Immune System and Microbial Disease: Core Concepts for Microbiology

IMMUNE SYSTEM OVERVIEW

Key Components of the Immune System

The immune system is a complex network that protects the body from pathogens and maintains homeostasis. It consists of various cells, tissues, and molecules working together to identify and eliminate foreign invaders.

• 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 of cells (e.g., monocytes, macrophages) that filter blood and lymph, removing pathogens.

• 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

Physical and Cellular Barriers

The body employs multiple layers of defense against infection, ranging from physical barriers to highly specific immune responses.

• First Line (Nonspecific Physical Barriers):

  • Skin, mucous membranes, tears (contain lysozyme), saliva, urine flow, 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

Key Terms and Functions

Recognition of pathogens relies on molecular markers and specialized proteins.

• 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, B cells; recognized by CD4 (helper T cells).

ANTIBODY CLASSES

Structure, Function, and Clinical Relevance

Antibodies (immunoglobulins) are produced by plasma cells and play diverse roles in immunity.

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

Types and Mechanisms

Hypersensitivity reactions are exaggerated or inappropriate immune responses to antigens.

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

Examples and Targets

Autoimmune diseases occur when the immune system attacks self-tissues.

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

Blood Types and Transfusion Reactions

Understanding blood compatibility is essential for safe transfusions.

• Universal donor: O-

• Universal recipient: AB+

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

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

PATHOGENS & DISEASE

Definitions and Epidemiological Terms

Pathogens are organisms that cause disease, and their impact is measured using epidemiological terms.

• 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

Major Cell Types and Functions

Immune cells have specialized roles in defense and 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

Key Terms and Organizations

Epidemiology studies the distribution and determinants of disease in populations.

• 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

Types and Immune Response

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 before approval.

• Primary vs. Secondary response:

  • Primary: IgM first, slower.

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

EXAM SURVIVAL STRATEGY

Study Tips for Microbiology Exams

Effective study strategies can improve retention and understanding of immune system 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.

Additional info:

• Key terms such as agglutination, hemolysis, anamnestic response, and xenograft are important for exam preparation.

• Understanding the difference between primary and secondary immune responses is crucial for vaccine-related questions.