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 for a faster response upon re-exposure.

• Example: The skin acting as a barrier to prevent microbial entry is an example of innate immunity.

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 Factors:

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

  • Mucous Membranes: Line the respiratory, gastrointestinal, and genitourinary tracts; trap and remove microbes.

  • Lacrimal Apparatus: Produces tears that wash away microbes from the eyes.

• Chemical Factors:

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

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

  • Acidity: Low pH of skin, gastric juice, and vaginal secretions inhibits microbial growth.

• Role of 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 of defense, the second line involves cellular and chemical responses to eliminate invaders.

• Formed Elements of Blood:

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

  • Leukocytes (White Blood Cells): Key players in innate immunity, classified as granulocytes and agranulocytes.

  • Platelets: Involved in blood clotting and inflammation.

• Granulocytes:

  • Neutrophils: Phagocytic cells that ingest and destroy microbes.

  • Eosinophils: Combat parasitic infections and participate in allergic responses.

  • Basophils: Release histamine and other mediators of inflammation.

• Agranulocytes:

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

  • Lymphocytes: Include natural killer (NK) cells, which destroy infected or abnormal cells.

• Phagocytosis: The process by which phagocytes ingest and destroy pathogens.

  • Stages: Chemotaxis, adherence, ingestion (formation of phagosome), fusion with lysosome (forming phagolysosome), digestion, and exocytosis.

• Inflammation: Localized response to infection or injury, characterized by redness, heat, swelling, and pain. Can be acute or chronic.

• Fever: Systemic response to infection; elevated body temperature inhibits microbial growth and enhances immune responses. Induced by pyrogens.

• Complement System: A group of plasma proteins that enhance phagocytosis, lyse pathogens, and promote inflammation.

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

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

Table: Leukocytes and Their Roles in Immunity

Leukocyte Type	Main Function	Granulocyte/Agranulocyte
Neutrophil	Phagocytosis of bacteria and fungi	Granulocyte
Eosinophil	Defense against parasites; allergic reactions	Granulocyte
Basophil	Release histamine; inflammation	Granulocyte
Monocyte	Differentiate into macrophages; phagocytosis	Agranulocyte
Lymphocyte (NK cell)	Destruction of virus-infected and tumor cells	Agranulocyte

Chapter 16: Adaptive Immunity

Introduction to Adaptive Immunity

Adaptive immunity is a specific immune response 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 the production of antibodies that target extracellular pathogens.

• Cell-Mediated Immune Response: Involves T lymphocytes that target infected or abnormal cells.

Elements of Adaptive Immunity

• Antigen: Any substance that can trigger an immune response. Includes exogenous (external), endogenous (internal), and autoantigens (self-antigens).

• 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 cells.

Cells of Adaptive Immunity

• B Lymphocytes (B cells): Produce antibodies; differentiate into plasma cells and memory B cells.

• T Lymphocytes (T cells):

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

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

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

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:

Class	Main 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	Mucous, saliva, tears, breast milk	~13%	No
IgE	Allergic responses, defense against parasites	Bound to mast cells, basophils	<1%	No
IgD	B cell receptor	B cell surface	<1%	No

Antigen–Antibody Reactions

• Neutralization: Antibodies block pathogen attachment or toxin activity.

• Agglutination: Antibodies cause pathogens to clump together for easier removal.

• Opsonization: Antibodies enhance phagocytosis by marking pathogens.

• Activation of Complement: Antibody binding triggers the complement cascade, leading to pathogen lysis.

• Antibody-Dependent Cellular Cytotoxicity (ADCC): Antibodies recruit immune cells to destroy target cells.

Primary vs. Secondary Immune Responses

• Primary Response: Occurs upon first exposure to an antigen; slower and produces mainly IgM.

• Secondary Response: Occurs upon subsequent exposures; faster, stronger, and mainly produces IgG due to memory cells.

Types of Acquired Immunity

• Active Immunity: Results from direct exposure to antigen (natural infection or vaccination); long-lasting.

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

Key Terms and Definitions

• Antigen: Substance that induces an immune response.

• Epitope: Specific part of an antigen recognized by immune cells.

• Cytokines: Signaling proteins that regulate immune responses.

• Memory Cells: Long-lived lymphocytes that respond rapidly upon re-exposure to an antigen.

Summary Table: Comparison of Innate and Adaptive Immunity

Feature	Innate Immunity	Adaptive Immunity
Specificity	Non-specific	Highly specific
Memory	None	Present
Response Time	Immediate	Delayed (days)
Main Components	Barriers, phagocytes, complement, NK cells	B and T lymphocytes, antibodies

Additional info: Where content was brief or only terminology was provided, academic context and definitions were added for completeness and clarity.