Innate and Adaptive Immunity: Mechanisms, Cells, and Molecular Basis

Innate and Adaptive Immunity

Basic Properties of the Immune System

The immune system protects organisms from infection through two interconnected branches: innate immunity and adaptive immunity. Innate immunity provides immediate, non-specific defense, while adaptive immunity offers specific, long-lasting protection against particular pathogens.

• Immunity: The ability of an organism to resist infection.

• Innate Immunity: Noninducible, preexisting defense mechanisms that recognize and destroy a broad range of pathogens. Rapid response (hours), no prior exposure required.

• Adaptive Immunity: Acquired ability to recognize and destroy specific pathogens or their products. Slower response (days), highly specific, and results in immune memory.

• Antigen: A molecule or molecular structure that can be recognized by the immune system, often unique to a particular pathogen.

• Phagocytes: Cells that ingest and destroy pathogens (e.g., neutrophils, macrophages, dendritic cells, eosinophils).

• Mast cells, basophils, and NK cells: Other innate immune cells involved in inflammation and destruction of compromised host cells.

• Adaptive immune cells: B lymphocytes (B cells) and T lymphocytes (T cells), which recognize specific antigens and mediate humoral and cellular immunity, respectively.

Example: If a virus infects a host, innate immunity attempts to eliminate it immediately. If unsuccessful, adaptive immunity is activated, producing a targeted response and memory for future encounters.

Barriers to Pathogen Invasion

Physical and Chemical Barriers

The first line of defense against pathogens includes physical and chemical barriers that prevent colonization and infection.

• Normal microbiota: Resident microbes on skin and mucosal surfaces outcompete pathogens for nutrients and attachment sites (competitive exclusion).

• Physical barriers: Skin, mucous membranes, tight junctions between epithelial cells, mucus, and cilia (which expel trapped microbes).

• Chemical barriers: Low pH of skin (pH 5), stomach acid (pH 2), digestive enzymes (e.g., pepsin), lysozyme in tears and secretions, and antimicrobial peptides (defensins).

• Host factors: Age, nutrition, stress, and underlying health affect susceptibility to infection.

• Tissue and species specificity: Pathogens often require specific receptors or environments to establish infection (e.g., Clostridium tetani causes disease only in wounds, not when ingested).

Example: The acidity of the stomach kills many ingested pathogens, while lysozyme in tears digests bacterial cell walls.

The Blood and Lymphatic Systems

Circulation and Immune Cell Development

Immune cells are produced in the bone marrow and gut from hematopoietic stem cells and circulate via the blood and lymphatic systems.

• Blood: Contains erythrocytes (red blood cells), leukocytes (white blood cells), and plasma (liquid component with proteins and solutes).

• Lymphatic system: Drains fluid from tissues, transports immune cells, and filters antigens through lymph nodes and mucosa-associated lymphoid tissue (MALT).

• Spleen: Filters blood, contains red pulp (RBCs, macrophages) and white pulp (lymphocytes, phagocytes).

• Secondary lymphoid organs: Lymph nodes, MALT, and spleen—sites of antigen encounter and initiation of adaptive responses.

• Serum: Fluid remaining after blood clots, rich in antibodies.

Example: Lymph nodes swell during infection due to accumulation of immune cells responding to antigens.

Leukocyte Production and Diversity

Types of Leukocytes

Leukocytes (white blood cells) are central to both innate and adaptive immunity and arise from two main lineages: myeloid and lymphoid.

• Myeloid lineage:

  • Monocytes: Differentiate into macrophages and dendritic cells (phagocytic, antigen-presenting).

  • Granulocytes: Neutrophils (abundant, rapid responders), eosinophils (target parasites), basophils and mast cells (inflammation, allergy).

• Lymphoid lineage:

  • B cells: Mature in bone marrow, produce antibodies, present antigen.

  • T cells: Mature in thymus, mediate cellular immunity.

  • Natural killer (NK) cells: Innate immunity, destroy virus-infected and tumor cells.

Example: Neutrophils migrate rapidly to infection sites, while B cells produce antibodies specific to encountered antigens.

Pathogen Challenge and Phagocyte Recruitment

Phagocyte Response to Infection

When pathogens breach barriers, phagocytes are recruited to the site of infection through chemokine signaling.

• Invasion: Pathogens must attach, multiply, and invade tissues to cause disease.

• Cytokines and chemokines: Released by activated macrophages and damaged cells, recruit neutrophils and other immune cells.

• Margination and diapedesis: Neutrophils adhere to blood vessel walls (margination) and migrate into tissues (diapedesis) in response to chemokine gradients.

Example: Chemokine CXCL8 attracts neutrophils to infection sites, amplifying the inflammatory response.

Pathogen Recognition and Phagocyte Signal Transduction

PAMPs and PRRs

Phagocytes recognize pathogens using pattern recognition receptors (PRRs) that bind pathogen-associated molecular patterns (PAMPs).

• PAMPs: Conserved microbial structures (e.g., LPS, peptidoglycan, flagellin, dsRNA) absent from host cells.

• PRRs: Receptors on/in phagocytes (e.g., Toll-like receptors, TLRs) that recognize PAMPs and trigger immune responses.

• Signal transduction: PAMP–PRR binding activates intracellular signaling cascades (e.g., via TLR-4 and MyD88), leading to gene expression of cytokines and other defense proteins.

• NOD-like receptors (NLRs): Cytoplasmic PRRs that detect intracellular PAMPs and activate inflammation via inflammasome formation.

Example: TLR-4 recognizes LPS from gram-negative bacteria, activating NF-κB and inducing proinflammatory cytokine production.

Phagocytosis and Phagocyte Inhibition

Mechanisms and Evasion

Phagocytosis involves engulfment and destruction of pathogens, but some microbes have evolved mechanisms to evade or inhibit this process.

• Phagolysosome: Fusion of phagosome (containing pathogen) with lysosome (containing toxic enzymes and reactive oxygen species) leads to pathogen destruction.

• Reactive oxygen species: Hydrogen peroxide, superoxide, hydroxyl radicals, singlet oxygen, hypochlorous acid, and nitric oxide are used to kill pathogens.

• Pathogen evasion:

  • Carotenoids (e.g., in Mycobacterium): Neutralize toxic oxygen species.

  • Leukocidins: Kill phagocytes (e.g., Streptococcus pyogenes, Staphylococcus aureus).

  • Capsules: Prevent phagocyte binding and engulfment (e.g., Streptococcus pneumoniae).

  • M protein: Surface protein that inhibits phagocytosis (e.g., S. pyogenes).

• Opsonization: Coating of pathogens with antibodies or complement proteins (C3b) enhances phagocytosis.

Example: Vaccines using capsule polysaccharides induce antibodies that promote opsonization and clearance of encapsulated bacteria.

Inflammation and Fever

Inflammatory Response

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

• Proinflammatory cytokines: IL-1, IL-6, TNF-α increase vascular permeability and recruit immune cells.

• Fever: Elevated body temperature induced by endogenous pyrogens (e.g., IL-1, IL-6, TNF-α) inhibits pathogen growth and enhances immune function.

• Systemic inflammation: If uncontrolled, can lead to septic shock (life-threatening loss of blood pressure and organ failure).

Example: Endotoxin (LPS) from gram-negative bacteria can trigger systemic inflammation and septic shock.

The Complement System

Activation Pathways and Functions

The complement system consists of plasma proteins that enhance immune responses through a cascade of activation events.

• Classical pathway: Triggered by antibodies bound to antigens; leads to formation of C3 convertase, C5 convertase, and the membrane attack complex (MAC) that lyses pathogens.

• Mannose-binding lectin (MBL) pathway: Initiated by MBL binding to bacterial polysaccharides; activates complement without antibodies.

• Alternative pathway: Triggered by spontaneous C3 hydrolysis and direct binding to pathogen surfaces; part of innate immunity.

• Opsonization: C3b coats pathogens, enhancing phagocytosis.

• Anaphylatoxins: C3a and C5a promote inflammation and recruit immune cells.

• MAC: C5b–C9 complex forms pores in pathogen membranes, causing lysis.

Example: Complement activation can lyse gram-negative bacteria and enhance phagocytosis of gram-positive bacteria via opsonization.

Adaptive Immunity: Specificity, Memory, and Tolerance

Key Features and Lymphocyte Selection

Adaptive immunity is mediated by B and T lymphocytes and is characterized by specificity, memory, and tolerance.

• Specificity: Each B or T cell recognizes a unique antigen via its receptor (BCR or TCR).

• Memory: Primary exposure generates memory cells; secondary exposure elicits a faster, stronger response.

• Tolerance: Immune system avoids attacking self-antigens through clonal deletion and anergy during lymphocyte development.

• T cell selection: Positive selection (retains cells that recognize self-MHC), negative selection (eliminates self-reactive cells).

• B cell selection: Clonal deletion of self-reactive B cells in bone marrow; clonal expansion upon antigen encounter.

Example: Vaccination induces memory B and T cells, providing long-term protection against specific pathogens.

Immunogens and Classes of Immunity

Antigen Properties and Types of Immunity

Not all antigens are equally effective at inducing immune responses. Immunogens are antigens that elicit adaptive immunity.

• Immunogen properties: Large size, molecular complexity, insolubility, and appropriate dose/route enhance immunogenicity.

• Epitope: The specific part of an antigen recognized by antibodies or TCRs.

• Active immunity: Acquired through infection (natural) or vaccination (artificial); generates memory.

• Passive immunity: Transfer of antibodies or immune cells (e.g., maternal antibodies, antiserum); no memory generated.

Example: Antivenom therapy is an example of artificial passive immunity.

Antibody Production and Structural Diversity

Antibody Classes and Functions

Antibodies (immunoglobulins) are produced by B cells and plasma cells in response to antigen exposure. They mediate pathogen neutralization, opsonization, and complement activation.

• IgG: Most abundant in serum; activates complement; crosses placenta.

• IgM: First antibody produced; pentameric; strong complement activator.

• IgA: Found in secretions (saliva, tears, breast milk); protects mucosal surfaces.

• IgE: Binds mast cells and eosinophils; involved in allergy and defense against parasites.

• IgD: Functions mainly as a B cell receptor.

• Primary response: Initial exposure, mainly IgM, slower and lower titer.

• Secondary response: Subsequent exposure, rapid and high-titer IgG or IgA (class switching), due to memory cells.

Example: Secretory IgA in breast milk protects infants from gastrointestinal infections.

Antigen Binding and Antibody Diversity

Genetic Mechanisms of Diversity

Antibody diversity is generated by somatic recombination of gene segments (V, D, J, C), junctional diversity, and somatic hypermutation.

• Variable (V) domains: Contain complementarity-determining regions (CDRs) that form the antigen-binding site.

• VDJ recombination: Random joining of V, D, and J gene segments creates unique heavy and light chains.

• Allelic exclusion: Each B cell expresses antibody genes from only one parental allele.

• Somatic hypermutation: Point mutations in V regions after antigen exposure increase affinity (affinity maturation).

• Estimated diversity: Millions of possible antibodies can be generated from limited gene segments.

Example: Affinity maturation leads to higher-affinity antibodies during secondary immune responses.

MHC Proteins and Their Functions

Antigen Presentation and T-Helper Cell Differentiation

Major histocompatibility complex (MHC) proteins present peptide antigens to T cells, guiding adaptive immune responses.

• MHC class I: Expressed on all nucleated cells; present antigens to cytotoxic T cells (CD8+).

• MHC class II: Expressed on antigen-presenting cells (B cells, macrophages, dendritic cells); present antigens to helper T cells (CD4+).

• T-helper (Th) cell subsets:

  • Th1: Activate macrophages and promote cell-mediated immunity (produce IL-2, IFN-γ, TNF-α).

  • Th2, Th17: Other subsets with distinct cytokine profiles and functions.

Example: Th1 cells stimulate macrophages to kill intracellular pathogens and can also target tumor cells.

Table: Comparison of Innate and Adaptive Immunity

Feature	Innate Immunity	Adaptive Immunity
Specificity	Non-specific (broad recognition of PAMPs)	Highly specific (unique antigen recognition)
Response Time	Immediate (hours)	Delayed (days)
Memory	None	Yes (memory cells)
Major Cells	Phagocytes, NK cells, mast cells, basophils	B cells, T cells
Key Molecules	PRRs, complement, cytokines	Antibodies, TCRs, cytokines

Key Equations and Concepts

• VDJ Recombination Diversity (approximate):

• $\text{Number of possible antibodies} = (\text{Number of heavy chain combinations}) \times (\text{Number of light chain combinations})$

• Opsonization Enhancement:

• $\text{Phagocytosis enhancement} \approx 10\text{-fold (antibody)} \times 10\text{-fold (C3b)} = 100\text{-fold}$

Additional info: Some details, such as the exact number of gene segments for antibody diversity, were inferred based on standard immunology knowledge.