Innate (Non-Specific) Immunity

Overview of Innate Immunity

Innate immunity refers to the body's immediate, non-specific defense mechanisms against pathogens. These defenses are present at birth and do not require prior exposure to a pathogen to be activated. They act as the first line of defense and respond to a wide variety of threats.

• Surface Barriers: Physical and chemical barriers such as skin, mucous membranes, and secretions (e.g., antimicrobial proteins in saliva, tears, and gastric juice) prevent pathogen entry.

• Phagocytes: Neutrophils and macrophages ingest and destroy pathogens through phagocytosis.

• Natural Killer (NK) Cells: Lymphocytes that target and destroy virus-infected and cancerous cells.

• Complement System: A group of plasma proteins that enhance phagocytosis, inflammation, and cell lysis.

• Inflammation: A localized response to tissue injury or infection, characterized by redness, heat, swelling, and pain.

• Fever: An increase in body temperature that inhibits pathogen growth and enhances immune function.

• Interferons: Proteins released by virus-infected cells that inhibit viral replication and activate immune cells.

Phagocytosis and Inflammatory Response

Phagocytes, such as neutrophils and macrophages, are essential for engulfing and destroying pathogens. The inflammatory response is triggered by tissue damage or infection and involves several steps:

• Local Macrophage Activation: Macrophages release cytokines and chemokines to recruit other immune cells.

• Cardinal Signs of Inflammation: Redness, heat, swelling, pain, and sometimes loss of function.

• Chemotaxis: Movement of immune cells toward the site of infection in response to chemical signals.

• Diapedesis: The process by which leukocytes exit blood vessels to reach affected tissues.

Complement System

The complement system consists of plasma proteins that enhance immune responses. It can be activated via several pathways and leads to:

• Opsonization: Coating pathogens to enhance phagocytosis.

• Cell Lysis: Formation of membrane attack complexes (MAC) that destroy pathogen cell membranes.

• Inflammation: Attracting immune cells to the site of infection.

Fever and Acute Phase Response

Fever is a systemic response to infection, mediated by pyrogens such as interleukin-1 (IL-1). It increases body temperature, which can inhibit pathogen growth and enhance immune cell activity.

• Pyrogens: Substances that cause fever by acting on the hypothalamus.

• Acute Phase Proteins: Proteins produced by the liver in response to inflammation (e.g., C-reactive protein).

Interferons

Interferons are cytokines released by virus-infected cells. They interfere with viral replication and activate NK cells and macrophages.

• Type I Interferons: Produced by most cells in response to viral infection.

• Type II Interferons: Produced by immune cells, especially T cells and NK cells.

Adaptive (Specific) Immunity

Overview of Adaptive Immunity

Adaptive immunity is a specific defense mechanism that develops after exposure to particular antigens. It involves the activation of lymphocytes and the production of antibodies, providing long-term protection and immunological memory.

• Antigen: Any substance that can elicit an immune response.

• Epitope: The specific part of an antigen recognized by immune cells.

• Self vs. Non-Self: The immune system distinguishes between the body's own cells and foreign invaders.

Types of Adaptive Immunity

• Cell-Mediated Immunity: Involves T lymphocytes (T cells) that directly attack infected or abnormal cells.

• Humoral Immunity: Involves B lymphocytes (B cells) that produce antibodies to neutralize pathogens.

Antigen Presentation and Lymphocyte Activation

Antigen-presenting cells (APCs), such as dendritic cells and macrophages, process and present antigens to T cells, initiating the adaptive immune response.

• Major Histocompatibility Complex (MHC): Proteins on cell surfaces that display antigens for recognition by T cells.

• Clonal Selection: Activation and proliferation of lymphocytes specific to an encountered antigen.

Immunological Memory

Adaptive immunity provides long-term protection through the formation of memory cells. Upon re-exposure to the same antigen, the immune response is faster and more robust.

• Primary Response: Initial response to a new antigen; slower and less intense.

• Secondary Response: Rapid and strong response upon subsequent exposures due to memory cells.

Self-Tolerance and Autoimmunity

The immune system is normally tolerant to self-antigens, preventing attacks on the body's own tissues. Breakdown of self-tolerance can lead to autoimmune diseases.

• Self-Tolerance: Mechanisms that prevent immune responses against self-antigens.

• Autoimmunity: Immune responses directed against the body's own cells.

Key Terms and Definitions

• Antigen: Any molecule that can be recognized by the immune system and elicit a response.

• Epitope: The specific region of an antigen recognized by antibodies or T cell receptors.

• Clonal Selection: The process by which lymphocytes that recognize a specific antigen proliferate and differentiate.

• Immunological Memory: The ability of the immune system to respond more rapidly and effectively to pathogens that have been encountered previously.

Formulas and Equations

• Cardinal Signs of Inflammation:

• Complement Activation:

• Antigen-Antibody Binding:

Examples and Applications

• Example: Vaccination exposes the immune system to a harmless form of a pathogen, stimulating adaptive immunity and memory cell formation.

• Application: Fever and inflammation are common signs of infection and are mediated by innate immune mechanisms.

Additional info: Some explanations and definitions have been expanded for clarity and completeness based on standard Anatomy & Physiology textbooks.