, where immune responses are coordinated and executed.

Clonal Selection Theory

• Mechanism:

  • Each B and T lymphocyte possesses a unique receptor specificity established at birth.

  • Upon antigen encounter, specific B cells proliferate and differentiate into effector and memory cells.

  • This diversity in receptors is generated prior to antigen exposure, allowing a more effective and targeted immune response upon re-encounter with the antigen.

Primary vs Secondary Response

• Primary Response:

  • Characterized by a lag phase lasting 5-10 days before antibodies are detectable, with a lower peak of antibody response dominated by IgM.

• Secondary Response:

  • Significantly shorter lag phase (1-3 days), leading to a much higher peak in antibody production, predominantly IgG, which is more effective in neutralizing pathogens.

Lymphatic System

• Functions:

  1. Immunity: Filters foreign particles from lymph, circulates lymphocytes to sites of infection.

  2. Fluid Balance: Collects and returns interstitial fluid to the circulatory system, thus maintaining fluid levels in tissues.

  3. Nutrient Transport: Transports dietary fats and fat-soluble vitamins from the digestive system via lacteals (lymphatic capillaries in the intestine).

• Structure:

  • Made up of lymphoid organs (like lymph nodes and spleen), lymphatic vessels, and the lymphatic fluid that circulates throughout the body.

Antibodies (Immunoglobulins)

• Structure:

  • Y-shaped molecules consisting of two heavy chains and two light chains, interconnected by disulfide bonds, allowing for antigen binding specificity.

• Types:

  • IgM:

  • Initially produced in response to an infection, effective at agglutinating pathogens.

  • IgG:

  • Most prevalent antibody type in serum; has the longest half-life and can cross the placenta, providing passive immunity to the fetus.

  • IgA:

  • Found in mucosal areas (like saliva and tears), offering protection against pathogens at mucosal surfaces before they enter the body.

  • IgD:

  • Though its function is less understood, it may play a role in activating B cells.

  • IgE:

  • Involved in allergic reactions and defense against parasitic infections; binds to allergens and triggers histamine release.

Immune Responses to Infection

• Types of Infection Locations:

  1. Interstitial spaces, blood, and lymph where pathogens may reside freely.

  2. Epithelial surfaces where pathogens can enter the body.

  3. Cytoplasm of cells housing intracellular pathogens (such as viruses).

  4. Vesicles and other cellular compartments where pathogens may replicate or escape immune detection.

• Innate Anti-Viral Defenses:

  • Key components include pathogen-associated molecular patterns (PAMPs), pattern recognition receptors (PRRs), cytokines, and NK cells.

  • Critical signaling molecules, particularly interferons (IFN-α and IFN-β), are essential for establishing an antiviral state within neighboring unaffected cells and activating immune cells.

Comparing NK Cells and Cytotoxic T Cells

• NK Cells:

  • Function as part of innate immunity, recognizing and eliminating cells displaying reduced or abnormal MHC expression, often indicative of viral infection or malignancy.

• Cytotoxic T Cells:

  • Part of adaptive immunity, specifically recognizing peptide/MHC complexes, leading to targeted cytotoxicity against infected or neoplastic cells; capable of forming long-lasting memory cells after activation.

Activation of the Adaptive Immune Response

• If innate defenses fail to contain an infection, the adaptive response activates after several days.

  • Involves B and T cell recognition of processed antigen presented by dendritic cells and other antigen-presenting cells (APCs).

  • Epitopes:

  • Specific regions on an antigen recognized by B/T cell receptors and antibodies; for T cell activation, epitopes must be presented by Major Histocompatibility Complex (MHC) molecules, which are critical for T cell recognition in viral infections.

This expanded transcript discusses various aspects of the immune system in detail, emphasizing the structure, functions, types, responses to infections, and the roles of different immune cells. It also elaborates on the distinctions between innate and adaptive immunity, clarifying how each operates in response to pathogens while maintaining the homeostasis of the body.