BackInnate and Adaptive Immunity: Study Notes
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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 prior 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 for a faster response upon re-exposure.
Example: The skin acting as a physical barrier to block pathogen entry.
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 and mucous membranes form a protective layer.
Chemical Barriers: Substances like sebum, lysozyme, and acidic secretions inhibit microbial growth.
Normal Microbiota: Compete with pathogens for nutrients and space, a process known as competitive inhibition.
Example: Lysozyme in tears breaks down bacterial cell walls.
The Body’s Second Line of Defense
If pathogens bypass the first line, the second line of defense involves cellular and molecular responses to eliminate invaders.
Formed Elements: Blood components including erythrocytes, leukocytes, and platelets.
Leukocytes: White blood cells classified as granulocytes (eosinophils, basophils, neutrophils) and agranulocytes (monocytes, lymphocytes).
Phagocytes: Cells such as neutrophils and monocytes that ingest and destroy pathogens.
Phagocytosis Stages: Chemotaxis, adherence, ingestion (formation of phagosome), fusion with lysosome (phagolysosome), digestion, and exocytosis.
Inflammation: Localized response to infection or injury, characterized by redness, heat, swelling, and pain.
Fever: Systemic increase in body temperature, often induced by pyrogens, to inhibit pathogen growth.
Complement System: A group of proteins that enhance phagocytosis, lyse pathogens, and promote inflammation.
NK Cells (Natural Killer Cells): Lymphocytes that destroy infected or abnormal cells without prior sensitization.
Interferons: Proteins released by virus-infected cells to warn neighboring cells and inhibit viral replication.
Toll-like Receptors (TLRs): Proteins on immune cells that recognize pathogen-associated molecular patterns (PAMPs) and trigger immune responses.
Acute vs. Chronic Inflammation: Acute is short-term and beneficial; chronic is prolonged and can cause tissue damage.
Example: Neutrophils rapidly migrate to sites of infection to engulf bacteria.
Classification of Innate Immunity Mechanisms
Line of Defense | Mechanism | Examples |
|---|---|---|
First | Physical Barriers | Skin, mucous membranes |
First | Chemical Barriers | Sebum, lysozyme, acidic pH |
Second | Cellular Defenses | Phagocytes, NK cells |
Second | Molecular Defenses | Complement, interferons |
Second | Physiological Responses | Inflammation, fever |
Roles of Leukocytes in Immunity
Leukocyte | Type | Function |
|---|---|---|
Neutrophil | Granulocyte | Phagocytosis of bacteria and fungi |
Eosinophil | Granulocyte | Defense against parasites; modulate allergic responses |
Basophil | Granulocyte | Release histamine in inflammation and allergic reactions |
Monocyte | Agranulocyte | Differentiate into macrophages and dendritic cells for phagocytosis |
Lymphocyte | Agranulocyte | NK cells (innate); B and T cells (adaptive) |
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 targeting extracellular pathogens.
Cell-mediated Immune Response: Involves T lymphocytes targeting infected or abnormal cells.
Example: Production of antibodies against the influenza virus after vaccination.
Elements of Adaptive Immunity
Antigen: Any substance that can induce an immune response; includes exogenous, endogenous, and autoantigens.
Major Histocompatibility Complex (MHC): Cell surface proteins essential for antigen presentation to 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.
T Cell Receptor (TCR): Recognizes antigen-MHC complexes on APCs.
B Lymphocytes: Produce antibodies and differentiate into plasma cells and memory cells.
Cytokines: Signaling proteins that regulate immune responses.
Types of T Cells
Type | Function |
|---|---|
Helper T Cells (Th) | Activate B cells and cytotoxic T cells; secrete cytokines |
Cytotoxic T Cells (Tc) | Destroy infected or abnormal cells |
Regulatory T Cells (Treg) | Suppress immune responses to prevent autoimmunity |
Antibodies (Immunoglobulins)
Antibodies are proteins produced by B cells that specifically bind to antigens. Each antibody has a variable region (Fab) for antigen binding and a constant region (Fc) for effector functions.
Class | Function | Location | Prevalence | Placental Transfer |
|---|---|---|---|---|
IgM | First antibody produced; agglutination | Blood, lymph | ~6% | No |
IgG | Main antibody in secondary response; opsonization, neutralization | Blood, extracellular fluid | ~80% | Yes |
IgA | Mucosal immunity | Secretions (tears, saliva, mucus) | ~13% | No |
IgE | Allergic responses, defense against parasites | Bound to mast cells, basophils | ~0.002% | No |
IgD | B cell receptor | B cell surface | ~1% | No |
Outcomes of Antigen–Antibody Reactions
Agglutination: Clumping of antigens for easier removal.
Opsonization: Coating of pathogens to enhance phagocytosis.
Neutralization: Blocking of pathogen attachment or toxin activity.
Activation of Complement: Leads to pathogen lysis.
Antibody-Dependent Cellular Cytotoxicity (ADCC): Target cell destruction by immune cells.
Primary vs. Secondary Immune Responses
Primary Response: First exposure to antigen; slower, mainly IgM produced.
Secondary Response: Subsequent exposures; faster and stronger, mainly IgG produced due to memory cells.
Example: Booster vaccinations elicit a secondary immune response.
Types of Acquired Immunity
Active Immunity: Results from direct exposure to antigen (natural infection or vaccination).
Passive Immunity: Transfer of antibodies from another source (maternal antibodies, immunoglobulin therapy).
Key Terms and Definitions
Phagosome: Vesicle formed around a particle engulfed by a phagocyte.
Phagolysosome: Fusion of phagosome with lysosome for digestion of pathogens.
Plasma: Liquid component of blood containing proteins, nutrients, and waste products.
Platelets: Cell fragments involved in blood clotting and inflammation.
Memory Cells: Long-lived lymphocytes that respond rapidly upon re-exposure to antigen.
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