Innate Immunity

Overview of Innate Immunity

Innate immunity is the body's first line of defense against pathogens, providing immediate but non-specific protection. It includes physical, chemical, and cellular barriers that prevent the entry and spread of infectious agents.

• Definition: Innate immunity refers to the non-specific defense mechanisms that come into play immediately or within hours of an antigen's appearance in the body.

• First Line of Defense: Consists of surface barriers such as skin and mucous membranes.

• Second Line of Defense: Includes internal defenses like phagocytes, inflammation, fever, and antimicrobial proteins.

Surface Barriers

Surface barriers are the body's initial protection against pathogens, preventing their entry.

• Types: Mechanical (skin, mucous membranes), chemical (enzymes, acids), and biological (normal flora).

• Examples: Skin acts as a physical barrier; lysozyme in saliva and tears destroys bacterial cell walls.

Phagocytes and Phagocytosis

Phagocytes are immune cells that engulf and destroy pathogens through the process of phagocytosis.

• Types of Phagocytes: Neutrophils, macrophages, dendritic cells.

• Phagocytosis Steps:

  1. Chemotaxis: Movement toward infection site.

  2. Adherence: Attachment to pathogen.

  3. Ingestion: Engulfment of pathogen.

  4. Digestion: Breakdown by lysosomal enzymes.

  5. Exocytosis: Expulsion of debris.

• Cytoplasmic Extensions: Pseudopods that help engulf pathogens.

• Lysosomes: Organelles containing digestive enzymes that destroy pathogens.

• Phagolysosome: Fusion of phagosome and lysosome for digestion.

• Pathogen Killing: Some pathogens resist lysosomal enzymes; additional mechanisms like respiratory burst may be used.

Inflammation

Inflammation is a localized response to infection or injury, characterized by redness, heat, swelling, and pain.

• Purpose: To contain infection, remove debris, and initiate tissue repair.

• Cardinal Signs: Redness (rubor), heat (calor), swelling (tumor), pain (dolor).

• Mechanisms: Vasodilation, increased vascular permeability, migration of leukocytes.

• Hyperemia: Increased blood flow to affected area; beneficial for delivering immune cells and nutrients.

• Exudate Formation: Fluid containing proteins and immune cells accumulates at site of inflammation.

Antimicrobial Proteins

Antimicrobial proteins such as interferons and complement enhance the innate immune response.

• Interferons (IFNs): Proteins produced by virus-infected cells; protect neighboring cells by inducing antiviral states.

• Complement System: Group of plasma proteins that enhance immune responses by promoting inflammation, opsonization, and cell lysis.

• Fever: Systemic response to infection; triggered by pyrogens (e.g., cytokines, chemicals from pathogens).

• Benefits of Fever: Inhibits pathogen growth, enhances immune cell activity.

Adaptive Immunity

Overview of Adaptive Immunity

Adaptive immunity provides specific, long-lasting protection against pathogens through the action of lymphocytes and the production of antibodies.

• Definition: Adaptive immunity is the body's ability to recognize and remember specific pathogens and mount stronger attacks on subsequent exposures.

• Cell Types: B lymphocytes (humoral immunity), T lymphocytes (cell-mediated immunity).

Antigens and Antigen Recognition

Antigens are substances that provoke an immune response. They are recognized by immune cells via specific receptors.

• Definition: An antigen is any molecule that can be recognized by the immune system and elicit an immune response.

• Examples: Pathogenic antigens (bacteria, viruses), non-pathogenic antigens (pollen, food proteins).

• Epitope: The specific part of an antigen recognized by antibodies or T cell receptors.

Major Histocompatibility Complex (MHC)

MHC proteins present antigens to T cells and are essential for immune recognition.

• MHC Class I: Found on all nucleated cells; present endogenous antigens to CD8+ cytotoxic T cells.

• MHC Class II: Found on antigen-presenting cells (APCs); present exogenous antigens to CD4+ helper T cells.

• APCs: Dendritic cells, macrophages, B cells.

T Lymphocytes

T cells are central to cell-mediated immunity, recognizing antigens presented by MHC molecules.

• Types: CD4+ Helper T cells, CD8+ Cytotoxic T cells.

• Development: Originate in bone marrow, mature in thymus.

• Selection: Positive selection ensures T cells recognize self-MHC; negative selection eliminates self-reactive T cells.

• Functions: Helper T cells activate other immune cells; cytotoxic T cells kill infected cells.

B Lymphocytes and Humoral Immunity

B cells mediate humoral immunity by producing antibodies that neutralize pathogens.

• Development: Originate and mature in bone marrow.

• Activation: Upon encountering antigen, B cells differentiate into plasma cells (antibody production) and memory B cells.

• Primary vs. Secondary Response: Primary response occurs upon first exposure; secondary response is faster and stronger due to memory cells.

Antibodies (Immunoglobulins)

Antibodies are proteins produced by B cells that specifically bind antigens and facilitate their removal.

• Structure: Y-shaped molecule with two heavy chains and two light chains.

• Binding Sites: Variable regions bind specific antigens; constant regions determine antibody class.

• Classes: IgG, IgA, IgM, IgD, IgE.

• Function: Neutralization, agglutination, opsonization, activation of complement.

Key Equations and Concepts

• Antibody-Antigen Binding:

• Complement Activation (Classical Pathway):

Additional info:

• Some context and terminology were inferred based on standard Anatomy & Physiology immunology curriculum.

• Tables and stepwise processes were reconstructed for clarity and completeness.