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Questions and Answers
What is the outcome when C3b becomes deposited on the surface of a microbe?
The alternative pathway is triggered
What is formed when C3b binds to factor B on the microbial surface?
C3 convertase
What role does properdin play in the alternative pathway?
Stabilization of C3 convertase
How does C3b attached to the microbial surface affect the rest of the complement components?
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What type of immunity is the alternative pathway involved in?
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What is the difference in initiation between the classical and alternative pathways?
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What is the role of C5a in the complement system?
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What is the result of opsonization by C3b?
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What is the difference in specificity between the classical and alternative pathways?
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What is the final product of the alternative pathway?
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Study Notes
Physical Barriers
- The first line of defense prevents or retards the entry of cells and molecules into the body
- Key features of the skin:
- Physical barrier that prevents or retards the entry of cells and molecules into the body
- Key features of the eyes:
- Tears contain antimicrobial substances (e.g., lysozyme) that wash away microbes on the eye surface
- Secretions of lacrimal glands contain lysozyme that protects the eyes
Chemical Barriers
- Antimicrobial substances (e.g., lysozyme) in tears and lacrimal gland secretions that protect the eyes
- Antimicrobial substances in respiratory, urinary, and gastrointestinal tracts
Biological Barriers
- Microbes in the respiratory, urinary, and gastrointestinal tracts
Antigens and Epitopes
- Antigen: a substance that can induce an immune response
- Antigenicity: the ability to combine with the final products of the immune response (antibodies and/or T-cell receptors)
- Epitope (Antigenic Determinant): the region of an antigen that binds to a T-cell receptor or a B-cell receptor (antibody)
- T and B cells recognize different epitopes on an antigen
Complement System
- A system of proteins that work together to help eliminate pathogens from the body
- Three pathways: Classical, Alternative, and Lectin pathways
- Proteins are denoted by the capital letter C with numbers (e.g., C1, C2, C3) or alphabets (e.g., B, D)
Classical Complement Pathway
- Activation:
- Antigen-antibody complexes activate C1 (C1q, r, s) to form a protease
- C1 cleaves C2 and C4 to form C4b,2a, which cleaves C3 into C3a and C3b
- C3b binds to C4b,2a, producing a new enzyme, C5 convertase (C4b,2a,3b)
- C5 convertase cleaves C5 to form C5a and C5b
- C5b binds to C6 and C7 to form a complex that interacts with C8 and C9 to produce the membrane attack complex (MAC)
- Osmotic lysis: cell death due to the insertion of the MAC into the membrane
Lectin Complement Pathway
- Activation:
- Mannose-binding lectin (MBL) recognizes and binds to pathogens through repeating carbohydrate sequences
- MBL binding causes a conformational change, activating associated serine proteases (MASPs) that initiate the lectin pathway
- This pathway bypasses the antibody-requiring step, providing early protection in infection (innate immunity)
Alternative Complement Pathway
- Activation:
- C3 is abundant in plasma and spontaneously cleaved to C3b
- C3b attaches to the surface of a microbe, endotoxin, or yeast cell wall
- C3b forms a complex with factor B, which is the C3 convertase of the alternative pathway
- Factor D and properdin help generate and stabilize the C3 convertase
- C3 convertase breaks down more C3, attaching more C3b to the microbial surface, which activates the rest of the complement components
Biologic Effects of Complement
- Opsonization: C3b promotes phagocytosis of microbes by phagocytes with C3b receptors
- Chemotaxis: C5a attracts neutrophils
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Description
This quiz covers the physical, chemical, and biological barriers that protect the human body from external threats, including the skin and eyes.
