Comprehensive Study Notes: Immune System, Pathogens, and Public Health

IMMUNE SYSTEM OVERVIEW

Introduction

The immune system is a complex network of cells, tissues, and molecules that protects the body from pathogens and maintains homeostasis. It consists of various components, each with specialized functions in defense and regulation.

• Haematopoiesis: Formation of all blood cells, including red blood cells (RBCs), white blood cells (WBCs), and platelets.

• Plasma: The fluid portion of blood; contains proteins such as albumin, globulins, and fibrinogen.

• Mononuclear phagocyte system (MPS): Network that filters blood and lymph, houses macrophages.

• Resident Biota: Microorganisms found in skin, mouth, nasal passages, large intestine, etc., providing protection from pathogens (mutualistic relationship).

FIRST, SECOND, AND THIRD LINES OF DEFENSE

Introduction

The body employs a multi-layered defense system against pathogens, categorized into nonspecific (innate) and specific (adaptive) responses.

First Line (Nonspecific Physical Barriers)

• Includes skin, mucous membranes, tears (lysozyme), saliva, urine flow, and nasal hairs.

• Prevents pathogen entry; not antigen-specific.

Second Line (Nonspecific Cellular Defenses)

• Phagocytosis: Engulfment and destruction of microbes by neutrophils and macrophages.

• Inflammation: Redness, heat, swelling, pain; increases vascular dilation and permeability (edema).

• Fever: Inhibits microbe multiplication and enhances immune activity.

• Complement System: Blood proteins that lyse pathogens and enhance phagocytosis.

Third Line (Specific Immune Response)

• B cells → Plasma cells → Antibodies

• T cells → Cytotoxic T cells, Helper T cells, Regulatory T cells

• Develops memory and specificity for antigens.

ANTIGENS, ANTIBODIES, AND MHC

Introduction

Antigens and antibodies are central to immune recognition, while MHC molecules present antigens to immune cells.

• Antigen: Foreign molecule that triggers an immune response.

• Epitope: The part of the antigen recognized by lymphocyte receptors.

• Hapten: Small molecule that becomes antigenic when attached to a carrier.

• MHC I: Found on all nucleated cells; recognized by CD8 (cytotoxic T cells).

• MHC II: Found on antigen-presenting cells (APCs) like macrophages, dendritic cells, and B cells; recognized by CD4 (helper T cells).

ANTIBODY CLASSES

Introduction

Antibodies (immunoglobulins) are produced by plasma cells and have distinct classes with specialized functions.

Class	Key Feature	Function
IgM	First antibody made	Activates complement
IgG	Most abundant	Crosses placenta, long-term immunity
IgA	Found in secretions (saliva, tears, mucus)	Protects mucosal surfaces
IgE	Allergic responses & parasitic defense	Binds to basophils & mast cells
IgD	Receptor on B cells	Activation role

HYPERSENSITIVITY REACTIONS

Introduction

Hypersensitivity reactions are exaggerated immune responses that can cause tissue damage.

Type	Mediated by	Example	Mechanism
I	IgE	Hay fever, asthma, anaphylaxis	Mast cell degranulation
II	IgG/IgM + complement	Blood transfusion reaction	Lysis of cells
III	Immune complexes	Rheumatoid arthritis	Deposition of Ag-Ab complexes
IV	T cells	Contact dermatitis, TB test	Delayed response

AUTOIMMUNE DISEASES

Introduction

Autoimmune diseases occur when the immune system attacks self-tissues, leading to chronic inflammation and tissue damage.

Disease	Target
Systemic lupus erythematosus (SLE)	Multiple tissues
Rheumatoid arthritis	Joints
Graves' disease	Thyroid
Type I diabetes mellitus	Pancreatic β cells
Multiple sclerosis	Myelin sheath of neurons

BLOOD AND TRANSFUSIONS

Introduction

Blood transfusions require careful matching to prevent immune reactions and complications.

• Universal donor: O-

• Universal recipient: AB+

• Hemolytic disease of newborn: Mother Rh-, baby Rh+; prevented by RhoGAM.

• Transfusion mismatch: Antibodies cause agglutination and hemolysis, leading to kidney failure, shock, and death.

PATHOGENS & DISEASE

Introduction

Pathogens are microorganisms that cause disease, and their virulence determines the severity of infection.

• Opportunistic pathogen: Causes disease in immunocompromised hosts.

• Virulence: Degree of pathogenicity.

• Bacteremia: Presence (not multiplication) of bacteria in blood.

• Toxemia: Toxins in the blood.

• Reportable diseases: Must be documented by law to the CDC (reported in MMWR).

IMMUNE CELLS AT A GLANCE

Introduction

Immune cells have specialized roles in defense, inflammation, and immune regulation.

Cell Type	Function
Neutrophils	Phagocytosis; first responders
Eosinophils	Attack parasites; involved in allergy
Basophils/Mast Cells	Release histamine; inflammation
Monocytes → Macrophages	Phagocytosis and antigen presentation
Lymphocytes (B/T)	Adaptive immunity
Plasma cells	Produce antibodies (~2,000/sec)
NK cells	Kill virus-infected and cancer cells

EPIDEMIOLOGY & PUBLIC HEALTH

Introduction

Epidemiology studies disease patterns, risk factors, and public health interventions.

• Incidence rate: New cases over time.

• Prevalence: Total existing cases.

• Morbidity rate: Illness rate in a population.

• Mortality rate: Death rate.

• CDC (Centers for Disease Control and Prevention): Tracks diseases in the U.S.

• Florence Nightingale: Introduced sanitary reforms in hospitals.

VACCINES & IMMUNITY

Introduction

Vaccines stimulate the immune system to develop protection against specific pathogens.

• DNA vaccines: Introduce plasmid DNA that expresses antigenic proteins, inducing immune response.

• Four clinical trial phases: Conducted before approval.

• Primary vs. Secondary response:

  • Primary: IgM first, slower.

  • Secondary: IgG dominates; faster and stronger (anamnestic response).

EXAM SURVIVAL STRATEGY

Introduction

Effective study strategies can improve retention and understanding of microbiology concepts.

• Make flashcards for antibody classes and hypersensitivity types.

• Review the immune system flow: antigen → APC → T helper → B cells → plasma cells.

• Practice applying definitions to examples (e.g., xenograft: swine heart valve).

• Quiz in small groups using the "why" behind each correct answer.

KEY TOPICS FOR REVIEW

• MHC & immune cell functions

• Lines of defense and inflammation

• Epidemiological terms

• Autoimmune diseases