Page 59 - Concise Pathology for Exam Preparation ( PDFDrive )

44 SECTION I General Pathology

3. Mannose-binding lectin pathway or MBL–MASP (MBL-associated serine proteases;
homologous to the classical pathway; Flowchart 2.9):

Binding of MBL to mannose residues on pathogen surface

Activation of the MBL-associated serine proteases, MASP-1 and MASP-2 (very similar to C1r and C1s,
respectively)

Splitting of C4 into C4a and C4b and C2 into C2a and C2b

Binding together of C4b and C2a to form the C3-convertase as in the classical pathway
FLOWCHART 2.9. Complement activation by MBL–MASP pathway.

Effector Functions of Complement Proteins
• C3b: Opsonization
• C5a: Chemotaxis
• C3a and C5a: Increased permeability of the capillary beds
• The early complement components break down and eliminate the antigen-antibody
complexes from the body, failure of which can lead to immune complex diseases.
• Killing of microbes through direct lysis is mediated by the MAC, C5b-9 (Complement-
mediated lysis can cause serious disorders such as Rh disease, immune haemolytic anaemia
and immune thrombocytopenic purpura).
• Complement promotes antibody formation through breakdown products. Break-
down of C3b generates a fragment (C3d) that binds to antigens to aid in their uptake
by antigen-presenting cells and B cells.

Q. Enumerate the steps involved in activation of kinin and clotting
systems.

Ans. Activated Hageman factor initiates four interrelated systems (Flowchart 2.10),
namely,
• Kinin system
• Clotting system
• Fibrinolytic system
• Complement system
Activation of prekallikrein activator by factor XIIa eventually generates bradykinin,
which has the following functions:
• Smooth muscle contraction
• Vasodilatation
• Increased vascular permeability
• Generation of pain

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