Introduction to the Immune System

• Immunology is the study of the immune system, which defends the body against pathogens and other foreign substances
• The immune system distinguishes between self and non-self to target threats accurately

Immune System Overview

• The immune system is a complex network of cells, tissues, and organs that work together to protect the body
• Consists of two main branches: innate and adaptive immunity
• Innate immunity provides a rapid, non-specific response
• Adaptive immunity is slower but provides a specific and long-lasting defense

Innate Immunity

• The first line of defense against pathogens
• Includes physical barriers such as skin and mucous membranes, as well as cellular and molecular defenses
• Key components:
  • Phagocytes (e.g., macrophages, neutrophils) engulf and destroy pathogens
  • Natural killer (NK) cells eliminate infected or cancerous cells
  • Complement system enhances the ability of antibodies and phagocytic cells to clear microbes and damaged cells
  • Cytokines mediate inflammation and regulate immune responses
• Inflammation is a critical process in innate immunity, characterized by redness, swelling, heat, and pain

Adaptive Immunity

• A specific and long-lasting defense against pathogens
• Mediated by lymphocytes: T cells and B cells
• T cells:
  • Helper T cells (CD4+ T cells) coordinate immune responses by releasing cytokines and activating other immune cells
  • Cytotoxic T cells (CD8+ T cells) directly kill infected or cancerous cells
• B cells produce antibodies that neutralize pathogens or mark them for destruction by other immune cells
• Antigens are substances that can elicit an adaptive immune response
• The adaptive immune system exhibits immunological memory, allowing for a faster and stronger response upon re-exposure to the same antigen

Antigen-Antibody Interactions

• Antibodies, also known as immunoglobulins, are produced by B cells in response to antigens
• Antibodies bind to specific regions on antigens called epitopes
• Antigen-antibody interactions are highly specific, like a lock and key
• Antibody functions include:
  • Neutralization: preventing pathogens from infecting cells
  • Opsonization: marking pathogens for phagocytosis
  • Complement activation: triggering the complement cascade to destroy pathogens
  • Antibody-dependent cell-mediated cytotoxicity (ADCC): enhancing the killing of infected cells by NK cells
• There are five main classes of antibodies:
  • IgG: most abundant, provides long-term immunity
  • IgM: first antibody produced during an infection
  • IgA: found in mucosal secretions, protects against pathogens at mucosal surfaces
  • IgE: involved in allergic reactions and defense against parasites
  • IgD: function not fully understood, present on B cells

Vaccination and Immunization

• Vaccination is the process of introducing weakened or inactive pathogens (or their components) into the body to stimulate an immune response
• Immunization is the process by which a person becomes protected against a disease through vaccination
• Vaccines work by inducing immunological memory, so that the body can mount a rapid and effective response upon subsequent exposure to the actual pathogen
• Types of vaccines:
  • Live attenuated vaccines: weakened form of the pathogen
  • Inactivated vaccines: killed pathogens
  • Subunit vaccines: specific components of the pathogen
  • Toxoid vaccines: inactivated toxins produced by the pathogen
  • mRNA vaccines: contain genetic material that instructs cells to produce a viral protein, triggering an immune response

Autoimmune Diseases

• Autoimmune diseases occur when the immune system mistakenly attacks the body's own tissues and organs
• Can affect a variety of organs and systems
• Mechanisms: Breakdown of T cell tolerance, molecular mimicry and genetic susceptibility
• Examples of autoimmune diseases:
  • Type 1 diabetes: immune destruction of insulin-producing cells in the pancreas
  • Rheumatoid arthritis: chronic inflammation of the joints
  • Multiple sclerosis: immune destruction of the myelin sheath around nerve fibers
  • Systemic lupus erythematosus (SLE): can affect multiple organs, including the skin, joints, kidneys, and brain
  • Inflammatory bowel disease (IBD): chronic inflammation of the gastrointestinal tract
• Treatment typically involves immunosuppressive drugs to reduce the activity of the immune system

Immunodeficiency

• Occurs when the immune system is weakened or absent, making individuals more susceptible to infections
• Can be congenital (present at birth) or acquired
• Primary immunodeficiency diseases are genetic disorders that affect the development or function of immune cells
• Acquired immunodeficiency syndrome (AIDS) is caused by HIV, which infects and destroys CD4+ T cells

Hypersensitivity Reactions

• Hypersensitivity reactions are inappropriate or excessive immune responses that cause tissue damage
• Four types of hypersensitivity reactions:
  • Type I (immediate): IgE-mediated, e.g., allergies
  • Type II (cytotoxic): antibody-mediated, e.g., hemolytic anemia
  • Type III (immune complex): immune complex-mediated, e.g., serum sickness
  • Type IV (delayed-type): T cell-mediated, e.g., contact dermatitis

Immunotherapy

• Aims to treat diseases by modulating the immune system
• Types of immunotherapy:
  • Checkpoint inhibitors: block inhibitory signals on T cells, enhancing their ability to kill cancer cells
  • Adoptive cell therapy: involves isolating and modifying a patient's immune cells to enhance their ability to fight cancer
  • Cytokine therapy: uses cytokines to stimulate the immune system
  • Monoclonal antibodies: target specific proteins on cancer cells or immune cells
  • CAR T-cell therapy: genetically engineers T cells to express a receptor that targets a specific antigen on cancer cells