Overview of the Immune System

Introduction

The immune system is a complex network of cells, tissues, and organs that protects the body from harmful pathogens and foreign substances. It is divided into two main branches: innate (nonspecific) immunity and adaptive (specific) immunity. Both systems work together to defend the body and maintain health.

Major Categories of Human Pathogens

Types and Examples

Pathogens are disease-causing organisms that trigger immune responses. They include viruses, fungi, protozoa, and bacteria.

Innate (Nonspecific) Immune System

First Line of Defense: External Barriers

The first line of defense consists of physical and chemical barriers that prevent pathogen entry.

• Skin: Acts as a physical barrier to pathogens.

• Mucous membranes: Trap and expel microbes.

Second Line of Defense: Internal Defenses

If pathogens bypass external barriers, the second line of defense is activated.

• Phagocytes: Cells such as neutrophils and macrophages that engulf and destroy pathogens.

• Natural Killer (NK) Cells: Destroy virus-infected and cancerous cells.

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

• Antimicrobial Proteins: Interferons and complement proteins that inhibit pathogen replication and enhance immune responses.

• Fever: Elevated body temperature that inhibits pathogen growth and speeds up tissue repair.

Key Features of Innate Immunity

• Fast response (minutes to hours)

• Nonspecific: Responds to a wide range of pathogens

• No memory: Does not improve with repeated exposure

Adaptive (Specific) Immune System

Third Line of Defense

The adaptive immune system provides a targeted response to specific pathogens and retains memory for future protection.

• Lymphocytes: B cells and T cells are the main adaptive immune cells.

• Humoral Immunity: B cells produce antibodies that bind to antigens and mark them for destruction.

• Cell-Mediated Immunity: T cells directly attack infected or abnormal cells.

Key Features of Adaptive Immunity

• Slower to develop (days)

• Highly specific to individual antigens

• Memory: Faster and stronger response upon re-exposure

Leukocytes (White Blood Cells)

Types and Functions

Leukocytes are essential for immune defense. They are classified as granular or agranular based on the presence of granules in their cytoplasm.

• Leukopenia: Abnormally low white blood cell count, increasing infection risk.

• WBCs are found in the buffy coat layer of centrifuged blood.

• Neutrophils are the most abundant WBC and the first to respond to infection.

• B lymphocytes make antibodies.

Antigens and Antibodies

Definitions and Functions

• Antigen: Any substance that can trigger an immune response; usually a protein or glycoprotein.

• Antibody (Immunoglobulin): Protein produced by B cells that binds specifically to an antigen.

Antibodies have two important properties:

• Immunogenicity: Ability to stimulate lymphocyte proliferation and antibody production.

• Reactivity: Ability to react with lymphocytes and antibodies.

Antibodies mark antigens for destruction by phagocytes or neutralize toxins and viruses.

Major Immunoglobulin Classes

• IgM: First antibody produced; effective in agglutination and complement activation.

• IgG: Most abundant; provides long-term immunity; crosses placenta.

• IgA: Found in mucous membranes and secretions; protects body surfaces.

• IgE: Involved in allergic reactions and defense against parasites.

Immune System Development and Homeostatic Imbalances

Developmental Aspects

• Immune stem cells develop in the liver and spleen during fetal life; bone marrow is the primary source after birth.

• Immune function can be influenced by the nervous system, diet (vitamin D), and age.

• Immunity wanes with age, increasing disease susceptibility.

Homeostatic Imbalances

• Immunodeficiency: Impaired function or production of immune cells (e.g., SCID, Hodgkin’s disease, AIDS).

• Autoimmune Diseases: The Immune system attacks self-tissues (e.g., rheumatoid arthritis, type 1 diabetes, multiple sclerosis).

• Hypersensitivities: Exaggerated immune response to non-pathogen antigens (e.g., allergies, anaphylactic shock).

Summary Table: Innate vs Adaptive Immunity