Introduction to Immunology and the Immune System

Posted on May 12, 2024 in Biology

Immunology

Immunology studies our defense mechanisms against infections of pathogenic microorganisms or damaged or dangerous cells, such as cancer cells.

Immunity

Immunity is the state of resistance or protection of the organism to an infection caused by a particular antigen, pathogen, or infectious disease.

The Immune System

The immune system is a complex network of organs, cells, and molecules that defends the body against infection. The immune system keeps a record of every germ (microbe) it has ever defeated so it can recognize and destroy the microbe quickly if it enters the body again.

Function:

• Perception of the presence of pathogens in the body.
• Removal (or destruction) of pathogens located in different tissues and organs.
• Most often, to prevent another infection caused by the same pathogen.

Exterior Barriers

Exterior barriers are important for making it difficult for pathogens to enter the body.

Types of Barriers:

• Anatomical or Mechanical Barrier: The skin is a physical barrier that contains fatty acids and commensals.
• Chemical Barrier: Several substances prevent the development of bacteria: sweat, gastric urine, basic duodenal fluid.
• Biochemical Barrier: Some fluids (tears, nasal mucosa, saliva, etc.) contain anti-bacterial enzymes.
• Biological Barrier: Some commensal microorganisms of the native bacterial flora live symbiotically in the skin, digestive tract, and urogenital tract.

Cells of the Immune System

1. Types and Secretions (Function):

Phagocytes:

• Monocytes: Transform into macrophages or dendritic cells, phagocytizing foreign substances.
• Neutrophils: Engulf pathogens, particularly bacteria, and contribute to the formation of abscesses.
• Eosinophils: Combat worm parasites and pathogens through phagocytosis.

Auxiliary Cells:

• Basophils: Release granules triggering inflammation, important in allergic and inflammatory responses.
• Mast Cells: Contain granules rich in histamine and heparin, integral to immune and neuroimmune systems.

2. Origin of Leukocytes:

• All blood and white cells of the immune system originate from stem cells in the red bone marrow.
• T cells additionally mature in the thymus.
• Stem cells differentiate into common myeloid progenitor or common lymphoid progenitor, producing distinct cell lines.
• Monocytes mature into dendritic cells or macrophages, and B cells differentiate into plasma cells.

Innate Immune System: The Local Inflammatory Response

The innate immune system mounts a local inflammatory response when tissues are damaged by pathogens or other causes. Phagocytes, such as macrophages and neutrophils, play a key role by engulfing and destroying pathogens. Inflammatory mediators like histamine, eicosanoids, cytokines, and interferons are released, causing redness, swelling, and increased blood flow to the infected area. This response aims to isolate and destroy the pathogen. Pus, formed from dead tissue and phagocytes, may accumulate at the site. Inflammation may also lead to fever, which helps to combat pathogens. Once the pathogen is destroyed, fibrocytes and macrophages repair the tissue. If the infection persists, further immune responses are activated.

Adaptive Immune System

The adaptive immune system is a specific defense mechanism that activates when pathogens breach innate immunity. It involves leukocytes called lymphocytes, organs, tissues, and specific molecules.

Molecules:

1. Antigens: Foreign substances inducing immune responses or B lymphocyte reactions, typically macromolecules present on pathogenic cells or viruses.
   Epitope: Small regions of antigens recognized by the immune system, crucial for binding to antibodies or immune cells.
2. Immunoglobulins (Antibodies): Synthesized by B lymphocytes, these proteins identify pathogenic molecules.
   Structure: Consists of two long heavy chains and two shorter light chains connected by disulfide bridges, forming a Y-shape. Antigen binding sites at the ends facilitate specific linking to epitopes.
3. Major Histocompatibility Complex (MHC): A set of peptides essential for the adaptive immune system, present on cell membranes. They bind to antigens during infection, distinguishing between self and foreign molecules.
4. Complement Immune System: A system of soluble proteins produced by monocytes, involved in bacterial infections by facilitating capillary dilation and complementing the action of antibodies.
5. Interferons (IFNs): Signaling proteins produced by host cells in response to viral infections, triggering antiviral defenses and modulating immune responses. Induce antiviral defenses in neighboring cells, inhibit viral replication, and enhance immune responses by activating macrophages and natural killer cells.
6. Cytokines: Regulatory proteins produced by various immune cells that modulate immune responses and mediate communication between immune cells. Regulate the proliferation, differentiation, and activation of immune cells, coordinate immune responses, and modulate inflammation and immune tolerance.

Humoral and Cellular Immunity: Mechanisms of Action and Phases. Function:

Lymphocyte Maturation and Activation:

• Lymphocytes originate from stem cells in the red bone marrow, where B lymphocytes mature, while T lymphocytes mature in the thymus.
• Each lymphocyte acquires antigenic receptors on its surface during maturation, allowing recognition of specific epitopes.
• Secondary lymphoid organs, such as spleen and lymph nodes, facilitate the final differentiation and activation of lymphocytes upon encountering antigens.

T Lymphocytes:

• T4 Lymphocytes (TH): Initiate immune responses by activating other immune cells, including macrophages and B lymphocytes, through cytokine signaling.
• Cytotoxic T Lymphocytes (T8 CTL): Target and destroy infected or abnormal cells by binding to specific antigens on their surfaces, leading to cell lysis.
• Regulatory T-Lymphocytes (TS): Suppress immune responses by inhibiting antibody synthesis and T-lymphocyte activity.
• Natural Killer Cells (NK): Differentiate from immature T lymphocytes. Lack specific antigen receptors and exhibit nonspecific immune responses. Attack infected cells and tumor cells by inducing cell death.

B Lymphocytes and Plasma Cells:

-B lymphocytes mature in the bone marrow and interact with T4 lymphocytes for activation. -Upon activation, B lymphocytes differentiate into plasma cells, which produce specific antibodies targeting particular antigens and release antibodies into the bloodstream.