Immune System Defenses

Overview of Defense Systems

The human body is protected by two main types of immune defenses: innate (non-specific) and adaptive (specific). These systems work together to prevent and combat infections from microorganisms and other foreign substances.

• Innate (Non-specific) System: Responds within minutes; first line of defense against a wide range of pathogens.

• Adaptive (Specific) System: Targets specific foreign substances; slower to respond but provides long-lasting protection.

Innate (Non-specific) Defenses

Surface Barriers

Surface barriers are the body's first line of defense, preventing entry of pathogens.

• Skin and Mucous Membranes: Cover body surfaces and line cavities open to the environment (digestive, respiratory, urinary, reproductive tracts).

• Act as physical barriers and produce protective chemicals:

  • Acidic secretions: Skin, vaginal secretions, and stomach acid create hostile environments for microbes.

  • Enzymes: Saliva and tears contain lysozyme, which destroys bacteria.

  • Mucus: Traps foreign substances in respiratory and digestive tracts.

  • Small cuts or abrasions: Can allow invasion if surface barrier is breached.

Internal Defenses: Cells and Chemicals

When pathogens bypass surface barriers, internal defenses are activated.

• Phagocytes: Cells that engulf and digest pathogens.

  • Macrophages: Develop from monocytes; some are fixed in tissues, others roam freely.

  • Neutrophils: Most abundant phagocytes; especially effective against bacteria.

  • Eosinophils: Attack parasitic worms; release chemicals to digest them.

• Natural Killer (NK) Cells: Lymphocytes that target infected or cancerous cells by detecting abnormal "self" markers (lack of MHC).

• Inflammation: Tissue response to injury; prevents spread of damaging agents, disposes of pathogens, and sets the stage for repair.

  • Signs: Redness, heat, swelling, pain.

  • May cause temporary loss of function in severe cases.

• Antimicrobial Proteins:

  • Interferons: Proteins secreted by virus-infected cells to protect neighboring cells.

  • Complement Proteins: Plasma proteins that enhance inflammation and destroy pathogens.

• Fever: Systemic response; increases body temperature to inhibit pathogen growth and speed healing.

Inflammatory Chemicals and Events

Inflammatory chemicals (histamine, kinins, prostaglandins, complement) trigger vasodilation, increase blood flow, and attract immune cells to the site of injury.

• Increase permeability of capillaries, allowing immune cells and proteins to enter tissues.

• Cause swelling, pain, and sometimes loss of function.

• Some pathogens release chemicals that resist phagocytosis or promote abscess formation.

Flow Chart of Inflammation Events

• Pain and swelling due to increased blood flow and capillary permeability.

• Clotting factors form a fibrin mesh to isolate the area.

• Phagocytes arrive to remove debris and pathogens.

• Healing and repair begin after removal of harmful substances.

Adaptive (Specific) Defenses

Characteristics of Adaptive Immunity

The adaptive immune system recognizes and targets specific pathogens. It is systemic and has memory, allowing for a stronger response upon re-exposure to the same antigen.

• Antigen: Any substance recognized as "non-self" by the immune system (e.g., microorganisms, cancer cells, transplanted tissues).

• Self-Antigens (MHC Proteins): Unique protein markers on cells that help the immune system distinguish self from non-self.

Cells of the Immune System

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

• T Lymphocytes (T cells): Responsible for cell-mediated immunity (direct attack on infected cells).

Lymphocyte Maturation

Lymphocytes become immunocompetent (able to recognize a specific antigen) in primary lymphoid organs:

• B cells: Mature in bone marrow.

• T cells: Mature in thymus.

• Each lymphocyte is committed to recognize one specific antigen.

• Naive lymphocytes have not yet encountered their antigen.

• Genes determine which antigens lymphocytes can recognize.

Antigen-Presenting Cells (APCs)

APCs (macrophages, dendritic cells, B lymphocytes) engulf foreign particles and present antigen fragments on their surface to T cells, initiating the adaptive immune response.

• Process:

  1. Antigen adheres to APC.

  2. APC engulfs the particle and forms a phagocytic vesicle.

  3. Enzymes break down the antigen; fragments are displayed on the APC's surface.

  4. T cells recognize the antigen-MHC complex and become activated.

Humoral Immune Response

Immunological Memory

The humoral immune response involves the production of antibodies by B cells. It has two phases:

• Primary Immune Response: First exposure to antigen; B cells activate, clone, and differentiate into plasma cells that secrete antibodies. Response is slower and less intense.

• Secondary Immune Response: Subsequent exposure; memory cells respond rapidly and with greater intensity.

Active and Passive Humoral Immunity

• Active Immunity: Body produces antibodies in response to antigen exposure (natural infection or vaccination).

• Passive Immunity: Antibodies are received from another source (e.g., maternal antibodies, antibody injections); no antigen exposure required.

Key Terms and Definitions

• Phagocytosis: Process by which phagocytes engulf and digest pathogens.

• Antigen: Any substance that triggers an immune response.

• Antibody: Protein produced by B cells that binds to specific antigens.

• Complement: Group of plasma proteins that enhance immune responses.

• Interferon: Protein released by virus-infected cells to protect neighboring cells.

• Inflammation: Localized tissue response to injury or infection.

Table: Comparison of Innate and Adaptive Immunity

Key Equations and Processes

• Phagocytosis:

  • Engulfment:

  • Digestion:

• Antibody-Antigen Binding:

Examples and Applications

• Example: Vaccination introduces a harmless form of a pathogen, stimulating active immunity and memory cell formation.

• Application: Antibiotics target bacteria, but the immune system is required to clear viral infections through interferons and cytotoxic T cells.