Immune system

Immune System Overview

• Pathogen Definition: A pathogen is a disease-causing agent that includes harmful organisms like bacteria and viruses.

• Immune System Composition: The immune system comprises cells and tissues located throughout the body.

• Defense Mechanisms: The body employs both nonspecific and specific defense mechanisms to identify and eliminate pathogens, helping to prevent or lessen infections.

Common Infectious Diseases and Their Pathogens

Blood and Lymphatic System

• Role in Defense: Blood and lymph tissues function together as part of the mammalian defense system, interacting with various tissues and organs.

• Components: Lymphoid tissues include the thymus, bone marrow, spleen, and lymph nodes. Blood and lymph are composed of water, solutes, and cells.

Blood

• Composition of Blood Plasma: Yellowish solution consisting of ions, small molecular solutes, and soluble proteins. Blood contains red blood cells, white blood cells, and platelets.

Lymph

• Definition and Function: Lymph is a fluid derived from blood that accumulates in intercellular spaces and moves into lymphatic vessels, eventually returning to the circulatory system.

• Pathway of Lymph: Tiny lymph capillaries lead to larger lymphatic vessels, forming the thoracic duct that connects to major veins near the heart.

Lymphatic System and Functionality

• Circulation Type: The lymphatic system collects fluids leaking from capillaries, serving as an alternative circulation method.

• Valves: Lymphatic vessels have valves to prevent backflow, ensuring unidirectional flow towards the heart.

• Function of Lymph Nodes: These filter lymph, destroying foreign matter and returning cleaned fluid to the circulatory system.

Immune Defenses

Nonspecific Defense (Innate)

• Mechanism: Inherited mechanisms protecting the body from a variety of pathogens.

• Examples: The skin acts as a physical barrier. Components include:

  • Physical epithelial barriers

  • Phagocytic leukocytes (white blood cells)

  • Dendritic cells

  • Natural killer (NK) cells

  • Circulating plasma proteins

Specific Defense (Adaptive)

• Function: Targeted mechanisms tailored towards specific pathogens, including antibody production against invading viruses.

• Activation: Components are typically silent but become activated during specific infections, leading to proliferation and powerful neutralization of pathogens.

Immune Response Structure

Phases of Immune Response

1. Recognition Phase: The immune system identifies self and non-self.

2. Activation Phase: Mobilization of cells and molecules to confront the invader.

3. Effector Phase: Destruction of the invader by the mobilized cells and molecules.

Key Immune Cells

• Cytotoxic T Cells: Attack and kill infected host cells.

• B Cells: Label invaders for destruction.

• Helper T Cells: Activate both B cells and cytotoxic T cells.

Lymphocyte Development

• Origin: Stem cells in red bone marrow produce lymphocyte precursors that develop into T cells in the thymus and B cells in the bone marrow. Both types circulate through the blood to lymphatic organs like lymph nodes and the spleen.

Immune Response Mechanics

1. Viral Infection: Infected cells present viral antigens on their surface.

2. Macrophage Action: Macrophages engulf viruses and present their antigens.

3. Helper T Cell Activation: T cells bind to these antigens, resulting in a cascade of immune responses including B cell activation.

B Cell Response (Humoral Immunity)

• Activation: B cells can activate upon antigen binding, often requiring confirmation from helper T cells.

• Differentiation: Some B cells become memory cells while others turn into plasma cells that secrete antibodies.

T Cell Response (Cell-Mediated Immunity)

• Cytotoxic Action: Cytotoxic T cells identify and destroy infected cells by piercing their membranes and inducing apoptosis through perforin and granzymes.

Antibody Function and Types

• Neutralization: Antibodies attach to viruses preventing their entry into host cells.

• Opsonization: Antibodies mark bacteria for phagocytosis by immune cells.

• Functionality Example: Antibodies trigger the complement system that forms pores leading to cell lysis.

Classes of Antibodies

• IgG, IgM, IgA, IgE: Different classes of antibodies produced, with IgG being the most abundant.

• Antigen Specificity: Antibodies are created to recognize specific epitopes on antigens.

Immunological Concepts

Immunological Memory

• Long-term Protection: Some B cells and T cells become memory cells, enabling fast immune responses upon re-exposure to pathogens.

Active vs. Passive Immunity

• Active Immunity: Developed through natural infection or vaccination leading to memory cell formation.

• Passive Immunity: Short-term protection provided through maternal antibodies or direct injection of antibodies.

Antigen Shifting

• Mutation: Pathogens like the flu mutate frequently, allowing them to evade previously established immune responses, necessitating new antibody production.

HIV Infection

• AIDS Overview: AIDS emerged in the early 1980s, caused by the human immunodeficiency virus (HIV), which targets helper T cells, crippling the immune response.