The Immune System: Recognition, Response, and Defense Mechanisms

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Immune System: Recognition and Response

Overview of the Immune System

The immune system is a complex network of cells, tissues, and organs that protects animals from pathogens—agents that cause disease. It enables organisms to avoid or limit infections through a series of coordinated responses. Failure of the immune system can result in increased susceptibility to infections.

Pathogens: Disease-causing agents such as bacteria, viruses, fungi, and parasites.
All animals possess innate immunity, which is active immediately upon infection.
Vertebrates have both innate and adaptive immunity.
Invertebrates rely solely on innate immunity.
Plants do not have an immune system.

Innate Immunity

Characteristics of Innate Immunity

Innate immunity is present before exposure to pathogens and is effective from birth. It provides nonspecific defense mechanisms and consists of both external barriers and internal cellular and chemical defenses.

Nonspecific responses to a wide range of pathogens.
Includes barrier defenses and internal defenses.
Fever is a systemic response to infection, aiming to regulate cell survival and inhibit pathogen growth.

Barrier Defenses

Skin: Physical barrier preventing pathogen entry.
Mucous membranes: Line the respiratory, urinary, and reproductive tracts, trapping pathogens.
Secretions: Contain enzymes and antimicrobial peptides hostile to microbes.
Low pH of skin and digestive system inhibits bacterial growth.

Internal Defenses: White Blood Cells (Leukocytes)

Leukocytes are white blood cells derived from myeloid or lymphoid stem cells, playing a central role in defense.

Phagocytic cells: Engulf and digest pathogens.
Natural killer (NK) cells: Destroy abnormal cells nonspecifically.
Antimicrobial proteins: Attack microbes directly or impede their reproduction.
Inflammatory response: Mobilizes immune cells to sites of infection or injury.

Types of Phagocytic Leukocytes

Neutrophils: Most abundant, rapidly respond to infection.
Monocytes: Mature into macrophages (fixed or wandering) and dendritic cells (stimulate adaptive immunity).

Other Leukocytes

Basophils: Release histamine and anticoagulants during inflammation.
Eosinophils: Target large internal parasites with destructive enzymes.

Phagocytosis and Pathogen Recognition

Phagocytic cells recognize pathogens using toll-like receptors (TLRs), which bind to characteristic molecular fragments.

Complement System

The complement system consists of about 30 proteins activated by substances on microbe surfaces.

Enhances phagocytosis and inflammatory response.
Leads to cytolysis via formation of membrane attack complexes (MAC).
Fever may result from complement activation, making the environment less suitable for pathogens.

Inflammatory Response

Triggered by molecules released upon injury or infection.
Mast cells release histamine, causing blood vessels to dilate and become more permeable.
Macrophages and neutrophils release cytokines to modulate the immune response.
Enhanced blood flow delivers antimicrobial peptides and leukocytes, resulting in pus accumulation.
Inflammation can be local (e.g., redness around a wound) or systemic (e.g., fever).
Chronic inflammation can cause diseases such as Crohn’s Disease, Psoriasis, and Rheumatoid Arthritis.

Systemic Inflammation

Fever: Systemic response triggered by substances from macrophages; inhibits pathogen growth and stimulates immune activity.
Septic shock: Life-threatening condition from overwhelming inflammation.

Evasion of Innate Immunity

Some pathogens avoid phagocytosis (e.g., Mycobacterium tuberculosis resists lysosomal degradation).
Bacterial capsules can prevent phagocytosis.

Adaptive Immunity

Characteristics of Adaptive Immunity

Adaptive immunity develops after exposure to specific agents (microbes, toxins, or foreign substances) and involves a highly specific response. Also known as acquired immunity, it improves with repeated exposure to pathogens.

Humoral response: Antibodies defend against infection in body fluids.
Cell-mediated response: Cytotoxic cells defend against infection in body cells.

Lymphocytes

B lymphocytes (B cells): Originate in bone marrow; key to humoral immunity.
T lymphocytes (T cells): Originate in thymus; key to cellular immunity.
Natural Killer (NK) cells: Nonspecifically kill abnormal cells; act like innate immunity but only present in animals with adaptive immunity.

Lymphatic System

Interstitial fluid is collected as lymph and travels through lymphatic vessels.
Lymph nodes and lymphatic organs (e.g., spleen, thymus) contain lymphocytes and macrophages that monitor the blood.

Antigens and Antigen Recognition

Antigen: Any molecule or particle recognized as non-self by the immune system.
Recognition occurs when a B cell, T cell, or antibody binds to an antigen via an antigen receptor.
The specific binding site on the antigen is called an epitope.

B Cell Antigen Receptors and Antibodies

B cell antigen receptors are Y-shaped molecules with constant (C) regions and highly variable variable (V) regions that form the antigen-binding site.
Binding of antigen activates the B cell.
Activated B cells produce plasma cells that secrete antibodies (immunoglobulins, Ig).
Secreted antibodies are not membrane-bound and bind to antigens/pathogens.

Humoral Response: Antibody Function

Antibodies do not kill pathogens directly; they tag or mark them for destruction.
Neutralization: Antibodies bind to viral surface proteins, preventing infection of host cells.
Antibodies can also bind to toxins, preventing their entry into cells.
Antigen-antibody complexes may activate the complement system.

Cell-Mediated Response

T cells do not produce antibodies.
The variable regions of the T cell receptor form the antigen-binding site.

Antigen Presentation and T Cells

Antigen-presenting cells (APCs) display antigen-MHC complexes on their surface.
T cells recognize the antigen-MHC complex and become activated.

Helper T Cells

Activated by binding with APCs.
Trigger both humoral and cell-mediated responses.
Proliferate to form a clone of helper T cells.
Activate appropriate B cells and cytotoxic T cells.

Cytotoxic T Cells

Recognize APCs and bind to MHC via accessory proteins.
Secrete proteins that disrupt target cell membranes:
- Perforin: Creates pores in target cell membranes.
- Granzymes: Initiate apoptosis (programmed cell death) in target cells.

B & T Cell Development: Four Major Characteristics of Adaptive Immunity

Diversity of lymphocytes and receptors: Generated by rearrangement of immunoglobulin genes, producing many different antigen receptors.
Self-tolerance: Lymphocytes with self-reactive receptors are eliminated or inactivated during maturation (clonal deletion).
Proliferation after activation: Upon antigen recognition, lymphocytes undergo clonal selection, producing effector and memory cells.
Immunological memory: Long-lived memory cells enable a faster and stronger response upon subsequent exposures to the same antigen.

Table: Comparison of Innate and Adaptive Immunity

Feature	Innate Immunity	Adaptive Immunity
Specificity	Nonspecific	Highly specific
Response Time	Immediate	Slower (days to weeks)
Memory	None	Immunological memory present
Main Components	Barriers, phagocytes, NK cells, complement	B cells, T cells, antibodies

Example: Immune Response to a Bacterial Infection

Bacteria breach the skin (barrier defense).
Phagocytes (neutrophils, macrophages) engulf bacteria.
Complement system is activated, enhancing phagocytosis and inflammation.
Antigen-presenting cells activate helper T cells, which stimulate B cells to produce antibodies and cytotoxic T cells to destroy infected cells.
Memory cells are generated for faster future responses.

Additional info: Immunological memory is the basis for vaccination, which exposes the immune system to harmless forms of pathogens to elicit a memory response without causing disease.