The Immune System: Overview

The immune system is a complex network of cells, tissues, and molecules that protects the body from disease-causing agents. It performs three major functions:

• Recognition and removal of abnormal "self" cells

• Removal of dead or damaged cells

• Protection from pathogens

Objectives of Immune System Study

• Differentiating between innate and adaptive immune responses

• Describing physical, mechanical, and chemical barriers of defense

• Identifying types and functions of leukocytes

• Explaining molecular components of the innate immune response

• Describing the structure and function of antibodies

• Discussing antigen presentation and the role of MHC molecules

• Comparing active and passive immunity

• Explaining immune responses to bacterial and viral infections

Immune System Basics

Development of Immune Cells

All immune cells originate from pluripotent hematopoietic stem cells in the bone marrow. These differentiate into two main lineages:

• Myeloid lineage: Produces granulocytes (neutrophils, eosinophils, basophils), monocytes, macrophages, dendritic cells, mast cells, and others.

• Lymphoid lineage: Produces lymphocytes (B cells, T cells, and natural killer (NK) cells).

Molecules of the Innate Immune Response

• Chemotaxins: Attract immune cells to sites of infection.

• Opsonins: Proteins that coat pathogens, enhancing phagocytosis.

• Pyrogens: Substances that induce fever.

• Acute-phase proteins: Liver proteins (e.g., C-reactive protein) that increase during inflammation.

• Complement proteins: A group of plasma proteins that enhance immune responses, including the formation of the membrane attack complex (MAC).

Antigen Presentation and Recognition Molecules

• Major Histocompatibility Complex (MHC): Membrane proteins that display antigen fragments on cell surfaces. All nucleated cells have MHC-I; antigen-presenting cells (APCs) also have MHC-II.

• Antibodies (Immunoglobulins): Proteins secreted by B lymphocytes that bind specific antigens.

• T cell receptors: Recognize antigens presented by MHC molecules.

Barriers: First Line of Defense

Physical Barriers

• Skin, mucous linings, and ciliated epithelium form the body's first line of defense.

Mechanical Barriers

• Removal of pathogens by mechanisms such as mucociliary escalator, tears, coughing, sneezing, and GI motility.

Chemical Barriers

• Lysozyme in secretions, low pH in the stomach, and antimicrobial peptides help destroy pathogens.

Layers of Immune Defense

Innate vs Adaptive Immunity

Development of Immune Cells

Immune cells develop from hematopoietic stem cells in the bone marrow. The main types include:

• Granulocytes: Neutrophils, eosinophils, basophils

• Monocytes: Circulate in blood; differentiate into macrophages in tissues

• Dendritic cells: Antigen-presenting cells in tissues

• Lymphocytes: B cells, T cells, and NK cells

Leukocytes: Types and Functions

• Neutrophils: Phagocytose bacteria and debris; most abundant WBC

• Eosinophils: Combat parasites; involved in allergic responses

• Basophils and Mast Cells: Release histamine; mediate inflammation and allergic reactions

• Monocytes/Macrophages: Phagocytose pathogens; present antigens; secrete cytokines

• Dendritic Cells: Capture antigens and present them to T cells

• Lymphocytes: B cells (produce antibodies), T cells (cell-mediated immunity), NK cells (kill infected or abnormal cells)

Antibodies (Immunoglobulins)

Antibodies are Y-shaped proteins produced by B lymphocytes. They have two main regions:

• Fab region: Binds to specific antigens

• Fc region: Determines antibody class and mediates effector functions

There are five main classes of antibodies:

Antigen Presentation and MHC Molecules

• MHC-I: Present on all nucleated cells; present endogenous antigens to cytotoxic T cells

• MHC-II: Present on antigen-presenting cells; present exogenous antigens to helper T cells

Antigen presentation is essential for activating T lymphocytes and initiating adaptive immune responses.

Active vs Passive Immunity

Immune Response to Bacterial and Viral Infections

Bacterial Infections

• Innate immunity responds first: phagocytosis by neutrophils and macrophages, inflammation, complement activation

• Adaptive immunity: B cells produce antibodies; T cells help eliminate infection

Viral Infections

• Infected cells present viral antigens via MHC-I; cytotoxic T cells destroy infected cells

• NK cells target cells lacking MHC-I

• Antibodies neutralize viruses and prevent cell entry

• Memory cells ensure faster response upon re-exposure

Allergic Responses

• Allergy: Inflammatory response to nonpathogenic antigen (allergen)

• Immediate hypersensitivity: Mediated by antibodies (often IgE); rapid onset

• Delayed hypersensitivity: Mediated by T cells and macrophages; slower onset

• Anaphylaxis: Severe, systemic allergic reaction; massive histamine release, bronchoconstriction, vasodilation, shock

Summary Table: Key Immune System Concepts