The Immune System: Overview

Overview of the Immune System

• The immune system protects the body from harmful invaders such as microorganisms, toxins, and chemicals.

• It identifies harmful agents, responds to eliminate them, and retains memory for future encounters.

• Immunity refers to the state of being free from sickness or infection.

Components of the Immune System

• Cells: Various immune cells that fight infections.

• Molecules: Proteins and enzymes that facilitate immune responses (e.g., antibodies, cytokines)

• Tissues: Structures that house immune cells (e.g., lymph nodes, spleen).

Divisions of the Immune System

• Innate Immune System

  • Characteristics: Always present, responds quickly, no memory, nonspecific.

  • Response Time: Ranges from seconds to hours.

  • First Line of Defense:

    • Skin: Anatomical barriers, keratin layer, and antimicrobial chemicals prevent pathogens from entering the body.

    • Mucous Membranes: Found in areas like the mouth, nose, and digestive tract; produce mucus and contain cilia to trap and expel invaders.

• Second Line of Defense:

  • Involves white blood cells (except lymphocytes), cells' physiology, and chemical mediators.

  • Types of Cells:

    • Granulocytes:

      • Neutrophils: First responders; perform phagocytosis.

      • Eosinophils: Attack larger parasites.

      • Basophils: Involved in allergic reactions, release histamine.

      • Mast Cells: Release histamines and contribute to inflammation.

    • Monocytes: Transform into macrophages that also perform phagocytosis and present antigens.

    • Natural Killer Cells: Identify and induce apoptosis in infected or dysfunctional cells.

  • Physiological Responses:

    • Inflammation: Increased blood flow and immune cell recruitment due to chemical signals.

    • Fever: Regulated by the hypothalamus in response to pathogens, makes the body less hospitable for invaders.

  • Chemical Releases:

    • Interferons: Alert neighboring cells about viral infections.

    • Complement Proteins: Inactive proteins that activate in response to infection; assist in destroying pathogens.

• Adaptive Immune System

  • Characteristics: Specific response, slower initial activation, and retains memory for faster future responses.

  • Subdivisions:

    • Humoral Immune Response:

      • Involves B-cells that produce antibodies.

      • Upon activation by pathogens, they clone and form memory cells and plasma cells that release antibodies in the bloodstream.

    • Cell-Mediated Immune Response:

      • Involves T-cells (CD4+ helper T-cells and CD8+ cytotoxic T-cells).

      • T-cells recognize infected or dysfunctional cells via MHC molecules and activate appropriately.

  • Activation Process:

    • Encountering a specific antigen leads to the selection and clonal expansion of T-cells and B-cells.

    • Memory cells result from activation, allowing a quicker and more efficient response upon subsequent infections.

How the Immune System Functions to Maintain Health

• The innate immune response acts immediately without specificity, providing the first line of defense.

• The adaptive immunity learns from past infections, allowing the body to respond rapidly to previously encountered pathogens.

• Vaccines leverage this principle by introducing antigens to develop memory without causing disease, leading to quicker and stronger immune responses during actual infections.

Overview of the Immune System

The immune system is a complex network that protects the body from harmful invaders such as microorganisms (bacteria, viruses, fungi, and parasites), toxins, and chemicals. Its primary functions include identifying these harmful agents, mounting a response to eliminate them effectively, and retaining a memory of past encounters to provide long-lasting immunity against future infections. Immunity is the state of being free from sickness or infection, and the immune system plays a vital role in maintaining this state.

Components of the Immune System

• Cells: Various specialized immune cells that fight infections, including lymphocytes (T-cells and B-cells), phagocytes, and granulocytes. Each type has specific roles in immune defense.

• Molecules: A variety of proteins and enzymes that facilitate immune responses, including antibodies (which bind to specific antigens), cytokines (which signal various immune responses), and complement proteins (which help to destroy pathogens).

• Tissues: Structures that house immune cells and are critical to immune function, including lymph nodes (where immune cells can encounter antigens), the spleen (which filters blood and helps fight infections), and the bone marrow (where blood cells are produced).

Divisions of the Immune System

Innate Immune System

• Characteristics: The innate immune system is always present in the body, provides an immediate response to infections, does not have memory (meaning it responds the same way to repeated exposures), and is nonspecific (it reacts to a wide range of pathogens).

• Response Time: Its response can occur within seconds to hours after exposure to a pathogen.

First Line of Defense:

1. Skin: The epidermis acts as an anatomical barrier; it has a keratin layer that provides protection and produces antimicrobial chemicals to inhibit pathogen entry.

2. Mucous Membranes: These line various body tracts (such as the respiratory and digestive systems); they secrete mucus that traps pathogens and contain cilia that help expel them from the body.

Second Line of Defense:

• This includes various white blood cells (excluding lymphocytes), physiological responses, and chemical mediators that act if pathogens breach the first line of defense.

• Types of Cells:

  • Granulocytes:

    • Neutrophils: The first responders to infection; they perform phagocytosis to engulf pathogens.

    • Eosinophils: Particularly involved in fighting larger parasites and modulating allergic responses.

    • Basophils: Play a role in allergic reactions and inflammation by releasing histamine and other mediators.

  • Mast Cells: Found in tissues and release histamines that contribute to local inflammation.

  • Monocytes: Circulate in the blood and can differentiate into macrophages and dendritic cells that perform phagocytosis and present antigens to T-cells.

  • Natural Killer (NK) Cells: These recognize and induce apoptosis (programmed cell death) in infected or dysfunctional cells.

• Physiological Responses:

  • Inflammation: Triggered by chemical signals from damaged tissues and immune cells, leading to increased blood flow, heat, swelling, and recruitment of additional immune cells.

  • Fever: A systemic response regulated by the hypothalamus, enhancing the body’s ability to fight infections by creating a less favorable environment for pathogen replication.

• Chemical Releases:

  • Interferons: Signaling proteins produced in response to viral infections; they alert neighboring cells and enhance their antiviral defenses.

  • Complement Proteins: A series of inactive proteins that become activated during infection, assisting in the destruction of pathogens and promoting inflammation.

Adaptive Immune System

• Characteristics: This division of the immune system mounts a specific response to pathogens, has a slower initial activation (taking days to weeks), and retains memory through B and T memory cells for faster responses to subsequent infections.

Subdivisions:

1. Humoral Immune Response:

   • Involves B-cells that produce antibodies specific to pathogens. Upon activation by exposure to an antigen, B-cells clone themselves and differentiate into memory B-cells and plasma cells; the latter secrete large quantities of antibodies into the bloodstream to neutralize pathogens.

2. Cell-Mediated Immune Response:

   • Involves T-cells (including CD4+ helper T-cells and CD8+ cytotoxic T-cells), which recognize infected or dysfunctional cells via Major Histocompatibility Complex (MHC) molecules on cell surfaces. Helper T-cells activate B-cells and cytotoxic T-cells, while cytotoxic T-cells directly kill infected cells.

• Activation Process:

  • Upon encountering a specific antigen, T-cells and B-cells undergo selection and clonal expansion to produce numerous copies that effectively target the infection.

  • Memory cells are produced as part of this process, enabling a more rapid and effective response to future encounters with the same pathogen.

How the Immune System Functions to Maintain Health

The innate immune response acts immediately, addressing infections with a nonspecific approach and providing the first line of defense through physical barriers and immune cells. Meanwhile, the adaptive immune system learns from previous infections, which allowing for quicker responses to previously encountered pathogens, minimizing the risk and severity of disease. Vaccines are designed based on this principle — they introduce harmless antigens to stimulate an immune response, resulting in the formation of memory cells without causing disease, leading to a significantly quicker and stronger immune response when faced with actual infections.