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Which cells are primarily responsible for phagocytosis?
Which cells are primarily responsible for phagocytosis?
What is the primary function of neutrophils in relation to pathogens?
What is the primary function of neutrophils in relation to pathogens?
During the process of phagocytosis, what structure forms after the microbe is engulfed?
During the process of phagocytosis, what structure forms after the microbe is engulfed?
Natural killer (NK) cells are known for which of the following capabilities?
Natural killer (NK) cells are known for which of the following capabilities?
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What is the role of mast cells in the immune response?
What is the role of mast cells in the immune response?
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Where do B cells develop immunocompetence?
Where do B cells develop immunocompetence?
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What happens to lymphocytes after they leave the bone marrow?
What happens to lymphocytes after they leave the bone marrow?
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What is the primary role of mature immunocompetent lymphocytes?
What is the primary role of mature immunocompetent lymphocytes?
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Which of the following statements is true regarding naive immunocompetent cells?
Which of the following statements is true regarding naive immunocompetent cells?
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How do activated immunocompetent B and T cells circulate through the body?
How do activated immunocompetent B and T cells circulate through the body?
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What is one primary function of antimicrobial proteins?
What is one primary function of antimicrobial proteins?
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Which type of interferon do fibroblasts primarily secrete?
Which type of interferon do fibroblasts primarily secrete?
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What role does PKR play in the antiviral response?
What role does PKR play in the antiviral response?
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What is the primary source of gamma (γ) interferon?
What is the primary source of gamma (γ) interferon?
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Which of the following uses FDA-approved alpha interferon as a treatment?
Which of the following uses FDA-approved alpha interferon as a treatment?
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What is the primary characteristic of the innate immune system?
What is the primary characteristic of the innate immune system?
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Which component is NOT part of the first line of defense in the immune system?
Which component is NOT part of the first line of defense in the immune system?
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What role does inflammation play in the immune response?
What role does inflammation play in the immune response?
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What is the effect of skin acidity on bacteria?
What is the effect of skin acidity on bacteria?
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Which of the following best describes the second line of defense?
Which of the following best describes the second line of defense?
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What is a key function of the cilia in the upper respiratory tract?
What is a key function of the cilia in the upper respiratory tract?
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Which of these is NOT a chemical barrier produced by epithelial membranes?
Which of these is NOT a chemical barrier produced by epithelial membranes?
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Which cells are considered part of the body's internal defense mechanism?
Which cells are considered part of the body's internal defense mechanism?
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What is the primary role of complement proteins in the immune system?
What is the primary role of complement proteins in the immune system?
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Which pathway of complement activation is directly linked to the immune system?
Which pathway of complement activation is directly linked to the immune system?
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What is the consequence of C3b activation in the complement system?
What is the consequence of C3b activation in the complement system?
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What is the role of C-reactive protein (CRP) in the immune response?
What is the role of C-reactive protein (CRP) in the immune response?
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Which of the following accurately describes a characteristic of complete antigens?
Which of the following accurately describes a characteristic of complete antigens?
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What triggers a fever in response to invading microorganisms?
What triggers a fever in response to invading microorganisms?
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Which of the following best describes the function of B lymphocytes in the immune system?
Which of the following best describes the function of B lymphocytes in the immune system?
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What is the significance of antigenic determinants in immune response?
What is the significance of antigenic determinants in immune response?
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How do MHC proteins contribute to the immune system?
How do MHC proteins contribute to the immune system?
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Why are high fevers considered dangerous?
Why are high fevers considered dangerous?
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What distinguishes haptens from complete antigens?
What distinguishes haptens from complete antigens?
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What happens to T cells during the selection process in the thymus?
What happens to T cells during the selection process in the thymus?
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What determines whether a lymphocyte becomes a B cell or a T cell?
What determines whether a lymphocyte becomes a B cell or a T cell?
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Study Notes
Immunity: Two Intrinsic Defense Systems
- The immune system is composed of two defense systems: innate and adaptive.
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Innate is nonspecific and responds quickly.
- It functions as the body's first line of defense against invading microorganisms by blocking entry, especially through intact skin and mucous membranes.
- It acts as the second line of defense, using antimicrobial proteins, phagocytes, and other cells to inhibit the spread of invaders throughout the body.
- Its hallmark and most important mechanism is inflammation.
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Adaptive is specific and mounts an attack against specific foreign substances.
- This system takes longer to react than the innate system.
- It works in conjunction with the innate system.
Surface Barriers (First Line of Defense)
- The first line of defense is composed of skin, mucous membranes, and their secretions.
- Keratin in the skin presents a physical barrier to most microorganisms and is resistant to weak acids and bases, bacterial enzymes, and toxins.
- Mucosae provide similar mechanical barriers.
Epithelial Chemical Barriers
- Epithelial membranes produce various protective chemicals, including:
- Skin acidity (pH of 3 to 5) inhibits bacterial growth.
- Sebum contains chemicals toxic to bacteria.
- Stomach mucosae secrete concentrated HCl and protein-digesting enzymes.
- Saliva and lacrimal fluid contain lysozyme which breaks down bacterial cell walls.
- Mucus traps microorganisms entering the digestive and respiratory systems.
Respiratory Tract Mucosae
- Mucus-coated hairs in the nose trap inhaled particles.
- Mucosa of the upper respiratory tract is ciliated, allowing cilia to sweep dust- and bacteria-laden mucus away from the lower respiratory passages.
Internal Defenses (Second Line of Defense)
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The body uses nonspecific cellular and chemical devices to protect itself, specifically:
- Phagocytes
- Natural killer (NK) cells
- Inflammatory responses involving macrophages, mast cells, WBCs, and chemicals
- Antimicrobial proteins in blood and tissue fluids.
- Harmful substances are identified by the surface carbohydrates unique to infectious organisms.
1. Phagocytes
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Macrophages are the main phagocytic cells, wandering throughout regions in search of cellular debris.
- Fixed macrophages reside in the liver (Kupffer cells) and brain (microglia).
- Neutrophils become phagocytic when encountering infectious material.
- Eosinophils are weakly phagocytic against parasitic worms.
- Mast cells bind and ingest a wide range of bacteria.
Mechanism of Phagocytosis
- Microbes adhere to the phagocyte.
- Pseudopods engulf the particle (antigen) into a phagosome.
- Phagosomes fuse with a lysosome to form a phagolysosome.
- Invaders in the phagolysosome are digested by proteolytic enzymes.
- Indigestible and residual material is removed by exocytosis.
2. Natural Killer (NK) Cells
- NK cells can lyse and kill cancer cells and virus-infected cells.
- NK cells are a small, distinct group of large granular lymphocytes that react nonspecifically and eliminate cancerous and virus-infected cells.
- NK cells kill target cells by releasing perforins and other cytolytic chemicals.
- They also secrete potent chemicals that enhance the inflammatory response.
3. Antimicrobial Proteins
- Antimicrobial proteins enhance innate defenses by:
- Attacking microorganisms directly.
- Hindering microorganisms' ability to reproduce.
- Important antimicrobial proteins include:
- Interferon
- Complement proteins
4 a. Interferon (IFN)
- Genes that synthesize IFN are activated when a host cell is invaded by a virus.
- Interferon molecules leave the infected cell and enter neighboring cells, stimulating them to activate genes for PKR (an antiviral protein).
- PKR nonspecifically blocks viral reproduction in the neighboring cell.
Interferon Family
- Interferons are a family of related proteins with slightly different physiological effects.
- Lymphocytes secrete gamma (γ) interferon, while most other WBCs secrete alpha (α) interferon.
- Fibroblasts secrete beta (β) interferon.
- Interferons also activate macrophages and mobilize NKs.
- FDA-approved alpha IFN is used as an antiviral drug against hepatitis C virus and to treat genital warts caused by the herpes virus.
4 b. Complement
- Complement comprises 20 or so proteins circulating in the blood in an inactive form, including C1 through C9, factors B, D, and P, and regulatory proteins.
- It provides a major mechanism for destroying foreign substances in the body.
Complement
- Complement amplifies all aspects of the inflammatory response, kills bacteria and certain other cell types, and enhances the effectiveness of both nonspecific and specific defenses.
Complement Pathways
- Complement can be activated by two pathways: classical and alternative.
- The classical pathway is linked to the immune system, depending on the binding of antibodies to invading organisms. Subsequent binding of C1 to the antigen-antibody complexes (complement fixation) activates the pathway.
- The alternative pathway is triggered by an interaction among factors B, D, and P, and polysaccharide molecules present on microorganisms.
Complement Pathways
- Each pathway involves a cascade in which complement proteins are activated in an orderly sequence, each step catalyzing the next.
- Both pathways converge on C3, which cleaves into C3a and C3b.
- C3b initiates formation of a membrane attack complex (MAC), causing cell lysis by interfering with a cell’s ability to eject Ca2+.
- C3b also causes opsonization, and C3a causes inflammation.
C-reactive Protein (CRP)
- CRP is produced by the liver in response to inflammatory molecules, providing a clinical marker used to assess for the presence of an acute infection, the existence of an inflammatory condition, and its response to treatment.
Functions of C-reactive Protein
- CRP binds to the PC receptor of pathogens and exposed self-antigens.
- It plays a surveillance role in targeting damaged cells for disposal.
- It activates complement.
Fever
- Fever is an abnormally high body temperature in response to invading microorganisms.
- The body's thermostat is reset upwards in response to pyrogens, chemicals secreted by leukocytes and macrophages exposed to bacteria and other foreign substances.
Fever
- High fevers are dangerous as they can denature enzymes.
- Moderate fever can be beneficial by:
- Causing the liver and spleen to sequester iron and zinc (needed by microorganisms).
- Increasing the metabolic rate, which speeds up tissue repair.
Adaptive (Specific) Defenses (Third Line of Defense)
- The adaptive immune system is a functional system that:
- Recognizes specific foreign substances.
- Acts to immobilize, neutralize, or destroy foreign substances.
- Amplifies inflammatory response and activates complement.
Adaptive Immune Defenses
- The adaptive immune system is antigen-specific, systemic, and has memory.
- It is composed of two separate but overlapping arms:
- Humoral, or antibody-mediated (B Cell) immunity.
- Cellular, or cell-mediated (T Cell) immunity.
Antigens
- Antigens are substances that can mobilize the immune system and provoke an immune response.
- The targets of all immune responses are mostly large, complex molecules not normally found in the body (nonself).
Complete Antigens
- Important functional properties:
- Immunogenicity - the ability to stimulate proliferation of specific lymphocytes and antibody production.
- Reactivity - the ability to react with the products of the activated lymphocytes and the antibodies released in response to them.
- Complete antigens include foreign protein, nucleic acid, some lipids, and large polysaccharides.
Haptens (Incomplete Antigens)
- Small molecules, such as peptides, nucleotides, and many hormones:
- Are not immunogenic (do not stimulate a response), but are reactive when attached to protein carriers.
- If they link up with the body's proteins, the adaptive immune system may recognize them as foreign and mount a harmful attack (allergy).
- Haptens are found in poison ivy, dander, some detergents, and cosmetics.
Antigenic Determinants
- Only certain parts of an entire antigen are immunogenic.
- Antibodies and activated lymphocytes bind to these antigenic determinants.
- Most naturally occurring antigens have numerous antigenic determinants that mobilize several different lymphocyte populations and form different kinds of antibodies against it.
- Large, chemically simple molecules (e.g., plastics) have little or no immunogenicity.
Self-Antigens: MHC Proteins
- Our cells are dotted with protein molecules (self-antigens) that are not antigenic to us, but are strongly antigenic to others (the reason for transplant rejection).
- One type of these, MHC proteins, marks a cell as self.
- The two classes of MHC proteins are:
- Class I MHC proteins - found on virtually all body cells.
- Class II MHC proteins - found on certain cells in the immune response.
MHC Proteins
- These are coded for by genes of the major histocompatibility complex (MHC) and are unique to an individual.
- Each MHC molecule has a deep groove that displays a peptide, which is a normal cellular product of protein recycling.
- In infected cells, MHC proteins bind to fragments of foreign antigens, playing a crucial role in mobilizing the immune system.
Cells of the Adaptive Immune System
- Two types of lymphocytes oversee the adaptive immune system:
- B lymphocytes - oversee humoral immunity.
- T lymphocytes - non-antibody-producing cells that constitute the cell-mediated arm of immunity.
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Antigen-presenting cells (APCs):
- Do not respond to specific antigens.
- Play essential auxiliary roles in immunity.
Lymphocytes
- Immature lymphocytes released from bone marrow are essentially identical.
- Whether a lymphocyte matures into a B cell or a T cell depends on where in the body it becomes immunocompetent.
- B cells mature in the bone marrow.
- T cells mature in the thymus.
T Cell Selection in the Thymus
- The thymus selects for T cells that are both immunocompetent and self-tolerant.
- T cells mature in the thymus under negative and positive selection pressures.
- Negative selection eliminates T cells that are strongly anti-self.
- Positive selection selects T cells with a weak response to self-antigens.
B Cells
- B cells become immunocompetent and self-tolerant in bone marrow.
- Some self-reactive B cells are inactivated (anergy), while others are killed.
- Other B cells undergo receptor editing in which there is a rearrangement of their receptors.
Immunocompetent B or T cells
- Display a unique type of receptor that responds to a distinct antigen.
- Become immunocompetent before they encounter antigens they may later attack.
- Are exported to secondary lymphoid tissues where encounters with antigens occur.
- Mature into fully functional antigen-activated cells upon binding with their recognized antigen.
- It is genes, not antigens, that determine which foreign substances our immune system will recognize and resist.
Immunocompetent B or T cells
- Lymphocytes destined to become T cells migrate to the thymus and develop immunocompetence there.
- B cells develop immunocompetence in red bone marrow.
- After leaving the thymus or bone marrow as naive immunocompetent cells, lymphocytes “seed” the lymph nodes, spleen, and other lymphoid tissues where the antigen challenge occurs.
- Mature (antigen-activated) immunocompetent lymphocytes circulate continuously in the bloodstream and lymph and throughout the lymphoid organs of the body.
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Description
Explore the two main defense systems of the immune system: innate and adaptive. This quiz covers how these systems function, their mechanisms, and the role of surface barriers as the first line of defense against invaders. Test your understanding of these vital biological processes!
