Immune System

Overview of Immunity

The immune system is responsible for defending the body against infection and disease. It consists of a complex network of cells, tissues, and organs that work together to recognize and eliminate pathogens and abnormal cells.

• Immunity: The ability to resist infection and disease.

• Immune response: The body's reaction to infectious agents and abnormal substances.

• Pathogens: Disease-causing organisms such as viruses, bacteria, fungi, and parasites.

• Resistance: The body's ability to maintain its immunity.

Lymphocytes

Lymphocytes are a type of white blood cell essential for immune responses. There are three main types, each with distinct roles:

• B cells: Move slowly, found in lymph nodes, spleen, and other lymphoid tissues. Differentiate into plasma cells that release antibodies.

• T cells: Most lymphocytes are T cells. They recognize abnormal and foreign antigens, produce immune responses, and mature in the thymus. Types include:

  • Cytotoxic T cells: Directly attack antigens physically and chemically.

  • Helper T cells: Stimulate responses of both T and B cells.

  • Regulatory T cells: Moderate immune responses.

  • Memory T cells: Respond to previously encountered antigens by cloning themselves.

• NK (Natural Killer) cells: Patrol the body, searching for abnormal cells to destroy.

• Lymphocytopoiesis: Formation of lymphocytes in red bone marrow, thymus, and peripheral lymphoid tissues.

Types of Immunity

Innate (Nonspecific) Immunity

Innate immunity is present at birth and provides immediate, generalized defense against pathogens. It does not distinguish between different threats and has no memory of previous encounters.

• Physical barriers: Skin, mucous membranes, and secretions (contain lysosomes and antibodies) protect underlying tissues.

• Phagocytes: Remove debris and pathogens. Includes:

  • Microphages: Neutrophils and eosinophils.

  • Macrophages: Large phagocytes found in most tissues. Can be fixed (reside in specific tissues) or free (move throughout the body).

• Chemotaxis: Movement of phagocytes toward chemicals (cytokines) released by cells or pathogens.

• Phagocytosis: Begins with adhesion, followed by digestion of contents after fusion with lysosomes or peroxisomes.

• Immune surveillance: NK cells rapidly recognize and destroy abnormal cells, including cancer cells (which have tumor-specific antigens).

• Interferons: Small proteins released by activated macrophages and virus-infected tissues. Interfere with viral replication and stimulate macrophages and NK cells.

• Complement system: Enhances antibody and phagocyte action. Involves a cascade of 30 proteins, activated via:

  • Lectin pathway

  • Alternative pathway

  • Classical pathway (most common, triggered by antibody-antigen complexes)

• Inflammation: Localized tissue response to injury, characterized by redness, swelling, pain, and heat. Mediated by mast cells (release histamine, heparin, prostaglandins, interleukins).

• Fever: Body temperature above 37.2°C (99°F), induced by pyrogens. Increases metabolic rate and inhibits some pathogens.

Adaptive (Specific) Immunity

Adaptive immunity targets specific antigens and provides long-lasting protection. It involves B and T lymphocytes and has memory, specificity, versatility, and tolerance.

• B cells: Recognize and bind antigens in body fluids.

• T cells: Recognize processed antigen fragments presented by other cells.

• Types of adaptive immunity:

  • Cell-mediated immunity: Provided by cytotoxic T cells; defends against intracellular pathogens and abnormal cells.

  • Antibody-mediated (humoral) immunity: Provided by B cells; defends against extracellular pathogens in body fluids.

• Antigens: Substances that stimulate immune responses, usually proteins but can be lipids, polysaccharides, or nucleic acids. Epitopes are specific regions that trigger responses.

• Apoptosis: Programmed cell death, eliminates dangerous cells.

Forms of Adaptive Immunity

• Active immunity: Develops after exposure to an antigen. Can be:

  • Naturally acquired: Through environmental exposure.

  • Artificially acquired: Through vaccination.

• Passive immunity: Antibodies are transferred from another source. Can be:

  • Naturally acquired: From mother to baby via placenta or breast milk.

  • Artificially acquired: Injection of antibodies.

Properties of Adaptive Immunity

• Specificity: Targets specific antigens.

• Versatility: Large diversity of lymphocytes and antibodies.

• Memory: Remembers previously encountered antigens for faster, stronger responses.

• Tolerance: Ignores self-antigens, attacks only foreign antigens.

Cell-Mediated Adaptive Immunity

T Cell Activation and MHC

T cells are activated by exposure to antigens presented by antigen-presenting cells (APCs) via Major Histocompatibility Complex (MHC) molecules.

• MHC Class I: Present on all nucleated cells (except RBCs). Present endogenous antigens (from inside the cell) to cytotoxic T cells (CD8+).

• MHC Class II: Present on APCs. Present exogenous antigens (from outside the cell) to helper T cells (CD4+).

T Cell Types and Functions

• Cytotoxic T cells (CD8+): Destroy infected or abnormal cells by releasing perforin and inducing apoptosis.

• Memory T cells: Provide long-term immunity by responding rapidly to previously encountered antigens.

• Regulatory T cells (Tregs): Moderate immune responses by secreting suppression factors (inhibitory cytokines).

• Helper T cells (CD4+): Coordinate immune responses by producing cytokines and stimulating both cell-mediated and antibody-mediated immunity.

• Costimulation: Additional signal required for full T cell activation, preventing attacks on normal tissues.

Antibody-Mediated Adaptive Immunity

B Cell Activation and Antibody Production

• Sensitization: Antigen binds to B cell, preparing it for activation.

• Activation: Usually requires helper T cell confirmation.

• Plasma cells: Produce and secrete large quantities of antibodies.

• Memory B cells: Remain in reserve for future exposures to the same antigen.

Antibody Structure and Classes

Antibodies (immunoglobulins) are Y-shaped molecules composed of two heavy and two light polypeptide chains, each with constant and variable segments.

• Constant segments: Form the base, determine antibody class and distribution.

• Variable segments: Form antigen-binding sites, determine specificity.

Primary and Secondary Immune Responses

• Primary response: Initial response to an antigen; slower due to activation and proliferation of B cells.

• Secondary response: Faster and stronger due to memory B cells.

• Immunocompetence: Ability to produce an immune response after antigen exposure.

Immune Disorders

Hypersensitivities (Allergies)

• Allergens: Antigens that trigger allergic reactions.

• Anaphylaxis: Severe, systemic allergic reaction. Can cause airway constriction and circulatory collapse. Treated with antihistamines, corticosteroids, and epinephrine.

Autoimmune Disorders

• Celiac Disease: Immune reaction to gluten, damages intestinal lining.

• Systemic Lupus Erythematosus (Lupus): Chronic, multi-system inflammatory disease, more common in females.

• Type I Diabetes: Immune system destroys insulin-producing pancreatic cells.

Immunodeficiency Diseases

• Caused by developmental problems, viral infections (e.g., HIV), or immunosuppressive treatments.

Summary Table: Innate vs. Adaptive Immunity

Additional info: The immune system is closely linked to the lymphatic system, which provides the structural basis for immune cell development and transport. Disorders of the immune system can lead to increased susceptibility to infections, chronic inflammation, or inappropriate immune responses against self-tissues.