Page 113 - Concise Pathology for Exam Preparation ( PDFDrive )

98 SECTION I General Pathology

basophils are activated by (a) cross-linking of high-affinity IgE Fc receptors, (b) anaphyla-
toxins (C3a and C5a) and (c) IL-8. These cells express high-affinity receptors called FceRI
which are specific for the Fc portion of IgE antibodies and avidly bind them. When in a
previously sensitized individual, a mast cell bound to IgE antibodies is exposed to the
same antigen; there is activation of the cell and release of powerful mediators.
�Primary mediators: These are immediately released already formed stored mediators
1.
which induce smooth muscle contraction, increased vascular permeability, increased
mucous production (by nasal, bronchial, gastric glands) as well as platelet granule re-
lease. They include
Biogenic amines
(a)
(i) Histamine

(ii) Adenosine

(iii) 5-Hydroxytryptamine (5HT)
(b)
Chemotactic mediators
Eosinophil chemotactic factor (ECF)
(i)
(ii)
Neutrophil chemotactic factor (NCF)
(c)
Enzymes
in granule matrix, eg, proteases (tryptase, chymase) and acid
Contained �
hydrolases
(d)
Proteoglycans
Heparin and chondroitin sulphate (serve to package and store other mediators in
the granules)
2.
�Secondary mediators: These are synthesized de novo and released late.
(a)
Lipid mediators

(i) Leukotrienes
(vasoactive and spasmogenic)
- C 4 , D 4
(chemotactic for neutrophils, eosinophils and monocytes)
- B 4

(ii) Prostaglandin D 2
�
- Generated by cyclooxygenase pathway
�
- Induce bronchospasm and increased mucous secretion
�

(iii) Platelet-activating factor (PAF)
- Induces platelet activation and bronchospasm
- Releases histamine
- Increased vascular permeability
- Is chemotactic for neutrophils and eosinophils

- Activates inflammatory cells and causes their aggregation and degranulation

6
-
Cytokines include TNF-a, ILs-1, -3, -4, -5,
(b) {T H 2 response}, GMCSF, chemo-

kines, and macrophage inhibitory protein (MIP-1a and b). Cytokines recruit and

activate inflammatory cells and mast cells

Q. What are mast cells? How do they contribute to type I

hypersensitivity?
Ans. Mast cells are bone marrow-derived cells, widely distributed in tissues near blood
vessels and nerves and in subepithelial sites. They contain membrane-bound granules that
possess a variety of biological mediators including PAF, histamine, leukotrienes C 4 , D 4 , E 4 ,
prostaglandins, cytokines, ECF and NCF.
Q. Write briefly on the role of eosinophils in type I hypersensitivity.
Ans. Role of eosinophils in type I hypersensitivity is shown in Flowchart 5.3.
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