Immunology: The Body’s Defense Mechanisms

Innate (Non-Specific) Defense Mechanisms

The innate immune system provides immediate, non-specific defense against pathogens. It includes physical barriers, cellular responses, and chemical mediators.

• Physical Barriers: Skin and mucous membranes block pathogen entry.

• Phagocytic White Blood Cells: Macrophages and neutrophils engulf and digest pathogens.

• Interferons: Proteins released by virus-infected cells to inhibit viral replication in neighboring cells.

• Inflammatory Response: Mobilizes immune cells to sites of infection, increases blood flow, and causes redness, heat, swelling, and pain.

• Lymphatic System: Transports lymph, houses immune cells, and filters pathogens.

Specific (Adaptive) Immunity

Specific immunity targets particular pathogens using specialized cells and molecules. It is characterized by memory and specificity.

• Antigens: Foreign molecules that trigger an immune response.

• Antibodies: Proteins produced by B cells that bind to antigens and mark them for elimination.

• Active Immunity: Acquired through exposure to pathogens or vaccination; the body produces its own antibodies.

• Passive Immunity: Acquired by receiving antibodies from another source (e.g., maternal antibodies, antiserum).

Lymphocytes and Dual Defense

Lymphocytes mount a dual defense: humoral immunity (B cells) and cell-mediated immunity (T cells).

• Humoral Immunity: B cells produce antibodies to fight pathogens in blood and body fluids.

• Cell-Mediated Immunity: T cells attack infected body cells and coordinate immune responses.

Clonal Selection and Immune Memory

Clonal selection ensures that only lymphocytes specific to an antigen are activated and expanded.

• Primary Response: First exposure to antigen activates B cells, producing plasma cells (antibodies) and memory cells.

• Secondary Response: Subsequent exposure activates memory cells, resulting in a faster and stronger antibody response.

Cellular Components of the Immune System

Helper T Cells

Helper T cells are central coordinators of the immune response. They activate cytotoxic T cells, macrophages, and stimulate antibody production by B cells.

• Activation: Helper T cells recognize antigens presented by macrophages and release cytokines (e.g., interleukin-2) to stimulate immune responses.

Cytotoxic (Killer) T Cells

Cytotoxic T cells destroy infected or abnormal cells by recognizing self-nonself complexes and releasing proteins such as perforin.

• Perforin: Forms pores in the target cell membrane, allowing enzymes to enter and induce cell death.

• Role in Cancer: Cytotoxic T cells can recognize and kill cancerous cells.

Cells of the Immune System

The immune system consists of various cell types derived from stem cells, each with specialized functions.

• B Cells: Produce antibodies and memory cells.

• T Cells: Include helper, cytotoxic, and regulatory T cells.

• Macrophages: Engulf pathogens and present antigens.

• Mast Cells: Release histamines during allergic reactions.

• Natural Killer Cells: Destroy virus-infected and tumor cells.

Immune Responses and Disease

Autoimmune Diseases

Autoimmune diseases occur when the immune system attacks the body’s own cells, leading to inflammation and tissue damage.

• Examples: Lupus, Multiple Sclerosis (MS), Rheumatoid Arthritis.

• Mechanism: B cells produce antibodies against self-molecules.

Allergies

Allergies are hypersensitivity reactions to environmental antigens (allergens). Sensitization leads to antibody production, and mast cells release histamines, causing inflammation.

• Mast Cells: Release histamines upon allergen exposure.

• Symptoms: Sneezing, itching, swelling, and redness.

Immunodeficiency Diseases

Immunodeficiency diseases result from deficient or inactive lymphocytes, making individuals susceptible to frequent infections.

• Examples: Severe Combined Immunodeficiency (SCID), Acquired Immunodeficiency Syndrome (AIDS).

HIV and AIDS

HIV targets helper T cells, suppressing both humoral and cell-mediated immunity. The virus enters T cells, replicates, and destroys them, leading to AIDS.

• Mechanism: HIV proteins bind to helper T cell receptors, allowing viral RNA entry and replication.

• Effects: Loss of T cells impairs immune response, increasing susceptibility to infections.

Immune Response to Covid-19 (SARS-CoV-2)

Covid-19 Virus Structure and Entry

SARS-CoV-2 uses the ACE-2 receptor to enter host cells in the respiratory tract. The virus replicates inside cells, causing cell death and inflammation.

• Structure: Contains spike (S) protein, envelope glycoprotein, and RNA genome.

• Entry: Spike protein binds to ACE-2 receptor, facilitating infection.

Pathogenesis and Immune Response

Viral replication in the respiratory tract triggers immune activation, inflammation, and fluid build-up. Severe cases may lead to pneumonia and respiratory distress.

• Immune Cell Levels: Severe Covid-19 reduces B cells, T cells, and other white blood cells.

• Cytokine Storm: Excessive inflammatory response can cause blood clots, nerve damage, and organ failure.

Summary Table: Types of Immune Responses

Key Equations and Concepts

Antibody Production Rate

Plasma cells can produce up to 2000 antibodies per second.

Clonal Selection Model

Upon antigen exposure:

• B cell activation:

• Antibody production:

Immune Response Timeline

• Primary response: slower, less effective

• Secondary response: faster, more effective

Covid-19 Pathogenesis

• Virus entry:

• Immune activation:

Additional info: Academic context was added to clarify mechanisms, cell types, and disease examples for completeness.