Understanding the Human Immune System
The Human Immune System: A Comprehensive Overview
Nonspecific Barriers: First Line of Defense
Skin
The skin, a remarkably effective natural barrier, provides a protective covering for the entire body. This resilient and sensitive layer, composed of a constantly renewing keratinized stratified epithelium, forms the epidermis. Keratin-filled cells combine with an acid mantle, creating a hostile environment for bacteria and fungi.
Mucous Membranes
Cavities like the nostrils possess a specialized skin called mucosa. Ciliated cells within the mucosa guide mucus secretions outwards. These secretions, due to their chemical nature and pH, bolster the mucosa’s protective function. A symbiotic indigenous flora also contributes to defense.
Secondary Barriers: Responding to Breaches
When the skin or mucous membranes are compromised, pathogens can penetrate, potentially reaching internal tissues. Clean wounds typically prevent microbial entry. However, infected wounds become entry points, triggering secondary barriers. These involve mechanisms like the inflammatory reaction, phagocytosis, and the complement system.
Nonspecific Cells: Phagocytes
Phagocytosis, a crucial defense mechanism, involves specific white blood cells called phagocytes. Three main types exist:
- Monocytes: These cells circulate in the bloodstream for several days before migrating to the bone marrow, where they transform into macrophages.
- Macrophages: Large cells with high phagocytic capacity, forming the reticuloendothelial system.
- Neutrophils: More abundant than monocytes but with shorter lifespans, smaller size, and lobed nuclei.
Inflammatory Reaction and Phagocytosis
When tissues are affected by infection, they release substances like histamine and serotonin, triggering inflammation and redness. Swelling results from increased capillary permeability, allowing plasma to escape into interstitial space. Redness is caused by increased blood flow to the affected area. Heat and pain also accompany inflammation. Blood delivers numerous phagocytic cells to the site, leading to the formation of pus (a mixture of serum, dead bacteria, and white blood cells).
Specific Barriers: The Immune System
When phagocytosis is overwhelmed, the immune system activates. This complex network of molecules, cells, and tissues mounts a specific response to foreign molecules, leading to their destruction. Successful resolution of an infectious disease often confers immunity for a period, as the body “remembers” the pathogen. Lymphocytes play a central role in these immune processes.
Organization of the Immune System
The immune system comprises lymphocytes and antibodies, utilizing the circulatory and lymphatic systems for distribution and transport. Lymphoid organs concentrate immune system elements.
Primary Lymphoid Organs
- Bone Marrow: The birthplace of precursor stem cells, which mature into B cells or migrate to the thymus to become T lymphocytes.
- Thymus: Located beneath the sternum, the thymus is where bone marrow cells undergo rapid division, proliferation, and a rigorous selection process. Surviving cells mature into T lymphocytes (thymocytes).
Secondary Lymphoid Organs
- Spleen: A blood-filtering organ that removes defective blood cells.
- Lymph Nodes: Small, encapsulated masses of lymphoid tissue interspersed along lymphatic vessels. They filter and purify lymph and are concentrated in anatomical folds like armpits. Inflammation of lymph nodes indicates infection and immune response activation.
- Lymphoepithelial Structures: Diffuse lymphoid tissue associated with mucosal epithelia, particularly in the digestive tract (e.g., tonsils).
Lymphocytes: Key Players in Immunity
Lymphocytes, characterized by large nuclei and scant cytoplasm, lack granules and pseudopods, preventing phagocytosis. They can exit the bloodstream and enter tissues. Two main types exist:
- B-Lymphocytes: Originating and maturing in the bone marrow, these cells drive the humoral immune response. They recognize antigens via membrane receptors and produce antibodies to neutralize free antigens.
- T-Lymphocytes: Generated in the bone marrow but maturing in the thymus, these cells mediate cellular immunity. They detect antigens on other cells’ surfaces using membrane receptors (not antibodies). Subtypes include cytotoxic T cells, helper T cells, and suppressor T cells.
The Immune Response: A Coordinated Defense
The immune response encompasses the processes triggered by the entry of a foreign substance (antigen) that the body recognizes as non-self. The goal is to neutralize the invader and establish immunity. This response is highly specific.
Primary Immune Response
This initial response involves lymphocyte proliferation and the creation of memory cells. It has three phases:
- Lag Phase: Antigen identification and lymphocyte proliferation.
- Logarithmic Phase: Antibody production increases to a maximum.
- Decline Phase: Antibody concentration falls to zero, marking the elimination of infection.
Secondary Immune Response
This response differs significantly from the primary response:
- Shorter Lag Phase: Memory cells recognize the antigen and proliferate rapidly.
- Faster and More Intense Antibody Production: The rapid and effective secondary response, thanks to immunological memory, often eliminates the antigen before symptoms appear.
Antigens: Triggers of Immunity
Antigens are foreign substances capable of triggering an immune response. They can be heteroantigens, isoantigens, or autoantigens. Proteins, complex polysaccharides, and lipids can exhibit antigenicity, triggering antibody formation.
Antibodies: Specialized Defense Molecules
Antibodies, produced by B lymphocytes in response to antigens, bind specifically to their target antigen. These globular proteins, known as immunoglobulins, consist of four amino acid chains: two heavy (H) chains and two light (L) chains.
Types of Immunoglobulins (Ig)
- IgG: Can cross the placenta and are secreted in breast milk.
- IgA: Found in blood, vaginal secretions, etc.
- IgM: Present in blood and extracellular fluids.
- IgD: Found on the surface of B cells.
- IgE: Primarily found in tissues and involved in allergic reactions.
Antigen-Antibody Reaction: Specific Binding
The immune reaction occurs when antibodies or immune cells encounter an antigen. This highly specific reaction results in the formation of an antigen-antibody complex, which is then phagocytosed. Types of reactions include:
- Precipitation: Occurs with polyvalent antigens, forming large, insoluble complexes that precipitate.
- Agglutination: Antibodies bind to antigens on cell surfaces, forming cellular aggregates.
- Neutralization: Eliminates the negative effects of antigens, often by binding to viruses and reducing infectivity.
- Opsonization: Antibodies (opsonins) bind to microbes, facilitating phagocytosis.
Humoral Immune Response: Antibody-Mediated Immunity
Clonal Selection Theory
This theory proposes that a specific antigen stimulates the proliferation of cells with matching antibodies, creating a clone of lymphocytes. The antigen drives the formation of the lymphocyte clone responsible for its elimination.
Mechanism of Humoral Immune Response
When an antigen enters the body, it binds to a B cell with a matching antibody. This binding activates the cell, leading to rapid proliferation and the generation of two cell lines: plasma cells and memory cells.
Cellular Immune Response: T Cell-Mediated Immunity
This response involves T lymphocytes and macrophages but does not produce antibodies.
Mechanism of Cellular Immune Response
Macrophages act as antigen-presenting cells. They phagocytose antigens, break them down into peptides, and display these peptides on their surface bound to major histocompatibility complex (MHC) proteins.
T Lymphocyte Role in Immune Response
Recognition and binding to the antigen-MHC complex activates T lymphocytes, which divide and differentiate into four cell types:
- Cytotoxic T Lymphocytes: Target foreign peptides on any cell’s surface, releasing proteins that cause cell death.
- Helper T Lymphocytes: Recognize peptides bound to MHC proteins on macrophages and other antigen-presenting cells. They release lymphokines, promoting the proliferation of cytotoxic and B lymphocytes.
- Suppressor T Lymphocytes: Inhibit helper T cell activity.
- Memory Cells: Activated T cells that persist in lymphoid tissue, ready to respond rapidly upon re-exposure to the pathogen.