This document, "Xirius - Physiology of the Immune System 2 - PIO201," serves as a comprehensive educational resource for the PIO201 course, focusing on the advanced physiology of the immune system. It systematically explores the intricate mechanisms by which the body defends itself against pathogens and maintains internal homeostasis. The material is structured to provide a foundational understanding of both innate and adaptive immunity, detailing their respective components, cellular players, molecular processes, and the coordinated responses they mount against various threats.

The document begins with a concise introduction to the immune system, emphasizing its critical role in host defense and the distinction between its two main branches. It then delves deeply into innate immunity, describing physical barriers, phagocytic cells, natural killer cells, inflammatory responses, fever, and antimicrobial proteins. Subsequently, it transitions to adaptive immunity, highlighting its unique characteristics such as specificity, memory, diversity, and self/non-self discrimination. A significant portion is dedicated to the cellular components of adaptive immunity—B lymphocytes and various T lymphocyte subsets (helper, cytotoxic, regulatory)—and their roles in humoral and cell-mediated responses, respectively. The crucial role of the Major Histocompatibility Complex (MHC) in antigen presentation is also thoroughly explained.

Furthermore, the document addresses the clinical relevance of immune system function by discussing various immune system disorders, including autoimmune diseases, immunodeficiencies, and hypersensitivity reactions. It also covers the principles of vaccination as a cornerstone of preventive medicine and briefly touches upon the complex interplay between stress and immune function. Overall, the document aims to equip students with a detailed understanding of immune system physiology, from basic cellular interactions to complex systemic responses and their implications for health and disease.

The immune system is a complex network of cells, tissues, and organs designed to protect the body from pathogens (bacteria, viruses, fungi, parasites) and other harmful substances. Its primary functions include distinguishing self from non-self, eliminating foreign invaders, and maintaining tissue homeostasis. The immune system is broadly divided into two main branches:

Innate immunity represents the body's immediate, non-specific defense system, offering rapid protection against a broad spectrum of pathogens without requiring prior exposure.

- Physical and Chemical Barriers:

- Skin: The epidermis forms a robust physical barrier with tight junctions, while the dermis contains collagen and phagocytes.

- Mucous Membranes: Line the respiratory, digestive, and urogenital tracts, producing mucus that traps pathogens.

- Cilia: In the respiratory tract, cilia actively sweep mucus and trapped pathogens out.

- Chemical Secretions: Include lysozyme (in tears, saliva), gastric acid (stomach), defensins (antimicrobial peptides), and fatty acids (skin).

- Phagocytic Cells: These cells engulf and digest pathogens.

- Neutrophils: The most abundant white blood cells (WBCs), they are rapid first responders, short-lived, and contribute to pus formation.

- Macrophages: Develop from monocytes, are long-lived, act as antigen-presenting cells (APCs), and clear cellular debris.

- Dendritic Cells: Highly efficient APCs that bridge innate and adaptive immunity.

- Natural Killer (NK) Cells: These are lymphocytes that can kill virus-infected cells and tumor cells without prior sensitization, primarily by recognizing the absence of MHC class I molecules on target cells.

- Inflammation: A localized tissue response to injury or infection, characterized by:

- Cardinal Signs: Redness (rubor), heat (calor), swelling (tumor), pain (dolor), and loss of function (functio laesa).

- Process: Involves vasodilation, increased vascular permeability, recruitment of leukocytes (chemotaxis), and phagocytosis.

- Mediators: Histamine, prostaglandins, leukotrienes, and various cytokines.

- Fever: A systemic increase in body temperature.

- Mechanism: Triggered by pyrogens (exogenous like LPS, endogenous like IL-1, TNF-α) acting on the hypothalamus.

- Benefits: Inhibits microbial growth and enhances immune cell activity.

- Antimicrobial Proteins:

- Complement System: A group of plasma proteins that activate in a cascade.

- Functions: Opsonization (enhancing phagocytosis), promoting inflammation (via C3a, C5a), and direct cell lysis through the Membrane Attack Complex (MAC: C5b-C9).

- Pathways: Classical (activated by antibody-antigen complexes), Alternative (activated directly by pathogen surfaces), and Lectin (activated by mannose-binding lectin).

- Interferons (IFNs): Cytokines produced by virus-infected cells.

- Types: IFN-α and IFN-β induce an antiviral state in neighboring cells, while IFN-γ activates macrophages and enhances MHC expression.

Adaptive immunity is a highly specific defense system that targets particular pathogens and develops immunological memory for future encounters.

- Specificity: Recognizes and targets specific antigens.

- Memory: Remembers previous encounters, leading to faster and stronger secondary responses.

- Diversity: Capable of recognizing an immense array of antigens.

- Self/Non-self Discrimination: Normally avoids attacking host tissues.

- B Lymphocytes (B cells):

- Origin/Maturation: Originate and mature in the bone marrow.

- Function: Mediate humoral immunity. Upon activation, they differentiate into plasma cells (which produce antibodies) and memory B cells.

- B Cell Receptor (BCR): A surface antibody that binds specific antigens.

- T Lymphocytes (T cells):

- Origin/Maturation: Originate in the bone marrow and mature in the thymus.

- Function: Mediate cell-mediated immunity. They recognize antigens presented by MHC molecules.

- T Cell Receptor (TCR): Binds specific antigen-MHC complexes.

- Subsets:

- Helper T cells (CD4+ T cells): Recognize antigens presented by MHC class II. They secrete cytokines to activate B cells, cytotoxic T cells, and macrophages.

- Cytotoxic T lymphocytes (CTLs or CD8+ T cells): Recognize antigens presented by MHC class I. They directly kill infected cells and tumor cells.

- Regulatory T cells (Tregs): Suppress immune responses to prevent autoimmunity and excessive inflammation.

This branch of adaptive immunity is mediated by B cells and antibodies.

1. A B cell encounters its specific antigen and becomes activated (often with the help of T helper cells).

2. The activated B cell differentiates into plasma cells (antibody factories) and memory B cells.

3. Plasma cells secrete antibodies into the blood and lymph.

- Structure: Composed of two heavy chains and two light chains, linked by disulfide bonds. They have a variable region (for antigen binding) and a constant region (for effector functions).

- Classes (Isotypes):

- IgG: Most abundant, crosses the placenta, long-lived, involved in opsonization, neutralization, and complement activation.

- IgM: A pentamer, it is the first antibody produced in a primary immune response and is a potent complement activator.

- IgA: A dimer, found in secretions (mucus, tears, saliva, breast milk), crucial for mucosal immunity.

- IgE: Involved in allergic reactions and defense against parasites, binding to mast cells and basophils.

- IgD: Primarily functions as a B cell receptor; its full role is still being researched.

- Functions:

- Neutralization: Blocks pathogens from binding to host cells.

- Opsonization: Coats pathogens, marking them for enhanced phagocytosis.

- Agglutination: Clumps pathogens together, making them easier to clear.

- Complement Activation: Triggers the complement cascade.

- Antibody-Dependent Cell-mediated Cytotoxicity (ADCC): NK cells recognize and kill antibody-coated cells.

This branch is primarily mediated by T cells, especially cytotoxic T lymphocytes (CTLs).

1. Antigen-presenting cells (APCs) present antigens via MHC molecules to T cells.

2. Helper T cells activate other immune cells (e.g., B cells, macrophages, CTLs).

3. Cytotoxic T cells directly kill infected or cancerous cells.

The MHC is a set of genes encoding cell surface proteins that are crucial for antigen presentation to T cells.

- MHC Class I:

- Location: Found on almost all nucleated cells.

- Function: Presents endogenous antigens (peptides derived from intracellular pathogens or abnormal self-proteins) to CD8+ cytotoxic T cells.

- Structure: Composed of an alpha chain and beta-2 microglobulin.

- MHC Class II:

- Location: Primarily found on professional antigen-presenting cells (APCs) such as macrophages, dendritic cells, and B cells.

- Function: Presents exogenous antigens (peptides derived from extracellular pathogens) to CD4+ helper T cells.

- Structure: Composed of alpha and beta chains.

Dysfunction of the immune system can lead to various disorders:

- Primary (Congenital): Result from genetic defects (e.g., Severe Combined Immunodeficiency - SCID).

- Secondary (Acquired): Caused by external factors (e.g., HIV/AIDS, malnutrition, chemotherapy).

- Type I (Immediate/Anaphylactic): IgE-mediated, involving mast cell degranulation (e.g., hay fever, asthma, anaphylaxis).

- Type II (Cytotoxic): IgG/IgM-mediated, targets cell surface antigens, leading to cell lysis (e.g., transfusion reactions, Rh incompatibility).

- Type III (Immune Complex): IgG/IgM-mediated, soluble antigen-antibody complexes deposit in tissues, causing inflammation (e.g., serum sickness, lupus nephritis).

- Type IV (Delayed-Type Hypersensitivity - DTH): T