Understanding the Immune System

• The immune system is a complex network of cells, tissues, and organs that work together to defend the body against harmful invaders

Types of Immunity

• Innate immunity is the first line of defense, providing rapid, non-specific responses to pathogens
• Adaptive immunity is a slower but more specific response, targeting particular pathogens and creating immunological memory
• Both innate and adaptive immunity work together to protect the body from infection

Cells of the Innate Immune System

• Phagocytes such as macrophages and neutrophils engulf and destroy pathogens
• Natural killer (NK) cells recognize and kill infected or cancerous cells
• Dendritic cells (DCs) capture antigens and present them to T cells, bridging innate and adaptive immunity

Cells of the Adaptive Immune System

• T cells mediate cellular immunity, directly killing infected cells or activating other immune cells
• B cells produce antibodies that neutralize pathogens and mark them for destruction
• Helper T cells (Th cells) coordinate the adaptive immune response by activating B cells and cytotoxic T cells

Organs of the Immune System

• Primary lymphoid organs, such as the bone marrow and thymus, are where immune cells develop and mature
• Secondary lymphoid organs, such as the lymph nodes and spleen, are where immune responses are initiated
• Lymph nodes filter lymph and provide a site for immune cells to interact with antigens
• The spleen filters blood and removes damaged or old blood cells, while also serving as a site for immune responses against bloodborne pathogens
• Mucosa-associated lymphoid tissue (MALT) is found in mucosal linings and defends against pathogens entering through mucosal surfaces

The Inflammatory Response

• Inflammation is a localized response to infection or injury, characterized by redness, swelling, heat, and pain
• Inflammatory cytokines, such as TNF-α, IL-1, and IL-6, recruit immune cells to the site of infection and promote tissue repair
• While inflammation is essential for clearing infections, excessive or chronic inflammation can lead to tissue damage and disease

Antigen Recognition

• Antigens are molecules that can be recognized by the immune system, triggering an immune response
• B cells recognize antigens through their B cell receptors (BCRs), which are membrane-bound antibodies
• T cells recognize antigens presented by antigen-presenting cells (APCs) on major histocompatibility complex (MHC) molecules
• MHC class I molecules present antigens derived from intracellular pathogens to cytotoxic T cells
• MHC class II molecules present antigens derived from extracellular pathogens to helper T cells

Activation of T Cells

• T cell activation requires two signals: antigen presentation and co-stimulation
• The T cell receptor (TCR) must bind to the MHC-antigen complex on an APC
• Co-stimulatory molecules, such as B7 on the APC and CD28 on the T cell, provide a second signal for T cell activation
• Once activated, T cells proliferate and differentiate into effector cells, such as cytotoxic T cells and helper T cells

Activation of B Cells

• B cell activation also requires two signals: antigen binding and T cell help
• The B cell receptor (BCR) must bind to its specific antigen
• Helper T cells provide a second signal by binding to the B cell and releasing cytokines
• Once activated, B cells proliferate and differentiate into plasma cells, which produce antibodies
• Some activated B cells differentiate into memory B cells, which provide long-term immunity

Antibodies

• Antibodies, also known as immunoglobulins (Ig), are glycoproteins produced by plasma cells
• Antibodies bind to antigens with high specificity, neutralizing pathogens and marking them for destruction
• There are five classes of antibodies: IgG, IgM, IgA, IgE, and IgD, each with different functions and locations in the body
• IgG is the most abundant antibody in serum and provides long-term immunity
• IgM is the first antibody produced during an immune response
• IgA is found in mucosal secretions and protects against pathogens entering through mucosal surfaces
• IgE is involved in allergic reactions and defense against parasitic worms
• IgD is found on the surface of B cells and plays a role in B cell activation

The Complement System

• The complement system is a group of serum proteins that enhance the ability of antibodies and phagocytes to clear pathogens
• Complement activation can occur through three pathways: the classical pathway, the alternative pathway, and the lectin pathway
• Complement activation leads to the formation of the membrane attack complex (MAC), which lyses pathogens
• Complement proteins also act as opsonins, marking pathogens for phagocytosis, and as chemoattractants, recruiting immune cells to the site of infection

Cytokines

• Cytokines are signaling molecules that mediate communication between immune cells
• Cytokines can be classified as interleukins (ILs), interferons (IFNs), tumor necrosis factors (TNFs), and chemokines
• IL-2 promotes T cell proliferation
• IFN-γ activates macrophages and enhances antigen presentation
• TNF-α promotes inflammation
• Chemokines recruit immune cells to the site of infection

Immunological Memory

• Immunological memory is the ability of the immune system to mount a faster and stronger response upon subsequent encounters with the same antigen
• Memory T cells and memory B cells are responsible for immunological memory
• Vaccination induces immunological memory by exposing the body to a weakened or inactive form of a pathogen
• Upon subsequent exposure to the pathogen, memory cells rapidly differentiate into effector cells, providing long-term protection

Immune Dysfunction

• Immunodeficiency disorders occur when the immune system is weakened or absent, making individuals susceptible to infections
• Autoimmune diseases occur when the immune system attacks the body's own tissues
• Hypersensitivity reactions are excessive or inappropriate immune responses to antigens
• Allergies are hypersensitivity reactions to harmless environmental antigens
• Transplantation rejection occurs when the immune system attacks transplanted organs or tissues
• Cancer can evade the immune system and grow uncontrollably