Immune System Practice Flashcards

Comparison of Immune System Branches

• The immune system is not composed of specific organs or tissues in the traditional sense, but rather a series of processes focused on defense.

• Innate Immunity:

  • These are the processes we are born with.

  • They provide an immediate response to pathogens.

  • They do not require previous exposure or training and do not change after repeated encounters.

• Adaptive Immunity:

  • These defenses must be trained through exposure to specific pathogens.

  • We do not possess adaptive immunity at birth.

  • It develops over time; we do not have immunity to a pathogen we have never encountered.

Immunity and Clinical Testing

• Definition of Immunity: True immunity occurs when a person has been exposed to a substance, developed an immune response, and possesses specific antibodies.

• Titer Blood Test:

  • A diagnostic blood sample used to check for immunity by measuring the amount of specific antibodies in the system.

  • Used to verify vaccine records (e.g., Hepatitis B, Chickenpox) or check for past/recent exposure.

  • A result described as "off the charts" suggests a very high antibody count, potentially indicating a recent exposure.

• Booster Effect: Every subsequent exposure to a pathogen after immunity has been established serves to increase that immunity, effectively "doubling the armies" of the immune system.

Second Line Defenses: Innate Immune Cells

• Nonspecific Targeting: These cells search for, fight, and destroy foreign substances, viruses, bacteria, and cancer cells without having one specific target.

• Phagocytes: Groups of cells that kill or ingest foreign materials.

  • Neutrophils: Often the first to arrive; they are phagocytic but frequently die after consuming pathogens (forming pus).

  • Monocytes/Macrophages: Monocytes last approximately $5$ days in the blood before migrating to tissues where they live for months as macrophages. They act as the "demo crew," cleaning up debris and damaged cell components.

Second Line Defenses: Protective Proteins

• Complement Proteins:

  • Produced primarily by the liver.

  • They provide a nonspecific defense against anything foreign.

  • Functions include:

    • Opsonization: Targeting bacteria for destruction.

    • Neutralization: Binding to viruses to prevent them from entering host cells.

• Cytokines:

  • Proteins produced specifically by immune cells (unlike complement proteins produced in the liver).

  • Tumor Necrosis Factor (TNF):

    • Kills cancer cells; humans produce cancer cells daily through genetic mutations, but a healthy immune system destroys them.

    • Responsible for the "feeling like crud" sensation (fatigue, achiness) during illness.

    • Serves as a signal for the body to divert energy from physical activity/study toward fighting infection.

    • High levels are associated with septic shock risk; septic shock involves dangerously low blood pressure.

  • Interferons:

    • Released by cells already infected with a virus.

    • Act as a paracrine signal to warn neighboring cells to prepare defenses (putting on "masks"/sanitizer).

    • Trigger programmed cell death (apoptosis) in the infected cell.

    • Synthetic interferon is used clinically to treat conditions like Hepatitis C.

  • Interleukins (IL):

    • Approximately $29$ identified types, numbered by order of discovery (e.g., $IL-2$, $IL-6$).

    • Function in cell-to-cell communication to coordinate immune responses, such as stimulating neutrophils to perform chemotaxis or triggering apoptosis.

The Inflammatory Response

• Inflammation is a beneficial innate process that bridges the gap between injury and rebuild.

• Process Stages:

  1. Damaged cells release inflammatory mediators locally.

  2. These mediators trigger nociceptors (pain) and attract phagocytes to clean the area.

• Inflammatory Mediators:

  • Histamine: Primarily released by mast cells and basophils.

  • Serotonin: Found in the digestive tract.

  • Bradykinin.

  • Prostaglandins: Primarily responsible for the pain response; they act as a protective mechanism to prevent further injury to the area.

• The Cardinal Signs of Inflammation:

  • Redness: Caused by increased blood flow (Hyperemia) to the area via vasodilation.

  • Heat: Also a result of increased blood flow.

  • Swelling (Edema): Caused by increased capillary permeability, allowing fluid to leak into the tissue.

  • Pain.

• Cellular Recruitment:

  • Chemotaxis: Cells follow a chemical gradient (gradient of cytokines or cytoplasmic leaked components like mitochondria) to find the site of injury.

  • Margination: Neutrophils or monocytes stick to the interior wall of the blood vessel.

  • Diapedesis: The process where cells squeeze through the dilated vessel wall to enter the interstitial fluid.

• Pus: The accumulation of dead leukocytes, primarily neutrophils, that have died from exhaustion/overconsumption during a "respiratory burst."

Fever (Pyrexia)

• Definition: A core body temperature above $101.4\,^\circ F$.

• Mechanism:

  • The hypothalamus resets the body's "thermostat" to a higher setting (e.g., from $70\,^\circ C$ to $75\,^\circ C$ in an analogy).

  • Shivering: Triggered by the hypothalamus to generate heat when the body detects it is below the new, higher set point.

• Pyrogenic Bacteria: Certain bacteria induce fever to trigger immune responses.

• Benefits:

  • Warm temperatures optimize the function of phagocytes and Cell Adhesion Molecules (CAMS).

  • Smokes out/inhibits bacteria that are sensitive to temperature changes.

• Risks: Extreme fevers can denature proteins and lead to death. Clinical intervention (antipyretics like Tylenol) is usually recommended when temperatures exceed $102\,^\circ F$.

T Cell Mediated Adaptive Immunity

• T Cell Selection in the Thymus:

  • T cells originate in the bone marrow but mature in the thymus.

  • Positive Selection: T cells must develop a functional receptor (CD4 or CD8) through random genetic reorganization.

  • Negative Selection: T cells must not be self-reactive (must not attack "self-antigens").

  • $95\%$ of T cells fail this gauntlet and are destroyed by macrophages.

• Types of T Cells:

  • Helper T Cells ($T_h$ or $CD4$): The "General" of the immune system. They coordinate the entire response. They only bind to Major Histocompatibility Complex (MHC) Class II.

  • Cytotoxic T Cells ($T_c$ or $CD8$): Directly kill infected or cancerous cells. They only bind to MHC Class I.

• Antigen Presentation (MHC):

  • MHC Class I: Found on all nucleated cells. Used for the endogenous pathway (inside fragments). Bind to $CD8$ cells. (Rule of Eight: $1 \times 8 = 8$).

  • MHC Class II: Found on Antigen Presenting Cells (APCs). Used for the exogenous pathway (eaten fragments). Bind to $CD4$ cells. (Rule of Eight: $2 \times 4 = 8$).

• Antigen Presenting Cells (APCs):

  • Dendritic Cells: The most common activator. They can perform "cross-presentation" (co-presentation), activating $CD8$ cells without being killed themselves.

  • Macrophages.

  • B Lymphocytes.

• T Cell Activation: Naive cells become activated $\rightarrow$ undergo mitosis (cloning) $\rightarrow$ produce Effector T cells (current battle) and Memory T cells (future battles).

B Cell Mediated Adaptive (Humoral) Immunity

• B Cells: Develop and mature in the bone marrow. Named "B" for Bone Marrow.

• Activation: A naive B cell is activated by a Helper T cell ($CD4$) after encountering its specific antigen.

• Differentiation:

  • Plasma Cells: Antibody factories that sacrifice their components to produce antibodies until they burst.

  • Memory B Cells: Retains the "blueprint" for the antibody for future exposures.

• Antibody (Immunoglobulin) Classes:

  • IgG: Most common ($70-85\%$); can cross the placenta.

  • IgA: Found in secretions (tears, breast milk).

  • IgM: Large pentamer; first antibody produced in an initial infection; excellent at agglutination.

  • IgE: Triggers basophil degranulation; involved in allergies.

  • IgD: Acts as the antigen receptor on the surface of B cells.

Organ Grafts and Rejection

• Autograft: Tissue moved from one part of a person's body to another (e.g., skin graft from thigh to nose).

• Isograft: Grafts between genetically identical individuals (identical twins).

• Allograft: Grafts between non-identical members of the same species (most common). Requires immunosuppressant therapy for life.

• Xenograft: Grafts from a different species (e.g., bovine/pig heart valves).

• Clinical Logic: Bone marrow has the highest MHC matching requirement because donor immune cells can attack the recipient (Graft-versus-host-disease).

Types of Acquired Immunity

• Active Natural: Getting sick and making your own antibodies.

• Active Artificial: Vaccines; the body is triggered to make antibodies without getting the actual disease.

• Passive Natural: Receiving antibodies from a mother via the placenta ($IgG$) or breast milk ($IgA$).

• Passive Artificial: Receiving pre-made antibodies via plasma infusion or monoclonal antibody treatments.

Immune System Disorders

• Hypersensitivity: The immune system overreacts to harmless substances (e.g., allergies).

• Immunodeficiency: The immune response is failing or absent (e.g., HIV/AIDS targeting Helper T cells).

• Autoimmune Disorders: The immune system misidentifies self-antigens as foreign (e.g., Lupus, MS, Hashimoto's, Rheumatoid Arthritis, Raynaud's).