Lymphatic and Innate Immune System

Fluid Recovery: The lymphatic system is essential for returning fluids that leaked from blood vessels back to the bloodstream, critically maintaining fluid balance in the body.
Components:
1. Network of Lymphatic Vessels (Lymphatics): Circulates lymph throughout the body.
2. Lymph: A clear, yellowish fluid that exists in lymphatic vessels, composed of interstitial fluid, proteins, and white blood cells.
3. Lymph Nodes: Act as filters for lymph, cleansing it and helping to activate the immune response.
Lymphoid Organs and Tissues:
- Provide structural support for the immune system.
- House phagocytic cells (such as macrophages) and lymphocytes (B and T cells).
- Key structures include:
- Spleen: Central component involved in blood filtration and immune response.
- Thymus: Site for T cell maturation.
- Tonsils: Guard against pathogens entering through the mouth and nose.
- Other Lymphoid Tissues: Include Peyer’s patches and lymph nodes.

Fluid Return: Lymphatic vessels return about 3 liters of interstitial fluid and leaked plasma proteins back to the bloodstream daily.
Flow Direction: Lymph flow is unidirectional, moving towards the heart.
Types of Lymphatic Vessels:
1. Lymphatic Capillaries: Specialized vessels that are highly permeable, facilitating fluid absorption.
2. Collecting Lymphatics: Similar to veins but feature thinner walls and more valves to regulate flow.
3. Lymphatic Trunks: Formed by the merging of collecting lymphatics.
4. Lymphatic Ducts: Large vessels where lymph is delivered and emptied into the venous circulation.
- Right Lymphatic Duct: Drains the right upper limb, right side of the head, and thorax.
- Thoracic Duct: Drains the rest of the body, beginning at the cisterna chyli.

Structural Features:
- Formed from overlapping endothelial cells, resembling blind-ended tubes.
- Minivalves: Allow uptake of larger particles, such as proteins and pathogens.
- Anchoring Filaments: Prevent capillary collapse by anchoring them to surrounding connective tissues.
Absence: Lymphatic capillaries are not present in avascular tissues, including bones and teeth, and central nervous system.
Specialized Capillaries — Lacteals:
- Found in the intestinal mucosa, absorb dietary fats and deliver fatty lymph (chyle) to the bloodstream.

Lymph Transport Mechanisms:
1. Skeletal Muscle Contraction: Helps push lymph through the vessels.
2. Pressure Changes During Breathing: Aids lymph flow.
3. Valves: Prevent backflow to ensure unidirectional flow.
4. Pulsation from Nearby Arteries: Assists in the movement of lymph.
5. Smooth Muscle Contractions: In the walls of larger lymphatic vessels.

Key Players in Immune Response:
- Lymphocytes: Main effector cells of the adaptive immune system, arise from red bone marrow.
- Types:
  - T Lymphocytes (T Cells): Regulate immunity and can destroy infected cells.
  - B Lymphocytes (B Cells): Produce antibodies which target toxins and pathogens.
- Macrophages: Engulf pathogens and help activate T cells.
- Dendritic Cells: Capture antigens and present them to T cells, facilitating their activation.
- Reticular Cells: Produce reticular fibers that support lymphoid organs.

Functionality: Provides sites for lymphocyte proliferation and offers vantage points for immune surveillance by lymphocytes and macrophages.
- Types of Lymphoid Tissue:
1. Diffuse Lymphoid Tissue: Scattered throughout various organs.
2. Lymphoid Follicles: Solid, spherical structures composed of packed lymphocytes and reticular fibers, which may include germinal centers rich in B cell proliferation.

Overview:
- Major lymphoid organs, located along lymphatic vessels, especially in clusters in the inguinal, axillary, and cervical regions.
Functions:
1. Filter lymph to remove pathogens and debris via macrophages.
2. Activate the immune response by acting as sites of lymphocyte activation.
Structure:
- Capsule: An outer fibrous casing.
- Cortex: Contains lymphoid follicles filled with dividing B cells.
- Medulla: Houses B cells, T cells, and plasma cells.
Lymph Circulation:
- Afferent vessels bring lymph into the node, which flows through sinuses before exiting the node via efferent vessels.

Overview:
- Largest lymphoid organ, involved in both immune function and blood filtration.
Functions:
1. Lymphocyte proliferation.
2. Cleansing blood of aged cells and platelets.
3. Storage of breakdown products of red blood cells for reuse.
4. Storage of blood platelets and monocytes.
5. Fetal erythrocyte production.
Structure:
- White Pulp: Contains lymphocytes performing immune functions.
- Red Pulp: Rich in red blood cells, functions in the disposal of aged blood cells.

Function: Critical for T cell maturation, ensuring immunocompetence.
Development: Most active during childhood; diminishes in size as one ages.
Structure:
- Contains cortex and medulla with lymphocytes and regulatory structures. No B cells present; focuses solely on T cell development.

Definition: Lymphoid tissues located in mucous membranes that protect against pathogens.
Major Sites: Tonsils, Peyer's patches, and appendix, also present in respiratory and digestive tracts.

Types:
- Palatine Tonsils: Paired masses located at the back of the throat.
- Lingual Tonsils: Located at the base of the tongue.
- Pharyngeal Tonsil: Also known as adenoids, located in the nasopharynx.
- Tubal Tonsils: Surround the openings of the auditory tubes.
Function: Gather and remove pathogens that enter through air and food; trap and destroy bacteria.

Definition: The body's resistance to disease, mediated by the immune system, divided into two types:
1. Innate Immunity: Nonspecific, immediate response to infections.
2. Adaptive Immunity: Specific response tailored to specific pathogens, taking longer to engage but providing lasting protection.

Surface Barriers: Skin and mucous membranes serve as the first line of defense.
Internal Defenses: Include phagocytes, natural killer cells, and inflammatory responses.
Phagocytosis Process:
- Attachment of phagocyte to pathogen, facilitated by opsonization (marking pathogens). Phagosome formation leads to pathogen destruction.

Function: Identify and induce apoptosis in cells lacking “self” markers, such as cancer cells or previously infected cells.
Chemical Release: Enhance the inflammatory response and assist in pathogen clearance.

Purpose: Protects against injury and pathogens by fostering healing and removing debris.
Signs of Inflammation: Redness, heat, swelling, pain, and function impairment.
Mechanism: Initiated by chemical signals (e.g., histamines) from injured tissues, leading to vascular changes and increasing blood flow to the area.

Interferons: Proteins produced in response to virus infections that signal neighboring cells to produce antiviral proteins, enhancing the overall immune response.
Complement System: Approximately 20 proteins that work together to destroy pathogens, enhance inflammation, and stimulate phagocytosis. Significantly important in both innate and adaptive responses, and interact with each other via three activation pathways: classical, lectin, and alternative.