Innate and Adaptive Immunity: Study Notes

Chapter 15: Innate Immunity

Introduction to Innate Immunity

Innate immunity is the body's first line of defense against pathogens and is present from birth. It provides immediate, non-specific protection and does not require previous exposure to a pathogen. Innate immunity includes physical, chemical, and cellular defenses that act rapidly to prevent infection.

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

• Adaptive immunity: Specific defense mechanisms that develop after exposure to antigens and involve memory cells (see Chapter 16).

The Body’s First Line of Defense

The first line of defense consists of physical and chemical barriers that prevent pathogens from entering the body.

• Physical barriers:

  • Skin: Acts as a tough, impermeable barrier; tightly packed cells and keratinized surface resist microbial invasion.

  • Mucous membranes: Line the respiratory, gastrointestinal, and genitourinary tracts; trap microbes in mucus and move them out via cilia.

• Chemical barriers:

  • Sebum: Oily substance produced by sebaceous glands; lowers skin pH and inhibits microbial growth.

  • Lysozyme: Enzyme found in tears, saliva, and mucus; breaks down bacterial cell walls.

  • Other secretions: Sweat, gastric juice, and vaginal secretions contain antimicrobial chemicals.

• Normal microbiota: Compete with pathogens for nutrients and space (competitive inhibition), produce substances harmful to pathogens, and stimulate the immune system.

The Body’s Second Line of Defense

If pathogens bypass the first line, the second line of defense involves cellular and molecular responses.

• Formed elements of blood:

  • Plasma: Fluid portion containing proteins (e.g., complement, antibodies), nutrients, and waste products.

  • Platelets: Involved in blood clotting and inflammation.

  • Leukocytes (white blood cells):

    • Granulocytes: Contain granules in cytoplasm.

      • Neutrophils: Phagocytic; most abundant; first responders to infection.

      • Eosinophils: Combat parasitic infections; involved in allergic responses.

      • Basophils: Release histamine; involved in inflammation and allergic reactions.

    • Agranulocytes: Lack visible granules.

      • Monocytes: Differentiate into macrophages and dendritic cells; phagocytic.

      • Lymphocytes: Involved in adaptive immunity (see Chapter 16).

• Phagocytosis: Process by which phagocytes (e.g., neutrophils, macrophages) ingest and destroy microbes.

  • Phagocyte: Cell that engulfs and digests pathogens.

  • Phagosome: Vesicle containing the ingested microbe.

  • Phagolysosome: Fusion of phagosome with lysosome; contains digestive enzymes.

• Natural Killer (NK) cells: Destroy infected or abnormal host cells by releasing cytotoxic granules.

• Inflammation: Localized response to infection or injury; characterized by redness, heat, swelling, and pain.

  • Acute inflammation: Short-term, beneficial response.

  • Chronic inflammation: Long-term, can cause tissue damage.

• Fever: Elevated body temperature induced by pyrogens; enhances immune responses and inhibits microbial growth.

• Interferons: Proteins produced by virus-infected cells; inhibit viral replication in neighboring cells.

• Complement system: Group of plasma proteins that enhance phagocytosis, lyse pathogens, and promote inflammation.

• Toll-like receptors (TLRs): Proteins on immune cells that recognize pathogen-associated molecular patterns (PAMPs) and trigger immune responses.

Summary Table: First and Second Lines of Defense

Defense Line	Components	Examples
First Line	Physical & Chemical Barriers	Skin, mucous membranes, sebum, lysozyme
Second Line	Cellular & Molecular Defenses	Phagocytes, NK cells, inflammation, fever, complement

Key Processes: Phagocytosis

1. Chemotaxis: Phagocyte moves toward chemical signals from microbes.

2. Adherence: Phagocyte attaches to microbe.

3. Ingestion: Microbe is engulfed into a phagosome.

4. Phagolysosome formation: Phagosome fuses with lysosome.

5. Digestion: Enzymes degrade the microbe.

6. Exocytosis: Debris is expelled from the cell.

Role of Normal Microbiota

• Compete with pathogens for nutrients and attachment sites (competitive inhibition).

• Produce substances (e.g., bacteriocins) that inhibit pathogens.

• Stimulate development of the immune system.

Chapter 16: Adaptive Immunity

Introduction to Adaptive Immunity

Adaptive immunity is a specific defense system that develops after exposure to antigens. It involves the activation of lymphocytes and the production of antibodies, providing long-lasting protection and immunological memory.

• Humoral immune response: Mediated by B lymphocytes and antibodies; targets extracellular pathogens.

• Cell-mediated immune response: Mediated by T lymphocytes; targets intracellular pathogens and abnormal cells.

Key Elements of Adaptive Immunity

• Antigen: Any substance that can trigger an immune response.

  • Exogenous antigen: Originates outside the cell (e.g., bacteria, toxins).

  • Endogenous antigen: Generated within infected or abnormal cells (e.g., viral proteins).

  • Autoantigen: Normal self-molecules that can trigger autoimmunity if recognized as foreign.

• Major histocompatibility complex (MHC): Cell surface proteins that present antigens to T cells.

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

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

• Antigen-presenting cells (APCs): Cells such as dendritic cells, macrophages, and B cells that process and present antigens to T cells.

• Epitopes: Specific regions of an antigen recognized by immune receptors.

Lymphocytes and Their Roles

• T lymphocytes (T cells):

  • Cytotoxic T cells (CD8+): Destroy infected or abnormal cells.

  • Helper T cells (CD4+): Activate B cells and other immune cells via cytokines.

  • Regulatory T cells: Suppress immune responses to prevent autoimmunity.

• B lymphocytes (B cells): Differentiate into plasma cells that secrete antibodies and memory B cells for long-term immunity.

Antibodies (Immunoglobulins)

Antibodies are proteins produced by plasma cells that specifically bind to antigens. Each antibody has a variable region (Fab) for antigen binding and a constant region (Fc) for effector functions.

• Five main classes of antibodies:

  • IgM: First antibody produced; effective in agglutination and complement activation; does not cross placenta.

  • IgG: Most abundant in serum; crosses placenta; provides long-term immunity.

  • IgA: Found in mucosal areas and secretions (e.g., saliva, breast milk); protects mucosal surfaces.

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

  • IgD: Functions mainly as a B cell receptor.

Antigen–Antibody Reactions

• Agglutination: Clumping of antigens by antibodies.

• Opsonization: Coating of pathogens to enhance phagocytosis.

• Neutralization: Blocking of pathogen attachment or toxin activity.

• Activation of complement: Leads to lysis of pathogens.

• Antibody-dependent cell-mediated cytotoxicity (ADCC): Destruction of target cells by immune cells.

Types of Acquired Immunity

• Active immunity: Results from direct exposure to antigen; long-lasting (e.g., infection, vaccination).

• Passive immunity: Transfer of antibodies from another source; temporary (e.g., maternal antibodies, antibody therapy).

Primary vs. Secondary Immune Responses

• Primary response: First exposure to antigen; slower, lower magnitude.

• Secondary response: Subsequent exposure; faster and stronger due to memory cells.

Summary Table: Classes of Antibodies

Class	Main Function	Location	Response	Prevalence	Crosses Placenta?
IgM	First response, agglutination, complement activation	Blood, lymph	Primary	~6%	No
IgG	Long-term immunity, opsonization, neutralization	Blood, extracellular fluid	Secondary	~80%	Yes
IgA	Mucosal protection	Secretions, mucous membranes	Primary/Secondary	~13%	No
IgE	Allergy, antiparasitic	Bound to mast cells, basophils	Secondary	<1%	No
IgD	B cell receptor	B cell surface	Primary	<1%	No

Example: Immune Response to a Viral Infection

• Innate immunity responds first with interferons, NK cells, and phagocytes.

• If the virus persists, adaptive immunity is activated: cytotoxic T cells destroy infected cells, and B cells produce antibodies to neutralize the virus.

Additional info: The above notes expand on the provided terminology and learning objectives, integrating standard academic explanations and logical structure for microbiology students.