Immunity Structure and Function

I. First Line of Defense: Non-Specific Immunity

Structure and Role:
Physical Barriers: Skin, mucous membranes, epithelium
Skin: The largest organ of the body, comprised of multiple layers, serves as the foremost barrier against environmental pathogens and helps to regulate essential body functions such as temperature and hydration.
Layers:

  • Epidermis: This outermost layer lacks its own blood supply and is composed mainly of keratinized cells, which provide a tough and resistant surface against pathogens. It also includes melanocytes that produce melanin to protect against UV radiation.

  • Dermis: Beneath the epidermis, this layer contains blood vessels, nerve endings, and sweat and sebaceous glands, which secrete substances that aid in maintaining skin moisture and barrier functions.

  • Hypodermis: The deepest layer that connects the skin to underlying muscles, it is composed of loose connective tissue and adipose tissue, which provides insulation and shock absorption.
    Biochemical Barriers: Saliva, tears, and digestive juices contain antimicrobial enzymes (like lysozyme) and compounds that inhibit microbial growth and neutralize pathogens.
    Digestive Tract & Immune Function:
    The digestive tract is lined with mucosa comprising specialized epithelial cells that secrete mucus and antimicrobial peptides, creating a hostile environment for pathogens.
    Contains lymphoid tissues (such as Tonsils, Peyer’s patches) that are rich in macrophages and lymphocytes, thus playing a critical role in immune surveillance and response.
    Respiratory System Defense:
    Nose hairs (vibrissae) trap large particles (dust, allergens) while ciliated epithelial cells in the respiratory tract move trapped pathogens and mucus towards the throat for expulsion, effectively cleansing the airways.

II. Second Line of Defense: Non-Specific Response

Response to Breach: This response is triggered by tissue injury or infection and serves to contain and eliminate pathogens.
Components: White Blood Cells (WBCs) and Defensive Proteins that work together to identify and neutralize threats.
Responses Include: Phagocytosis, cytotoxicity, inflammation, and fever.

  1. Phagocytosis:
    Involves the process by which WBCs (such as neutrophils and macrophages) ingest and destroy pathogens.

  • Neutrophils: The most abundant type of WBC (50-70%), these cells are short-lived and specialize in responding quickly to bacterial infections, using enzymes and reactive oxygen species to eliminate pathogens.

  • Macrophages: Derived from monocytes, these long-lived immune cells play a dual role in phagocytosis and antigen presentation, which helps to activate the adaptive immune response.

  • Dendritic Cells: Act as antigen-presenting cells (APCs) that capture and present antigens to T cells, bridging innate and adaptive immunity.

  1. Cytotoxicity:
    Refers to the ability of specific immune cells to destroy infected or cancerous cells.

  • Natural Killer Cells (NK Cells): A type of lymphocyte that can identify and eliminate virally infected or cancerous cells without needing prior activation, providing rapid defense early in an infection.

  • Cytotoxic T Cells (Tc Cells): These cells recognize and destroy infected cells by binding to specific antigens presented via MHC-I molecules on infected cells and require activation by antigen-presenting cells.

  1. Protective Proteins:
    Complement Proteins play vital roles in enhancing antibody actions, opsonization, and recruiting additional immune cells to sites of infection. They are part of the immune system that enhances the ability of antibodies and phagocytic cells to clear microbes and damaged cells.

  2. Inflammatory Response:
    This is a non-specific response to tissue damage characterized by the classic signs of inflammation: redness, heat, swelling, and pain.
    Histamine and prostaglandins are released by damaged cells, increasing blood flow and capillary permeability, thus attracting phagocytes to the site of injury or infection.

  3. Fever (Pyrexia):
    An increase in body temperature is a common systemic response to infection, triggered by substances known as pyrogens.
    While fever can inhibit pathogen growth and enhance the immune response, it can also lead to harmful effects if excessively elevated, such as enzyme denaturation or organ dysfunction.

III. Third Line of Defense: Specific Defenses

Acquired/Adaptive Immune System: This more complex immune response is tailored specifically to individual pathogens and involves meticulous recognition of specific antigens presented by pathogens. It features a memory component that enables the immune system to respond more effectively to subsequent exposures to the same pathogen.
Components: Predominantly B lymphocytes (B cells) and T lymphocytes (T cells), these cells orchestrate the body's immune response.
Recognition of self vs non-self: This discrimination is crucial for preventing autoimmunity and is key to initiating appropriate immune responses against invading pathogens.
Types of Immunity:

  • Inborn (Innate) Immunity: This is the first line of defense that is non-specific and present from birth, comprising physical and biochemical barriers, as well as phagocytic cells.

  • Acquired (Adaptive) Immunity: This immunity is specific and develops throughout an individual’s life, assisted by exposure to pathogens and vaccinations, engaging memory cells for long-term protection.

Active Immunity:

  • Natural: Acquired through infection, leading to the development of memory cells.

  • Artificial: Results from vaccination, which stimulates the immune response without causing disease.

Passive Immunity:

  • Natural: Transferred from mother to child through breast milk or placenta, providing immediate but temporary protection.

  • Artificial: Immunity conferred through injections of immune serum (antibodies) that provide immediate protection but do not induce memory.

IV. Cell Mediated vs Antibody Mediated Immune Responses

Cell Mediated Immunity: Primarily involves T cells responding to infected cells or tumors, thus playing a pivotal role in eliminating intracellular pathogens.
Antibody Mediated Immunity: Involves B cells that produce antibodies against specific antigens, which neutralize toxins and opsonize pathogens for destruction.
T Cells:

  • Types: Helper T cells (CD4+), Cytotoxic T cells (CD8+), and Memory T cells are critical for orchestrating and sustaining immune responses.
    B Cells:

  • Upon activation by T cells or directly through antigen binding, these cells produce antibodies and generate memory B cells for long-term immunity against specific pathogens.

V. Monoclonal Antibodies:
These are laboratory-produced molecules derived from a single clone of B cells that target a specific antigen. They have numerous therapeutic applications, including the treatment of cancers, autoimmune disorders, and infectious diseases.
Monoclonal antibodies, first developed by Georges Köhler and César Milstein, have revolutionized medical treatment and diagnostic practices through their specificity and efficacy in targeting disease mechanisms.