Immune System
The immune system refers to the body’s defense mechanisms which work together to recognize and eliminate harmful agents (like bacteria, viruses, fungi, parasites, toxins, and even abnormal host cells such as cancer cells) while maintaining tolerance to the body’s own tissues.
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1. Components of the Immune System
A. Organs & Tissues
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Primary lymphoid organs:
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Bone marrow – source of all blood cells (hematopoiesis); site of B cell maturation.
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Thymus – site of T cell maturation.
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Secondary lymphoid organs:
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Lymph nodes, spleen, tonsils, mucosa-associated lymphoid tissue (MALT) – sites where immune cells encounter antigens and initiate responses.
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B. Cells
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Innate immunity cells:
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Neutrophils, macrophages, dendritic cells, natural killer (NK) cells, mast cells, basophils, eosinophils.
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Adaptive immunity cells:
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B lymphocytes (produce antibodies).
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T lymphocytes (helper T cells [CD4+], cytotoxic T cells [CD8+], regulatory T cells).
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C. Molecular mediators
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Cytokines – small proteins that regulate immune communication.
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Chemokines – guide movement (chemotaxis) of immune cells.
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Complement system – plasma proteins that enhance phagocytosis, inflammation, and lysis of pathogens.
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Antibodies (immunoglobulins) – produced by B cells; recognize specific antigens.
2. Types of Immunity
A. Innate Immunity (First Line of Defense)
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Immediate, non-specific protection.
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Barriers: skin, mucous membranes, stomach acid, antimicrobial peptides.
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Cellular responses: phagocytosis (neutrophils, macrophages), NK cell killing.
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Inflammation: local response that increases blood flow, recruits immune cells, and enhances clearance of pathogens.
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Pattern recognition receptors (PRRs) like Toll-like receptors (TLRs) detect pathogen-associated molecular patterns (PAMPs).
B. Adaptive Immunity (Second Line, Specific Defense)
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Slower to develop (days) but highly specific and has memory.
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Humoral immunity – mediated by B cells → plasma cells → antibodies.
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Cell-mediated immunity – mediated by T cells:
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CD4+ helper T cells orchestrate responses by releasing cytokines.
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CD8+ cytotoxic T cells kill infected or cancerous cells.
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Memory cells: after infection, long-lived B and T cells ensure a faster, stronger response upon re-exposure.
3. Physiologic Processes of the Immune Response
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Recognition
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Innate system recognizes broad microbial patterns.
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Adaptive system recognizes specific antigens via T and B cell receptors.
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Activation
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Antigen-presenting cells (APCs, e.g., dendritic cells) present antigens to T cells.
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Co-stimulatory signals and cytokines activate lymphocytes.
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Effector Phase
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Phagocytosis, killing of infected cells, antibody neutralization of toxins and viruses, activation of complement.
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Resolution
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Regulatory T cells and anti-inflammatory cytokines (IL-10, TGF-β) dampen the response to prevent excessive damage.
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Memory Formation
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Memory B and T cells persist to provide long-term protection (basis of vaccines).
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4. Special Features
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Tolerance – immune system avoids attacking self (failure leads to autoimmune disease).
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Surveillance – detects and removes transformed (cancerous) cells.
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Plasticity – adapts to new pathogens through clonal selection and somatic hypermutation of antibodies.
✅ In summary:
The physiology of the immune system is the coordinated function of innate and adaptive defense mechanisms, involving recognition of pathogens, activation of immune responses, elimination of threats, and memory formation for long-term protection—all while maintaining tolerance to self.
flowchart showing how innate and adaptive immunity interact step by step—from pathogen entry, through innate recognition, adaptive activation, effector responses, and finally resolution with memory formation.
The immune system consists of three layers of defense.
The first line of defense is provided by a set of mechanical (e.g., skin), chemical (e.g., acidic environment of stomach), and biologic (e.g., commensal microbes) barriers that protect the body. If these barriers are breached, the second and third lines of protective systems are activated: first the innate immune system and then the adaptive immune system.
If the immune system malfunctions, it may attack the body’s own healthy cells, leading to autoimmune diseases.
The immune system distinguishes self from nonself .
The immune system also has the capacity to recognize and destroy abnormal cells that derive from host tissues. It clears the body’s own cells that have become senescent or abnormal.
Any molecule capable of being recognized by the immune system is considered an antigen (Ag).
Occasionally normal host tissues become the subject of inappropriate immune attack.
The immune system is the major defense system of the bodoy. Non specific barreirs also contribute to the defense systes as physical brriers
The skin, corneas, and mucosa of the respiratory, gastrointestinal, and genitourinary tracts form a physical barrier that is the body's first line of defense.
Some of these barriers also have active immune function
Outer, keratinized epidermis: Keratinocytes in the skin secrete antimicrobial peptides (defensins), and sebaceous and sweat glands secrete microbe-inhibiting substances (eg, lactic acid, fatty acids). Also, many immune cells (eg, mast cells, intraepithelial lymphocytes, antigen-sampling Langerhans cells) reside in the skin.
Corneas: Tears contain defensins. Macrophages and dendritic cells reside in the corneas and other immune cells, including T cells and phagocytic neutrophils, are recruited through the limbal vasculature during infection.
Mucosa of the respiratory, gastrointestinal, and genitourinary tracts: The mucus contains antimicrobial substances, such as lysozyme, lactoferrin, and secretory immunoglobulin (Ig) A antibody (SIgA).