Host Defense Mechanisms and Immune System Overview

Host Defense Mechanisms

Innate and Adaptive Immunity

The human body employs a multi-layered defense system to protect against infection. These mechanisms are classified as innate (natural) defenses, which are present at birth and provide nonspecific resistance, and adaptive immunities, which are specific and must be acquired through exposure.

• Innate Immunity: Nonspecific, present from birth, includes physical, chemical, and genetic barriers.

• Adaptive Immunity: Specific, develops after exposure to pathogens, involves lymphocytes and antibodies.

General Features of Host Defenses

Host defenses are organized into three lines, each with distinct characteristics and functions.

Line of Defense	Innate/Acquired	Specific or Nonspecific	Development of Immunologic Memory	Examples
First	Innate	Nonspecific	No	Physical barriers: skin, tears, coughing, sneezing Chemical barriers: low pH, lysozyme, digestive enzymes Genetic barriers: resistance inherent in host genetic makeup
Second	Innate	Mostly nonspecific	No	Phagocytosis, inflammation, fever, interferon, complement
Third	Acquired	Specific	Yes	Lymphocytes, antibodies

Defense Mechanisms of the Host

Overview of Lines of Defense

The immune system relies on a network of barriers, cells, and chemicals to prevent and eliminate infections.

• First line: Barriers that block invasion at the portal of entry; nonspecific.

• Second line: Protective cells and fluids; includes inflammation and phagocytosis; nonspecific.

• Third line: Acquired immunity; requires exposure to foreign substances; produces antibodies and memory cells; specific.

Physical or Anatomical Barriers

Role and Examples

Physical barriers are the body's first defense against pathogens, preventing their entry and colonization.

• Mucous membranes: Mucous impedes attachment and entry of bacteria.

• Cilia and nasal hair: Trap larger particles and prevent them from reaching deeper tissues.

Nonspecific Chemical Defenses

Antimicrobial Substances

Chemical defenses are produced by skin and mucous membranes to inhibit or destroy pathogens.

• Lysozyme: An enzyme that lyses the cell wall of bacteria.

• Defensins: Peptides that lyse bacteria and fungi.

• Lactic acid and electrolytes: High concentration in sweat inhibits microbial growth.

• Acidic pH of skin: Creates an inhospitable environment for many pathogens.

• Hydrochloric acid in stomach: Destroys ingested microbes.

• Digestive juices and bile: Further break down pathogens in the intestines.

• Semen: Contains antimicrobial chemicals.

• Vaginal pH: Acidic environment inhibits microbial growth.

Structure and Function of the Organs of Defense and Immunity

Immunology and Immune Functions

Immunology is the study of the body's second and third lines of defense. The immune system performs three primary functions:

1. Surveillance of the body

2. Recognition and differentiation of normal versus foreign material

3. Attack against and destruction of foreign substances

Role of White Blood Cells (Leukocytes)

White blood cells are central to immune function, with the innate ability to recognize and differentiate foreign material (nonself) from normal cells (self).

• Pathogen-associated molecular patterns (PAMPs): Molecules shared by microorganisms that signal their presence.

• Pathogen recognition receptors (PRRs): Receptors on WBCs that detect PAMPs and initiate immune responses.

Components and Connections of the Immune System

Body Compartments Involved in Immunity

Several body compartments participate in immune function, facilitating communication and transport of immune cells and molecules.

• Reticuloendothelial system (RES): Network of connective tissue fibers and phagocytic cells (macrophages) that attack microbes passing the first line of defense.

• Extracellular fluid (ECF): Fluid surrounding cells, allowing movement of immune components.

• Bloodstream: Transports immune cells and molecules throughout the body.

• Lymphatic system: Drains excess fluid and provides sites for immune cell activation.

Origin, Composition, and Functions of Blood

Blood Components

Blood consists of plasma and blood cells, each with specific roles in immunity and homeostasis.

• Serum: Liquid portion of blood after clotting; lacks clotting factors.

• Plasma: 92% water, contains proteins (e.g., globulins), chemicals, and gases.

Blood Cell Types and Hemopoiesis

Hemopoiesis (hematopoiesis) is the production of blood cells from stem cells. The primary cell lines include:

• Platelets (thrombocytes): Involved in clotting.

• Leukocytes (white blood cells): Responsible for immune functions.

Leukocyte Classification

• Granulocytes: Lobed nucleus, contain granules.

  • Neutrophils: 55-90% of circulating leukocytes; phagocytic.

  • Eosinophils: 1-3%; granules contain peroxidase, lysozyme, digestive enzymes, toxic proteins, and inflammatory mediators.

  • Basophils: 0.5%; granules contain chemical mediators that attract WBCs and cause blood vessel dilation.

  • Mast cells: Nonmotile, bound to connective tissue; function similar to basophils.

• Agranulocytes: Unlobed, rounded nucleus.

  • Lymphocytes: 20-35%;

    • B cells: Produced and matured in bone marrow; responsible for antibody production.

    • T cells: Cell-mediated immunity; control immune functions and kill foreign cells.

  • Monocytes: 3-7%; phagocytic.

    • Macrophages: Differentiated monocytes; perform specific and nonspecific phagocytosis, present antigens to lymphocytes, and release compounds to mediate immune responses.

    • Dendritic cells: Trap pathogens and participate in immune reactions.

Summary Table: Leukocyte Types and Functions

Cell Type	Percentage	Main Function
Neutrophils	55-90%	Phagocytosis of bacteria and debris
Eosinophils	1-3%	Defense against parasites; modulate allergic responses
Basophils	0.5%	Release histamine; mediate inflammation
Lymphocytes	20-35%	B cells (antibody production), T cells (cell-mediated immunity)
Monocytes/Macrophages	3-7%	Phagocytosis; antigen presentation; immune regulation

Example: Phagocytosis

Phagocytosis is a process by which certain white blood cells (e.g., neutrophils, macrophages) engulf and destroy pathogens. This is a key component of the second line of defense.

Additional info: The notes have been expanded to include definitions, examples, and logical groupings for clarity and completeness. All tables have been recreated in HTML format for study purposes.