X. Immunology
Key focus of this chapter: immunity system
This chapter focuses immunity system and gives concise summaries of the important things about lymphatic system, defense systme, nonspecific immunity, specific immunity, and immune responses.
A. Lymphatic system
• Interstitial fluid enters lymphatic capillaries and becomes lymph.
• Lymph nodes filter lymph and contain macrophages and lymphocytes that help remove pathogens and foreign material.
• Filtered lymph returns to the bloodstream through lymphatic vessels.
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B. Defense system
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C. Nonspecific Immunity
Rapid, nonspecific responses to infectious agents.
1. External defenses
a. Skin – Forms a physical barrier against invading pathogens.
b. Secretions – Lysozyme in tears and saliva helps destroy bacterial cell walls; other secretions also inhibit microbial growth.
c. Mucous membranes – Mucus traps microbes in the respiratory and digestive tracts.
2. Internal Defenses
a. Antimicrobial proteins
• Complement system
- Composed of a group of plasma proteins.
- Promotes inflammation, opsonization, and lysis of invading cells.
• Interferon
- Antiviral signaling proteins released by infected cells that help neighboring cells resist viral infection.
b. Inflammatory Response
• Histamine – Released mainly by mast cells; causes vasodilation and increased capillary permeability during inflammation.
• Chemokines – Attract immune cells, including phagocytes, to sites of infection or injury.
c. Phagocytic cells (macrophages)
Fig. 1
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d. Natural killer cells
• Kill virus-infected and tumor cells by releasing perforin and other cytotoxic molecules.
• Can trigger apoptosis (programmed cell death) in target cells.
D. Specific Immunity
Slower, antigen-specific responses that generate immunological memory.
1. Basic theory
a. Terminology
• Antibiotic
- Antimicrobial drug that kills microorganisms or inhibits their growth; antibiotics primarily target bacteria.
- Many antibiotics inhibit bacterial processes such as cell-wall synthesis or protein synthesis.
- Antibiotics do not treat viral infections.
• Cytokines
- Signaling proteins secreted by immune and other cells.
- Help activate and regulate immune cells and coordinate immune responses.
b. Structures of B cells and antigens
Fig. 2
• Antibodies
- Immunoglobulins produced by plasma cells; they circulate in blood and other body fluids.
- Bind specific antigens and help neutralize or mark them for destruction.
• Antigen
- A molecule or molecular structure specifically recognized by antibodies or lymphocyte receptors.
• Epitope
- A specific region of an antigen recognized by an antibody or lymphocyte receptor.
- Also called an antigenic determinant.
c. Lymphocytes
• Major lymphocyte types include B cells, T cells, and natural killer (NK) cells.
• Lymphocytes are central to adaptive immunity; NK cells are lymphocytes that function in innate immunity.
d. Structure of B-cell and T-cell receptors
Fig. 3
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e. Lymphocyte Development
Lymphocytes arise from hematopoietic stem cells in bone marrow. B cells mature in bone marrow, whereas T cells mature in the thymus.
Fig. 4
f. Clonal selection of lymphocytes
• Clonal selection – Antigen-driven activation and proliferation of lymphocytes with receptors specific to that antigen.
• Effector cells (e.g., plasma cells)
- Short-lived cells that carry out the immediate immune response.
- Plasma cells secrete antibodies specific to the activating antigen.
• Memory cells
- Long-lived cells.
- Carry receptors specific to the antigen and respond rapidly upon re-exposure.
• Primary immune response
- Antigen-specific B and T cells undergo clonal expansion and differentiate into effector and memory cells.
- The response develops relatively slowly during the first exposure.
• Secondary immune response
- Faster, stronger, and often longer-lasting response to the same antigen than the primary response.
2. Humoral and cell-mediated immunities
• Macrophage
- Engulfs pathogens and presents antigen fragments to helper T cells.
• Helper T cell
- Recognizes antigen presented by antigen-presenting cells on MHC class II molecules.
- Activates B cells, cytotoxic T cells, and other immune cells through cytokines and cell-to-cell signaling.
Fig. 5
a. Humoral Immunity
Humoral immunity is mediated by B cells and antibodies and is especially effective against extracellular pathogens and toxins.
Fig. 6
• B Cells
- Play the central role in humoral immunity.
- Recognize specific antigens and can be activated with help from helper T cells.
- Undergo clonal expansion to form memory B cells and plasma cells.
- Plasma cells secrete large amounts of antibodies into blood and lymph.
• Memory B cells
• Plasma cells – Produce antibodies
• Classes of antibodies
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b. Cell-Mediated Immunity
Activates cytotoxic T cells that destroy infected or cancerous cells.
Fig. 7
• Cytotoxic T cell
- Central to the cell-mediated immune response.
- Mature in the thymus and become activated after recognizing specific antigen.
- Kill target cells using perforin and granzymes.
• MHC class II molecules
- On antigen-presenting cells; present antigen to helper T cells.
• MHC class I molecules
- On nearly all nucleated cells; present antigen to cytotoxic T cells.
• T-cell receptor
- Recognizes antigen fragments presented by MHC.
E. Immune responses
- Immunity produced by the person's own immune response after infection or vaccination.
- Example: vaccination.
• Passive immunity
- Immunity provided by transfer of preformed antibodies from another source.
- Examples: maternal IgG crosses the placenta, and IgA is transferred in breast milk.
2. Blood group
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