Unit 4: Immunology – Structure and Function of the Immune System

Immune System Structure

Lymphatic Capillaries and Veins

The lymphatic system is a network of vessels and nodes that helps maintain fluid balance and defend the body against pathogens.

• Lymphatic capillaries: Small, thin-walled vessels that absorb excess tissue fluid (lymph) and transport it to larger lymphatic veins.

• Lymphatic veins: Carry lymph toward the heart, eventually returning it to the bloodstream.

• Function: Facilitate immune cell transport and filter pathogens.

Lymph Nodes

Lymph nodes are small, bean-shaped structures distributed throughout the lymphatic system.

• Function: Filter lymph and trap pathogens, which are then destroyed by immune cells.

• Contain: Large numbers of lymphocytes and macrophages.

• Example: Swollen lymph nodes during infection indicate active immune response.

Spleen

The spleen is an organ located in the upper left abdomen, playing a key role in immunity and blood filtration.

• Function: Removes old red blood cells, stores white blood cells, and filters pathogens from blood.

• Contains: Lymphocytes and macrophages.

Tonsils

Tonsils are lymphatic tissues located at the back of the throat.

• Function: Trap and destroy pathogens entering through the mouth or nose.

Thymus Gland

The thymus is a gland located behind the sternum, essential for immune cell development.

• Function: Site of T lymphocyte (T cell) maturation.

• Importance: Most active during childhood; shrinks with age.

First Line of Defense: Physical and Chemical Barriers

Skin and Membranes

The skin and mucous membranes act as physical barriers to prevent pathogen entry.

• Skin: Tough, impermeable layer; secretes antimicrobial substances.

• Mucous membranes: Line body cavities; trap pathogens.

Mucus and Cilia

Mucus and cilia work together to trap and remove pathogens from respiratory passages.

• Mucus: Sticky secretion that traps microbes.

• Cilia: Hair-like structures that sweep mucus and trapped pathogens out of the body.

Good (Indigenous) Bacteria

Normal flora (beneficial bacteria) occupy body surfaces and prevent colonization by harmful microbes.

• Function: Compete with pathogens for resources and produce antimicrobial substances.

Saliva and Tears

Saliva and tears contain enzymes and chemicals that destroy pathogens.

• Lysozyme: Enzyme in saliva and tears that breaks down bacterial cell walls.

Acidic pH

Acidic environments inhibit pathogen growth.

• Stomach acid: Destroys ingested microbes.

• Skin acidity: Discourages bacterial colonization.

Second Line of Defense: Inflammatory Response

Steps in the Inflammatory Response

The inflammatory response is a rapid, non-specific reaction to injury or infection.

• Step 1: Tissue damage releases chemical signals (e.g., histamine).

• Step 2: Blood vessels dilate and become more permeable, allowing immune cells to enter the affected area.

• Step 3: Phagocytes (e.g., neutrophils, macrophages) migrate to the site and engulf pathogens.

• Step 4: Area becomes red, swollen, and warm due to increased blood flow.

• Step 5: Healing begins as pathogens are eliminated and tissue is repaired.

Third Line of Defense: Adaptive Immunity

B and T Lymphocytes

B and T lymphocytes are specialized white blood cells responsible for adaptive immunity.

• B lymphocytes (B cells): Produce antibodies; mature in bone marrow.

• T lymphocytes (T cells): Attack infected cells; mature in thymus.

Lymphocyte Activation

Lymphocytes are activated when they encounter specific antigens.

• Activation: Leads to proliferation and differentiation into effector cells.

• Clonal selection: Only lymphocytes with receptors matching the antigen are activated.

Cell-Mediated Immunity

Cell-mediated immunity involves T cells directly attacking infected or abnormal cells.

• Cytotoxic T cells: Destroy infected cells.

• Helper T cells: Coordinate immune response by activating other cells.

Humoral (Acquired) Immunity

Humoral immunity is mediated by B cells and antibodies circulating in body fluids.

• Antibodies: Bind to antigens and neutralize or mark them for destruction.

• Plasma cells: B cells that produce large amounts of antibodies.

Antigens and Antibodies / Antigen-Antibody Complex

Antigens are molecules recognized as foreign by the immune system; antibodies are proteins that bind to antigens.

• Antigen-antibody complex: Formed when antibodies bind to antigens, leading to pathogen neutralization or destruction.

Primary vs. Secondary Response in Clonal Selection

The immune system responds more rapidly and effectively upon subsequent exposures to the same antigen.

• Primary response: First exposure; slower, fewer antibodies produced.

• Secondary response: Subsequent exposure; faster, stronger response due to memory cells.

• Example: Vaccination primes the immune system for a strong secondary response.

Immune System Disorders

Autoimmune Disorders

Autoimmune disorders occur when the immune system mistakenly attacks the body's own tissues.

• Examples: Rheumatoid arthritis, Type 1 diabetes, Multiple sclerosis.

• Mechanism: Loss of self-tolerance; immune cells target self-antigens.

Allergies

Allergies are exaggerated immune responses to harmless substances (allergens).

• Mechanism: Immune system produces IgE antibodies; mast cells release histamine, causing symptoms.

• Examples: Hay fever, food allergies, asthma.

Immunodeficiency

Immunodeficiency occurs when the immune system is weakened or unable to function properly.

• Examples: HIV/AIDS, congenital immunodeficiencies.

• Effects: Increased susceptibility to infections.

Summary Table: Types of Immune Responses

Type	Main Cells	Mechanism	Example
Physical/Chemical Barriers	Skin, mucous membranes	Prevent pathogen entry	Skin, stomach acid
Inflammatory Response	Phagocytes	Engulf pathogens, promote healing	Redness, swelling
Cell-Mediated Immunity	T cells	Destroy infected cells	Viral infection
Humoral Immunity	B cells, antibodies	Neutralize pathogens	Vaccination