PracticeVideosBlogFlashcardsEspañol

Free exam prep

TEAS 7 Science

Practice QuestionsStudy Guide

5.2 The Immune & Lymphatic Systems

Key Takeaways

  • Innate (nonspecific) immunity provides immediate, generalized defense through physical surface barriers, phagocytes, Natural Killer (NK) cells, inflammation, and the complement system.
  • Adaptive (specific) immunity develops memory and targeted responses through antibody-mediated humoral immunity (B-cells) and cell-mediated immunity (T-cells).
  • B-cells differentiate into plasma cells that produce five distinct immunoglobulin classes (IgG, IgM, IgA, IgE, IgD) to neutralize pathogens.
  • Cytotoxic T-cells (CD8+) kill virus-infected and tumor cells presenting foreign antigens on MHC Class I markers, while Helper T-cells (CD4+) release cytokines to coordinate adaptive immune responses.
  • Active immunity generates memory cells through direct infection or vaccination, whereas passive immunity provides immediate, temporary antibody protection without forming memory cells.
Last updated: July 2026

Overview of Immune & Lymphatic Physiology

The immune system is a complex functional network of cells, tissues, and soluble proteins that defends the body against pathogenic micro-organisms (bacteria, viruses, fungi, parasites) and surveils for transformed malignant cells. Working in tandem, the lymphatic system acts as a structural network that drains excess interstitial fluid, absorbs dietary lipids, and provides housing for immune surveillance.

Defenses are broadly categorized into two interconnected divisions:

  1. Innate (Nonspecific) Immunity: Present at birth, acts immediately, responds identically to all pathogens, lacks memory.
  2. Adaptive (Specific) Immunity: Acquired through exposure, highly specific to distinct molecular structures (antigens), generates long-lasting immunological memory.

Innate (Nonspecific) Immunity

1. First Line of Defense: Physical & Chemical Surface Barriers

  • Skin: Keratinized stratified squamous epithelium creates an anatomical barrier resistant to weak acids, bases, and bacterial enzymes. Sweat and sebum secretions form an acid mantle (pH 4.5–5.5) that inhibits microbial colonization.
  • Mucous Membranes: Line body cavities open to the exterior (respiratory, GI, urogenital tracts). Mucus traps microbes. Respiratory epithelial cells possess cilia that sweep mucus upward away from lungs (mucociliary escalator).
  • Chemical Defenses: Lysozyme (an enzyme in tears, saliva, and nasal secretions) cleaves bacterial peptidoglycan cell walls. Gastric juice contains hydrochloric acid (pH 1.5–3.5) and pepsin, destroying swallowed micro-organisms.

2. Second Line of Defense: Internal Innate Defenses

  • Phagocytes:
    • Neutrophils: Most abundant white blood cells; first responders to tissue injury that engulf bacteria and undergo apoptosis, forming pus.
    • Macrophages: Derived from circulating monocytes; large, long-lived tissue phagocytes (e.g., Kupffer cells in liver, microglial cells in brain) that ingest debris and present antigens to T-cells.
  • Natural Killer (NK) Cells: Specialized cytotoxic lymphocytes that patrol the body for virus-infected or tumor cells. NK cells detect cells lacking normal Major Histocompatibility Complex (MHC) Class I self-markers and release perforin (forms pores in target membranes) and granzymes (proteases entering pores to induce apoptosis).
  • Inflammatory Response: A systemic localized reaction triggered by tissue damage. Damaged tissues and mast cells release chemical mediators including histamine, bradykinin, and prostaglandins.
\text{Tissue Injury} &\rightarrow \text{Histamine Release} \\ &\rightarrow \text{Vasodilation} \rightarrow \text{Hyperemia (Redness \& Heat)} \\ &\rightarrow \text{Increased Permeability} \rightarrow \text{Exudate Leakage (Edema \& Pain)} \\ &\rightarrow \text{Phagocyte Chemotaxis} \end{aligned}$$ - **Complement System**: A cascade of ~30 plasma proteins ($C1 {--}C9$) synthesized by the liver. Activated via classical (antibody-bound), lectin, or alternative pathways, yielding three outcomes: 1. *Opsonization*: $C3b$ coats pathogen surfaces, enhancing phagocyte binding. 2. *Inflammation*: $C3a$ and $C5a$ stimulate histamine release and attract leukocytes. 3. *Cell Lysis*: $C5b {--}C9$ assembly forms a **Membrane Attack Complex (MAC)** that bores holes into bacterial cell membranes, causing osmotic lysis. - **Interferons**: Cytokines secreted by virus-infected host cells. Interferons bind to receptors on neighboring healthy cells, signaling them to synthesize antiviral proteins that block viral protein synthesis and replication. --- ## Adaptive (Specific) Immunity Adaptive immunity relies on lymphocytes recognizing specific foreign **antigens** presented by **Major Histocompatibility Complex (MHC)** glycoproteins: - **MHC Class I**: Found on all nucleated body cells; presents endogenous antigens (e.g., viral proteins synthesized inside cell) to $CD8^+$ Cytotoxic T-cells. - **MHC Class II**: Found exclusively on professional **Antigen-Presenting Cells (APCs)** (macrophages, dendritic cells, B-cells); presents exogenous digested antigens to $CD4^+$ Helper T-cells. ### 1. Humoral (Antibody-Mediated) Immunity Driven by **B-lymphocytes**, which mature in red bone marrow. When a naive B-cell binds its specific antigen via membrane-bound immunoglobulin (and receives $CD4^+$ Helper T-cell co-stimulation), it undergoes clonal selection, multiplying into **Plasma Cells** and **Memory B-Cells**. Plasma cells secrete soluble **Antibodies (Immunoglobulins)** structured as Y-shaped proteins with two heavy chains and two light chains. Antibodies feature a variable ($V$) region that binds specific antigen epitopes and a constant ($C$) region that determines antibody class. | Immunoglobulin Class | Structural Form | Key Physiological Functions | |---|---|---| | **IgG** | Monomer | Most abundant in blood (~80%). Only class that **crosses the placenta** to provide passive fetal immunity. Opsonizes bacteria and activates complement. | | **IgM** | Pentamer | First antibody secreted during primary immune response. Potent agglutinating agent and complement activator. | | **IgA** | Dimer | Located in body secretions (saliva, tears, sweat, breast milk, colostrum, mucus). Protects mucosal surfaces against pathogen entry. | | **IgE** | Monomer | Binds Fc receptors on mast cells and basophils. Triggers **histamine release in allergic reactions** and defends against parasitic worms. | | **IgD** | Monomer | Functions primarily as an antigen receptor on the surface of naive B-cells. | ### 2. Cell-Mediated Immunity Driven by **T-lymphocytes**, which originate in bone marrow but migrate to the **thymus** for maturation, undergoing positive and negative selection to ensure immunocompetence and self-tolerance. - **Helper T-Cells ($CD4^+$ $T_H$)**: Master regulators of adaptive immunity. Recognize antigens on MHC Class II of APCs. Secrete cytokines (interleukins) that activate B-cells, cytotoxic T-cells, and macrophages. - **Cytotoxic T-Cells ($CD8^+$ $T_C$)**: Recognize foreign antigens on MHC Class I of infected or altered host cells. Directly destroy target cells by secreting **perforin** and **granzymes** to induce programmed cell death (apoptosis). - **Regulatory T-Cells ($T_{reg}$)**: Suppress immune responses to prevent autoimmune reactions. - **Memory T-Cells**: Endure for decades to launch rapid secondary cell-mediated responses. --- ## Active vs. Passive Immunity Immunity can be acquired naturally or artificially through active or passive means: ``` ┌── Natural ─── Infection (e.g., catching measles) ┌── Active ────┤ │ └── Artificial ── Vaccine (e.g., MMR, tetanus toxoid) Immunity ─────┤ │ ┌── Natural ─── Maternal IgG across placenta / IgA in milk └── Passive ───┤ └── Artificial ── Anti-venom / Immunoglobulin injection ``` - **Active Immunity**: The host produces its own antibodies and memory cells after antigen exposure. Provides long-term protection. - **Passive Immunity**: Preformed antibodies are transferred into the host. Provides immediate protection but lasts only weeks to months because no memory cells are created. --- ## Anatomy of the Lymphatic System 1. **Lymphatic Vessels**: One-way vascular network returning ~3 liters per day of leaked ECF (interstitial fluid) back to blood circulation as **lymph**. - Microscopic **lymphatic capillaries** possess overlapping endothelial flap valves that open when tissue hydrostatic pressure rises. - Lymph flows into larger vessels, passing through **lymph trunks** to two main collecting ducts: - **Right Lymphatic Duct**: Drains lymph from the right arm, right thorax, and right side of head into the right subclavian vein. - **Thoracic Duct**: Arises from the **cisterna chyli**; drains lymph from the entire rest of the body into the left subclavian vein. 2. **Lymph Nodes**: Bean-shaped organs clustered along lymphatic vessels (cervical, axillary, inguinal regions). Lymph enters via multiple **afferent vessels**, filters through cortical lymphoid follicles (B-cells) and medullary cords (T-cells and macrophages), and exits via a single **efferent vessel** at the hilum. 3. **Spleen**: Largest lymphoid organ, located in the left upper quadrant. **White pulp** contains lymphocytes for immune surveillance of blood-borne pathogens. **Red pulp** contains macrophages that destroy aged, worn-out red blood cells and store platelets/iron. 4. **Thymus**: Bilobed organ in the superior mediastinum. Site of T-cell maturation. Secretes thymosin; highly active during childhood, undergoing post-pubertal atrophy (**involution**). 5. **MALT**: Mucosa-Associated Lymphoid Tissue (tonsils, Peyer's patches in ileum, appendix) guarding mucosal entryways.
Test Your Knowledge

Which antibody class is the most abundant in circulation and is uniquely capable of crossing the placenta to confer passive immunity to the fetus?

A
B
C
D
Test Your Knowledge

Which immune component is part of the innate (nonspecific) internal defenses and destroys virus-infected or tumor cells that lack normal Major Histocompatibility Complex (MHC) Class I markers?

A
B
C
D
Test Your Knowledge

Receiving a tetanus toxoid vaccine to stimulate the production of your own antibodies and memory cells is an example of which type of immunity?

A
B
C
D
Up Next

5.3 The Skeletal System & Bone Physiology

Continue learning