Innate and Adaptive Immunity: Lines of Defense and Immune System Components

Innate Immunity

Lines of Defense in the Human Body

The human body employs a multi-layered defense system against pathogens, organized into three main lines of defense:

• First Line of Defense: Physical and chemical barriers that prevent pathogen entry, such as skin and mucous membranes.

• Second Line of Defense: Non-specific internal responses including phagocytic cells, inflammation, fever, and antimicrobial proteins.

• Third Line of Defense: Specific, adaptive immune responses involving lymphocytes and the production of antibodies.

Functions of the Human Microbiome

• Microbial Antagonism: Normal microbiota compete with pathogens for nutrients and space, produce substances that inhibit pathogen growth, and stimulate the immune system.

• Protection Against Disease: By occupying niches, normal microbiota prevent colonization by harmful microbes.

Physical and Chemical Aspects of Skin

• Physical Barriers: The epidermis consists of tightly packed cells, with keratin providing toughness and water-resistance.

• Chemical Barriers: Sebum (oily secretion) lowers skin pH, inhibiting microbial growth; sweat contains lysozyme, which destroys bacterial cell walls.

Mucous Membranes: Physical and Chemical Protection

• Physical: Mucus traps microbes; cilia move mucus and trapped particles out of the body.

• Chemical: Secretions contain lysozyme and antimicrobial peptides.

Normal Microbiota and Microbial Antagonism

• Competitive Exclusion: Normal microbiota outcompete pathogens for resources.

• Production of Antimicrobials: Some microbiota produce bacteriocins that inhibit pathogens.

First vs. Second Line of Defense

• First Line: Non-specific, external barriers (skin, mucous membranes).

• Second Line: Non-specific, internal defenses (phagocytes, inflammation, fever, antimicrobial proteins).

• Physical Defenses: Barriers like skin and mucous membranes.

• Chemical Defenses: Substances like lysozyme, sebum, and antimicrobial peptides.

Components of Blood and Their Functions

• Plasma: Contains proteins (antibodies, complement), nutrients, and waste products.

• White Blood Cells (WBCs): Key players in immune defense.

Types of White Blood Cells and Their Roles

• Neutrophils: Phagocytose and destroy bacteria.

• Eosinophils: Combat parasitic infections and modulate allergic responses.

• Basophils: Release histamine during inflammation and allergic reactions.

• Monocytes/Macrophages: Phagocytose pathogens and present antigens to lymphocytes.

• Lymphocytes: Include B cells, T cells, and NK cells.

Role of Natural Killer (NK) Cells

• Function: NK cells recognize and destroy infected or abnormal cells by inducing apoptosis, without prior sensitization.

Complement Pathways

• Classical Pathway: Activated by antigen-antibody complexes.

• Alternative Pathway: Activated directly by pathogen surfaces.

Interferons in Innate Immunity

• Function: Interferons are cytokines that inhibit viral replication and activate immune cells.

Toll-like Receptors (TLRs)

• Function: TLRs recognize pathogen-associated molecular patterns (PAMPs) and activate innate immune responses.

Adaptive Immunity

Five Distinctive Attributes of Adaptive Immunity

• Specificity: Targets specific antigens.

• Inducibility: Activated by exposure to antigens.

• Clonality: Generates clones of activated lymphocytes.

• Unresponsiveness to Self: Does not attack the body's own cells.

• Memory: Remembers previous encounters for faster response.

Types of White Blood Cells in Adaptive Immunity

• B Lymphocytes (B cells): Responsible for antibody-mediated (humoral) immunity.

• T Lymphocytes (T cells): Responsible for cell-mediated immunity.

Divisions of Adaptive Immunity and Their Targets

• Humoral Immunity: Targets extracellular pathogens via antibodies produced by B cells.

• Cell-Mediated Immunity: Targets intracellular pathogens and abnormal cells via T cells.

Lymphatic System

• Function: Transports lymph, houses immune cells, and filters pathogens.

• Lymphatic Tissue in Small Intestine: Peyer's patches monitor intestinal bacteria and prevent pathogen growth.

• Flow of Lymph: Directed by muscle contractions and valves within lymphatic vessels.

Thymus and T Lymphocyte Development

• Importance: The thymus is essential for the maturation and differentiation of T cells.

Types of T Cells

• Helper T Cells (Th): Activate B cells and other immune cells.

• Cytotoxic T Cells (Tc): Destroy infected or abnormal cells.

• Regulatory T Cells (Treg): Suppress immune responses to prevent autoimmunity.

• Memory T Cells: Provide long-term immunity by remembering past infections.

B Lymphocyte Specificity

• Characteristic: Each B cell has unique membrane-bound antibodies (B cell receptors) that determine its antigen specificity.

Classes of Immunoglobulins (Antibodies)

• IgG: Most abundant; provides long-term protection.

• IgM: First antibody produced; effective in agglutination.

• IgA: Found in mucous secretions; protects mucosal surfaces.

• IgE: Involved in allergic responses and defense against parasites.

• IgD: Functions mainly as a B cell receptor.

Antigen-Antibody Complex Formation

• Elimination of Antigen: Complexes can neutralize toxins, agglutinate pathogens, and facilitate phagocytosis (opsonization).

Interferons in Adaptive Immunity

• Function: Interferons enhance the immune response by activating immune cells and increasing antigen presentation.

Characteristics of Effective Antigens

• Size: Larger molecules are more immunogenic.

• Complexity: Complex structures stimulate stronger responses.

• Foreignness: Non-self molecules are recognized as antigens.

Major Histocompatibility Complex (MHC) Proteins

• Class I MHC: Present on all nucleated cells; display endogenous antigens to cytotoxic T cells.

• Class II MHC: Present on antigen-presenting cells; display exogenous antigens to helper T cells.

• Role in Infection: MHC proteins alert the immune system to the presence of pathogens by presenting antigen fragments.

Line of Defense	Main Components	Type of Immunity	Examples
First	Skin, mucous membranes, normal microbiota	Innate	Physical barrier, lysozyme, sebum
Second	Phagocytes, inflammation, fever, complement, NK cells	Innate	Neutrophils, macrophages, interferons
Third	B cells, T cells, antibodies	Adaptive	IgG, cytotoxic T cells

Example: When a virus infects the body, the skin and mucous membranes act as the first barrier. If the virus bypasses these, phagocytes and NK cells attempt to destroy it. If the virus persists, B and T lymphocytes mount a specific adaptive response, producing antibodies and cytotoxic cells to eliminate the infection.

Additional info: The notes above expand on brief points by providing definitions, examples, and context for each immune system component and process, ensuring a comprehensive overview suitable for exam preparation.