Immunology: Core Concepts and Mechanisms

Immunology: Core Concepts and Mechanisms

Lecture 7: Innate and Adaptive Immunity

This section introduces the fundamental differences between innate and adaptive immunity, the roles of cytokines and chemokines, and the origin and function of blood and immune cells.

• Innate vs. Adaptive Immunity: Innate immunity provides immediate, non-specific defense against pathogens, while adaptive immunity is specific, has memory, and develops more slowly.

• Cytokines and Chemokines: These are signaling proteins that mediate and regulate immunity, inflammation, and hematopoiesis. Cytokines include interleukins and interferons; chemokines direct cell movement.

• Hematopoietic System: The bone marrow is the primary site for the production of all blood cells, including immune cells.

• Blood and Immune Cells: Blood contains red blood cells, white blood cells (leukocytes), and platelets. Leukocytes include lymphocytes (B cells, T cells, NK cells), monocytes, neutrophils, eosinophils, and basophils.

• Extracellular vs. Intracellular Pathogens: Extracellular pathogens (e.g., most bacteria) live outside host cells, while intracellular pathogens (e.g., viruses) replicate within host cells.

• Activation Timing: Innate immunity acts within minutes to hours; adaptive immunity takes days to become fully activated.

Example: Neutrophils (innate) respond rapidly to bacterial infection, while B cells (adaptive) produce specific antibodies after several days.

Lecture 8: Barriers, Phagocytosis, and Complement

This section covers the physical and chemical barriers to infection, the process of phagocytosis, and the complement system's role in innate immunity.

• Physical and Chemical Barriers: The skin and mucosa act as the first line of defense, preventing pathogen entry.

• Inflammation: When barriers are breached, inflammation is triggered as an early response to infection, recruiting immune cells to the site.

• Phagocytosis: Specialized cells (e.g., macrophages, neutrophils) engulf and destroy extracellular pathogens. The process involves recognition, ingestion, and digestion of microbes.

• Complement System: A group of plasma proteins that enhance ("complement") the ability of antibodies and phagocytic cells to clear microbes. Activation can occur via classical, alternative, or lectin pathways.

• Effector Mechanisms: Complement activation leads to opsonization, inflammation, and direct lysis of pathogens.

Example: The membrane attack complex (MAC) formed by complement proteins can lyse bacterial cells.

Lecture 9: Lymphocytes and Antigen Recognition

This section explores the differences between B and T lymphocytes, their receptors, and the development and selection of immune cells.

• B Cells vs. T Cells: B cells produce antibodies; T cells mediate cellular immunity. Both have unique antigen receptors generated by gene rearrangement.

• Antigen Receptors: B cell receptors (BCRs) bind free antigens; T cell receptors (TCRs) recognize antigens presented by MHC molecules.

• Development and Selection: B and T cells undergo selection to eliminate self-reactive clones (central tolerance). Only naïve, non-self-reactive cells enter the circulation.

• Clonal Selection Theory: Each lymphocyte bears a single type of receptor with unique specificity. Upon antigen encounter, only the specific clone is activated and proliferates.

• Immunoglobulin (Ig) Structure and Function: Antibodies (Igs) are produced by B cells and have variable regions for antigen binding and constant regions for effector functions.

Example: Upon vaccination, specific B cell clones expand and produce antibodies against the vaccine antigen.

Lecture 10: Antigen Processing and Presentation

This section details how antigens are processed and presented to T cells, the role of antigen-presenting cells (APCs), and the activation of helper and cytotoxic T cells.

• Antigen Movement: Antigens travel from infection sites to lymph nodes, where immune responses are initiated.

• Dendritic Cells: These APCs capture antigens, migrate to lymph nodes, and present antigens to T cells via MHC molecules.

• MHC Molecules: MHC Class I presents intracellular antigens to CD8+ T cells; MHC Class II presents extracellular antigens to CD4+ T cells.

• Helper T Cells (CD4+): Activate B cells, macrophages, and cytotoxic T cells; secrete cytokines to coordinate immune responses.

• Cytotoxic T Cells (CD8+): Kill infected or abnormal cells by recognizing antigen-MHC I complexes.

• Immunoglobulin Production: B cells undergo class switching and affinity maturation to produce high-affinity IgG, IgM, and IgA antibodies.

• Memory Cells: Some activated B and T cells become memory cells, providing rapid and robust responses upon re-exposure to the same antigen.

Example: Dendritic cells present viral peptides on MHC I to activate cytotoxic T cells during a viral infection.

Lecture 11: Vaccination and Immune Testing

This section discusses vaccination, the difference between active and passive immunity, and laboratory methods for detecting immune responses.

• Active vs. Passive Immunity: Active immunity results from exposure to antigen (infection or vaccination); passive immunity is conferred by transfer of antibodies (e.g., maternal IgG, antiserum).

• Vaccination: Stimulates active immunity by exposing the immune system to harmless forms of pathogens or their components. Booster shots enhance and prolong immunity.

• Risks and Benefits: Vaccination is generally safe and effective, but rare adverse reactions can occur.

• Laboratory Testing: Agglutination assays and antibody titers are used to detect the presence of specific antibodies in patient samples.

• Polyclonal vs. Monoclonal Antibodies: Polyclonal antibodies recognize multiple epitopes; monoclonal antibodies are specific for a single epitope.

Example: The rapid strep test uses antibodies to detect Streptococcus antigens in throat swabs.

Lecture 12: Hypersensitivity and Immunodeficiency

This section covers the four types of hypersensitivity reactions, their mechanisms, and immunodeficiencies.

• Types of Hypersensitivity:

  • Type I (Immediate): IgE-mediated; involves mast cell degranulation (e.g., allergies, anaphylaxis).

  • Type II (Cytotoxic): IgG/IgM-mediated; targets cells for destruction (e.g., hemolytic disease of the newborn, transfusion reactions).

  • Type III (Immune Complex): Immune complex deposition causes inflammation (e.g., serum sickness, some forms of glomerulonephritis).

  • Type IV (Delayed-Type): T cell-mediated; occurs 24-72 hours after exposure (e.g., contact dermatitis, tuberculin reaction).

• Localized vs. Systemic Reactions: Localized reactions affect a specific area; systemic reactions (e.g., anaphylaxis) can be life-threatening.

• Role of IgG and Complement: In Type II hypersensitivity, IgG and complement mediate cell lysis.

• Graft Rejection: Immune responses against transplanted tissues can be hyperacute, acute, or chronic, depending on timing and mechanism.

• Immunodeficiencies: Result from defects in immune components; can be congenital or acquired (e.g., HIV/AIDS).

• Immunosuppression: Drugs like cyclosporin inhibit T cell activation to prevent graft rejection.

Example: Allergic rhinitis is a localized Type I hypersensitivity; systemic anaphylaxis is a severe, rapid-onset reaction.

Table: Types of Hypersensitivity Reactions

Type	Immune Mechanism	Examples
I (Immediate)	IgE, mast cells	Allergies, anaphylaxis
II (Cytotoxic)	IgG/IgM, complement	Hemolytic anemia, transfusion reaction
III (Immune Complex)	Immune complexes, complement	Serum sickness, glomerulonephritis
IV (Delayed-Type)	T cells (CD4+, CD8+)	Contact dermatitis, tuberculin test

Additional info: The above notes expand on brief syllabus points to provide a self-contained study guide, including definitions, mechanisms, and examples for each topic.