Immune System Overview

Major Functions of the Immune System

The immune system is a complex network of cells, tissues, and organs that protects the body from pathogens and maintains homeostasis. Its primary functions include:

• Defense against pathogens: Identifies and eliminates bacteria, viruses, fungi, and parasites.

• Removal of dead or damaged cells: Clears cellular debris and promotes tissue repair.

• Surveillance against abnormal cells: Detects and destroys cancerous or mutated cells.

Barriers of the Immune System

The body employs several types of barriers to prevent pathogen entry:

• Physical barriers: Skin and mucous membranes act as the first line of defense.

• Mechanical barriers: Actions such as coughing, sneezing, and ciliary movement help expel pathogens.

• Chemical barriers: Secretions like stomach acid, lysozyme in saliva, and antimicrobial peptides inhibit microbial growth.

Steps of the Immune Response

If pathogens breach the barriers, the immune system responds in four main steps:

1. Detection: Recognition of pathogen-associated molecular patterns (PAMPs).

2. Communication: Signaling between immune cells via cytokines and chemokines.

3. Recruitment: Mobilization of immune cells to the site of infection.

4. Elimination: Destruction and removal of pathogens.

Innate vs. Adaptive Immunity

Comparison of Innate and Adaptive Immune Systems

The immune system is divided into two main branches:

• Innate Immunity: Non-specific, immediate response; includes physical barriers, phagocytic cells, and inflammation.

• Adaptive Immunity: Specific, slower response; involves lymphocytes (B and T cells) and memory formation.

Key Differences:

• Innate immunity is present from birth; adaptive immunity develops after exposure to antigens.

• Adaptive immunity has specificity and memory; innate immunity does not.

Neutrophil Mechanisms Against Microorganisms

Neutrophils are a type of white blood cell that destroy microorganisms using four main mechanisms:

• Phagocytosis: Engulfing and digesting pathogens.

• Degranulation: Release of antimicrobial granules.

• Production of reactive oxygen species (ROS): Toxic to microbes.

• Formation of neutrophil extracellular traps (NETs): Trapping and killing pathogens.

Cells of the Immune System

Monocytes and Macrophages

Monocytes are circulating white blood cells that differentiate into macrophages upon entering tissues. Their roles include:

• Phagocytosis of pathogens and debris

• Antigen presentation to T cells

• Secretion of cytokines to modulate immune responses

Locations: Monocytes are found in the bloodstream; macrophages reside in tissues throughout the body.

Dendritic Cells

Dendritic cells are antigen-presenting cells that bridge innate and adaptive immunity. They capture antigens and present them to T cells, initiating adaptive responses.

Recognition of Non-Self

Immune cells recognize non-self through pattern recognition receptors (PRRs) and antigen receptors. PRRs detect conserved microbial patterns, while B and T cell receptors recognize specific antigens.

Phagocytosis

Phagocytosis is the process by which cells engulf and digest particles. Cells capable of phagocytosis include:

• Neutrophils

• Macrophages

• Dendritic cells

Inflammation

Inflammation is a protective response to infection or injury. Its main functions are:

• Recruitment of immune cells

• Containment and elimination of pathogens

• Promotion of tissue repair

Cytokines

Cytokines are signaling proteins released by immune cells to regulate immunity, inflammation, and hematopoiesis.

Antigen Presentation and MHC

MHC Class I vs. MHC Class II

Major Histocompatibility Complex (MHC) molecules present antigens to T cells:

B and T Cell Antigen Binding

B cells bind free antigens directly via their B cell receptors (BCRs). T cells bind processed antigens presented by MHC molecules on APCs. This differs from PRRs of innate cells, which recognize general patterns rather than specific antigens.

Self-Tolerance

Self-tolerance is the immune system's ability to avoid attacking the body's own cells. It is enforced by:

• Elimination of self-reactive lymphocytes during development (central tolerance)

• Regulatory mechanisms that suppress autoimmunity (peripheral tolerance)

Failure of self-tolerance can lead to autoimmune diseases.

Additional info: Academic context and definitions have been expanded for clarity and completeness.