Host Defenses: Overview

Three Lines of Host Defenses

The human body employs a multilevel network of defenses against microbial invasion, classified into three lines:

• First Line of Defense: Physical and chemical barriers that block invasion at the portal of entry. These are general, nonspecific protections.

• Second Line of Defense: Internalized system of protective cells and fluids, including inflammation and phagocytosis. Acts rapidly at both local and systemic levels.

• Third Line of Defense: Acquired immunity, specific to each foreign substance encountered. Involves lymphocytes and provides long-term immunity.

Example: Skin acts as a physical barrier (first line), while phagocytic cells and inflammation represent the second line, and antibody production is part of the third line.

Immune System: Organization and Function

Markers and Their Importance

Markers (antigens) are molecules on cell surfaces, typically proteins and/or sugars, that allow immune cells to distinguish between self and nonself. Recognition of these markers is essential for immune function and prevents autoimmune reactions.

• Self: Body's own cells, recognized and tolerated.

• Nonself: Foreign material, targeted for destruction.

Example: Phagocytes use markers to identify and engulf pathogens.

Body Systems Participating in Immunity

• Lymphatic System

• Circulatory System

• Mononuclear Phagocyte System (MPS)

• Associated Lymphoid Tissues

These systems work together to provide surveillance, recognition, and destruction of foreign entities.

Lymphatic System: Structure and Function

Vessels and Fluids

The lymphatic system is a compartmentalized network of vessels, cells, and specialized organs. It transports lymph through a system of vessels and nodes, eventually draining into the circulatory system.

• Lymphatic Fluid: Plasma-like liquid formed when blood components move into extracellular space and lymphatic capillaries. Transports white blood cells, fats, cellular debris, and infectious agents.

• Lymphatic Vessels: Thin-walled, accompany blood capillaries, permeable to extracellular fluid, and found throughout the body except in certain organs (e.g., CNS, bone, placenta, thymus).

Lymphatic Organs

• Primary Organs: Red bone marrow (site of blood cell production and B cell maturation), Thymus (site of T cell maturation).

• Secondary Organs: Lymph nodes, spleen, associated lymphoid tissues (e.g., tonsils, Peyer's patches).

Example: Lymph nodes filter lymph and are sites of immune cell activation.

Mononuclear Phagocyte System (MPS)

Structure and Role

The MPS is a network of connective tissue fibers and phagocytic cells (macrophages, dendritic cells) that permeate tissues and organs. It is in direct contact with tissue cells and extracellular fluid, facilitating immune surveillance and response.

• Histiocytes: Differentiated macrophages and dendritic cells residing permanently in tissues (e.g., Kupffer cells in liver, alveolar cells in lungs, Langerhans cells in skin, microglia in brain).

Blood and Immune Cells

Blood Composition

• Blood Cells: Formed elements suspended in plasma.

• Plasma: Clear, yellowish fluid.

• Serum: Plasma from clotted blood, used in immune testing.

Hematopoiesis is the process of blood cell production from stem cells in the bone marrow.

White Blood Cells (Leukocytes)

• Granulocytes: Contain granules, e.g., neutrophils.

• Agranulocytes: Lack granules, e.g., lymphocytes, monocytes.

These cells are essential for both nonspecific (innate) and specific (adaptive) immunity.

Cytokines: Cell Communication

Definition and Function

Cytokines are small, active molecules secreted by various cells to regulate, stimulate, suppress, and control aspects of cell development, inflammation, and immunity.

• Pro-inflammatory cytokines: Interleukin-1 (IL-1), Tumor necrosis factor-β (TNF-β)

• Anti-inflammatory cytokines: Interleukin-10 (IL-10)

Pathogen Recognition: PAMPs and PRRs

Definitions

• Pathogen-associated molecular patterns (PAMPs): Shared markers found on microbial surfaces, not present in mammals. Examples: peptidoglycan, lipopolysaccharide, double-stranded RNA.

• Pattern recognition receptors (PRRs): Host cell receptors that recognize and bind PAMPs, triggering innate immune responses.

Associated Lymphoid Tissues

Types and Functions

• Skin-associated lymphoid tissue (SALT): Lymphocytes beneath the skin.

• Mucosa-associated lymphoid tissue (MALT): Lymphocytes beneath mucosal surfaces.

• Gut-associated lymphoid tissue (GALT): Includes appendix, lacteals, Peyer's patches.

• Tonsils: Source of lymphocytes in the pharynx.

These tissues provide first-strike potential against microbes entering via food, air, or skin.

Concept Check

• Lymphatic fluid contains all of the following except: red blood cells.

Additional Info

• Desquamation: Shedding of skin or hair follicle cells, aiding in removal of microbes.

• Autoimmune Disorders: Result from immune system attacking self tissues due to failure in self/nonself recognition.