The Immune System: Innate Immunity

• The immune system is a complex network of cells, tissues, and organs that work together to defend the body against harmful invaders like bacteria, viruses, fungi, and parasites.
• It recognizes and attacks foreign substances (antigens) while distinguishing them from the body's own tissues.

Types of Immunity

• Innate immunity: The body's first line of defense, providing rapid, non-specific responses.
• Adaptive immunity: A slower but more specific response, developing immunological memory for long-term protection.

Innate Immunity

• Present from birth and does not require prior exposure to an antigen.
• Involves physical barriers like skin and mucous membranes.
• Includes cellular components such as phagocytes (neutrophils, macrophages), natural killer (NK) cells, and dendritic cells.
• Also involves chemical mediators like complement, cytokines, and acute phase proteins.

Physical Barriers

• Skin: Acts as a primary barrier, preventing pathogen entry; its acidic pH and antimicrobial substances further inhibit microbial growth.
• Mucous membranes: Line the respiratory, digestive, and urogenital tracts, trapping pathogens in mucus; cilia help expel mucus and trapped pathogens.

Cellular Components of Innate Immunity

• Phagocytes: Engulf and destroy pathogens through phagocytosis.
• Neutrophils: Most abundant white blood cells, first responders to infection.
• Macrophages: Found in tissues, they phagocytose pathogens and present antigens to T cells.
• Dendritic cells: Located in tissues, they capture antigens and migrate to lymph nodes to activate T cells.
• Natural killer (NK) cells: Recognize and kill infected or cancerous cells by releasing cytotoxic granules.

Chemical Mediators of Innate Immunity

• Complement system: A group of proteins that enhance phagocytosis, promote inflammation, and directly kill pathogens.
• Cytokines: Small signaling proteins that regulate immune responses; examples include interleukins (ILs), interferons (IFNs), and tumor necrosis factor (TNF).
• Interferons (IFNs): Inhibit viral replication and activate immune cells.
• Acute phase proteins: Produced by the liver in response to inflammation, opsonizing pathogens and activating complement.

Inflammation

• A localized response to infection or injury, characterized by redness, heat, swelling, and pain.
• Initiated by the release of inflammatory mediators like histamine, prostaglandins, and leukotrienes.
• Serves to recruit immune cells to the site of infection and promote tissue repair.

Adaptive Immunity

• Develops over time in response to exposure to antigens.
• Involves lymphocytes (T cells and B cells) that recognize specific antigens.
• Results in immunological memory, allowing for a faster and stronger response upon subsequent exposure to the same antigen.

Lymphocytes

• T cells: Mature in the thymus and mediate cellular immunity.
• B cells: Mature in the bone marrow and produce antibodies.
• Both T and B cells have antigen receptors that recognize specific antigens.

Types of T Cells

• Helper T cells (CD4+): Secrete cytokines that activate other immune cells, including B cells and cytotoxic T cells.
• Cytotoxic T cells (CD8+): Directly kill infected or cancerous cells.
• Regulatory T cells: Suppress immune responses to prevent autoimmunity.

B Cells and Antibodies

• B cells differentiate into plasma cells that produce antibodies (immunoglobulins).
• Antibodies bind to specific antigens, neutralizing them or marking them for destruction.

Antibody Classes

• IgG: Most abundant antibody in serum, provides long-term immunity.
• IgM: First antibody produced during an infection.
• IgA: Found in mucosal secretions (e.g., saliva, tears, breast milk), providing protection at mucosal surfaces.
• IgE: Involved in allergic reactions and defense against parasites.
• IgD: Found on the surface of B cells, involved in B cell activation.

Antigen Presentation

• T cells recognize antigens presented on the surface of cells by major histocompatibility complex (MHC) molecules.
• MHC class I: Presents antigens to cytotoxic T cells (CD8+).
• MHC class II: Presents antigens to helper T cells (CD4+).
• Antigen-presenting cells (APCs) like dendritic cells, macrophages, and B cells, process and present antigens to T cells.

T Cell Activation

• T cell activation requires two signals: binding of the T cell receptor to the MHC-antigen complex and co-stimulatory signals.
• Activated T cells proliferate and differentiate into effector T cells, carrying out their specific functions.

B Cell Activation

• B cell activation occurs when a B cell receptor binds to its specific antigen.
• Helper T cells provide co-stimulatory signals, promoting B cell proliferation and differentiation into plasma cells.

Immunological Memory

• After an adaptive immune response, memory T cells and memory B cells are generated.
• These cells provide long-term immunity, allowing for a faster and stronger response upon subsequent exposure to the same antigen.

Active vs. Passive Immunity

• Active immunity: Acquired through exposure to an antigen, leading to the development of immunological memory.
• Passive immunity: Acquired through the transfer of antibodies from another individual (e.g., mother to fetus) or through administration of pre-formed antibodies (e.g., antivenom).

Immune System Disorders

• Autoimmune diseases: The immune system attacks the body's own tissues (e.g., rheumatoid arthritis, lupus, type 1 diabetes).
• Immunodeficiency disorders: The immune system is weakened or absent, increasing susceptibility to infections (e.g., severe combined immunodeficiency (SCID), HIV/AIDS).
• Hypersensitivity reactions: Excessive or inappropriate immune responses that cause tissue damage (e.g., allergies, anaphylaxis).
• Allergies: Immune responses to harmless substances (allergens).
• Anaphylaxis: A severe, life-threatening allergic reaction.

Factors Affecting Immune Function

• Age: Infants and elderly individuals have weakened immune systems.
• Nutrition: Malnutrition can impair immune function.
• Stress: Chronic stress can suppress immune responses.
• Environmental factors: Exposure to toxins and pollutants can affect immune function.
• Genetic factors: Some individuals are genetically predisposed to certain immune disorders.