Immune System

Immune System

Overview of the Immune System

  • The immune system provides resistance to disease.

  • Composed of two intrinsic systems:

    • Innate (nonspecific) defense system:

    • Constitutes the 1st and 2nd lines of defense.

    • Adaptive (specific) defense system:

    • Constitutes the 3rd line of defense.

Innate (Nonspecific) Defense System

First Line of Defense: External Body Membranes

  • Surface Barriers: Skin and mucous membranes, along with their secretions.

    • Functions:

    • Acts as a physical barrier to most microorganisms.

    • Keratin:

      • Resistant to weak acids and bases, bacterial enzymes, and toxins.

    • Mucosa provides similar mechanical barriers (e.g., gastric juice, lacrimal secretion).

Second Line of Defense: Cellular and Chemical Defense

  • Components:

    • Phagocytes

    • Natural Killer (NK) cells

    • Inflammatory response

    • Antimicrobial proteins

    • Fever

Phagocytosis

  • Steps in Phagocytosis:

    1. Recognition and Adhesion:

    • Pathogen's antigen (Ag) is recognized; process termed opsonization.

    1. Engulfing:

    • Pseudopods engulf the particle, forming a phagosome (vesicle).

    1. Fusion with Lysosome:

    • Phagosome fuses with lysosome, creating a phagolysosome.

    1. Acidification and Digestion:

    • Acidification of the phagolysosome and enzymatic digestion occurs.

    1. Exocytosis:

    • Residual materials are expelled by exocytosis from the phagocyte.

Natural Killer (NK) Cells

  • Mechanism of Action:

    • Induces apoptosis in cancer cells and virus-infected cells.

    • Capable of secreting potent chemicals that enhance the inflammatory response.

Inflammation

  • Definition:

    • Inflammation is the body's response to tissue injury.

  • Benefits of Inflammation:

    • Prevents the spread of damaging agents.

    • Disposes of cell debris and pathogens.

    • Alerts adaptive immune system.

    • Sets the stage for tissue repair.

Stages of Inflammation

  1. Chemical Signal Release:

    • Tissue injury triggers the release of chemical signals, such as histamine.

  2. Vasodilation and Increased Vascular Permeability:

    • Dilation and increased leakiness of local blood vessels occur; phagocyte migration to the area.

  3. Phagocyte Activation:

    • Phagocytes (macrophages and neutrophils) consume bacteria and cell debris; tissue healing begins.

  4. Phagocyte Mobilization:

    • Inflammatory chemicals act as chemotactic agents; affect leukocyte movement.

    • Phagocyte Recruitment: Neutrophils follow the chemical trail.

    • Leukocytosis: Neutrophils enter blood from bone marrow.

    • Margination: Neutrophils adhere to capillary walls.

    • Diapedesis: Neutrophils flatten and squeeze out of capillaries.

Antimicrobial Proteins

  • Functions:

    • Enhance innate defense by:

    • Attacking microorganisms directly.

    • Hindering microorganisms’ ability to reproduce.

  • Important Antimicrobial Proteins:

    • Interferons

    • Complement proteins

Fever

  • Definition:

    • A systemic response to invading microorganisms.

  • Mechanism:

    • Leukocytes and macrophages secrete pyrogens that act on the hypothalamus, raising body temperature.

  • Benefits of Moderate Fever:

    • Causes the liver and spleen to sequester iron and zinc, essential nutrients for microorganisms.

    • Increases metabolic rate, enhancing repair processes.

  • Normal Body Temperatures:

    • Adults: 97-99 °F

    • Babies and children: 97.9-100.4 °F

Adaptive (Specific) Defense System

Characteristics of Adaptive Immunity

  • Specificity:

    • Recognizes and targets specific antigens.

  • Systemic Nature:

    • Not restricted to the initial infection site.

  • Memory:

    • Mounts a stronger attack on “known” antigens during subsequent exposures.

  • Branches of Adaptive Immunity:

    • Humoral (antibody-mediated) immunity

    • Cellular (cell-mediated) immunity

Humoral Immunity

  • Mechanism:

    • Antibodies, produced by lymphocytes, circulate freely in body fluids.

    • They bind to target cells temporarily to:

    • Inactivate the target.

    • Mark for destruction by phagocytes or complement.

  • Targets:

    • Humoral immunity has extracellular targets (e.g., bacteria, fungi, parasites).

Types of Humoral Immunity

Type

Naturally Acquired

Artificially Acquired

Active

Infection; contact with pathogen

Vaccine; dead or attenuated pathogen

Passive

Antibodies passed from mother to fetus

Injection of exogenous antibodies (gamma globulin)

Cellular Immunity

  • Mechanism:

    • Lymphocytes target cells directly by killing infected cells and indirectly by:

    • Releasing chemicals that enhance inflammatory response.

    • Activating other lymphocytes or macrophages.

  • Targets:

    • Cellular immunity has cellular targets (e.g., virus-infected cells, cancer cells).

Overview of B and T Lymphocytes

Type of Lymphocyte

Type of Immune Response

Antibody Secretion

Primary Targets

Site of Origin

Site of Maturation

Effector Cells

B Lymphocytes

Humoral

Yes

Extracellular pathogens (e.g., bacteria, fungi, parasites)

Red bone marrow

Red bone marrow

Plasma cells

T Lymphocytes

Cellular

No

Intracellular pathogens (e.g., virus-infected cells, cancer cells)

Red bone marrow

Thymus

Cytotoxic T cells, Helper T cells, Regulatory T cells

Helper T (TH) Cells

  • Function:

    • Play a central role in the adaptive immune response.

    • Activate both humoral and cellular immunity.

    • After being primed by antigen-presenting cells (APC) presentation of antigen, helper T cells:

    • Aid in activating B cells and other T cells.

    • Induce T and B cell proliferation.

    • Secrete cytokines that recruit other immune cells.

  • Significance:

    • Without Helper T cells, there is no immune response.

Summary of Lines of Defense

  • 1st Line: Non-specific barriers

    • Broad, external defense (walls and moats): Skin and mucous membranes.

  • 2nd Line: Non-specific patrols

    • Broad, internal defense (patrolling soldiers): Leukocytes/phagocytic WBCs.

  • 3rd Line: True immune system

    • Specific, acquired immunity (elite trained units): Lymphocytes and antibodies (B cells & T cells).

Questions?

  • Invite any questions or clarifications regarding the immune system.