Page 80 - Textbook of Pathology, 6th Edition

64 NATURAL KILLER (NK) CELLS. NK cells comprise about or to B cells. Accordingly, the lymphoid cell would then deal
10-15% of circulating lymphocytes. These lymphocytes do with such antigen.
not have B or T cell markers, nor are these cells dependent
upon thymus for development unlike CD4+ and CD8+ T Basophils and Mast Cells
cells. NK cells carry surface molecules of CD2, CD16 and Basophils are a type of circulating granulocytes (0-1%) while
CD56, but negative for T cell marker CD3. NK cells are mast cells are their counterparts seen in tissues, especially
morphologically distinct from B and T cells in being large in connective tissue around blood vessels and in submucosal
granular lymphocytes. location. Basophils and mast cells have IgE surface receptor;
NK cells are part of the natural or innate immunity. These
SECTION I
cells recognise antibody-coated target cells and bring about thus on coming in contact with antigen binding to IgE (e.g.
allergic reaction to parasites), these cells get activated and
killing of the target directly; this process is termed as antibody- release granules i.e. degranulate. These granules contain
dependent cell-mediated cytotoxicity (ADCC). This mechanism substances such as: histamine, platelet activating factor,
is particularly operative against viruses and tumour cells.
heparin and certain chemical mediators (e.g. prostaglandins,
leukotrienes).
Monocytes and Macrophages
Mast cells and basophils are thus involved in mediating
The role of macrophages in inflammation consisting of inflammation in allergic reactions and have a role in wound
circulating monocytes, organ-specific macrophages and healing.
histiocytes has been described in Chapter 6. Circulating
monocytes are immature macrophages and constitute about Neutrophils
5% of peripheral leucocytes. They remain in circulation for Polymorphonuclear neutrophils (PMNs) are normally the
about 3 days before they enter tissues to become macrophages. most numerous of the circulating leucocytes (40-75%). The
The macrophage subpopulations like the dendritic cells found cytoplasm of PMNs contains lysosomal granules of three
in the lymphoid tissue and Langerhans’ cells seen in the types: primary (azurophilic), secondary, and tertiary.
epidermis, are characterised by the presence of dendritic PMNs have similar function to those of macrophages and
cytoplasmic processes and are active in the immune system. are therefore appropriately referred to as ‘microphages’ owing
Salient features and important immune functions of
macrophages are as follows: to their role as first line of defense against an invading foreign
organism in the body. However, these cells have limitation
1. Antigen recognition. They possess cell surface receptors of size and type of organisms to be engulfed e.g. while they
to several extracellular molecules— receptor for cytokines, are capable of acting against bacteria and small foreign
component of complement (C3b), selectins, integrins and Fc particulate material but not against viruses and large
General Pathology and Basic Techniques
(constant fragment) of antibody. These receptors recognise
the organisms and initiate intracellular mechanism in particles.
macrophages. Antigen to become recognisable can also get
coated by antibodies or complement, the process being Eosinophils
termed as opsonisation. Macrophages have capacity to Eosinophils are also circulating granulocytes (1-6%). These
distinguish self from non-self by presence of human leucocyte cells play a role in allergic reactions and in intestinal
antigens (HLA) or major histocompatibilty complex (MHC) helminthiasis. The granules of eosinophils contain lysosomal
discussed below. enzymes, peroxidases, and chemical mediators of
2. Phagocytosis. Antigen that has been recognised by the inflammation (e.g. prostaglandins, leukotrienes). On coming
macrophages due to availability of above-mentioned surface in contact with IgE opsonised antigen (e.g. helminths),
receptors, or the opsonised antigen, is ready to be engulfed eosinophils degranulate and release the chemicals stored in
by the process of cell-eating by macrophages explained on granules and incite inflammation.
page 134.
3. Secretory function. Macrophages secrete important HLA SYSTEM AND MAJOR
substances as follows:
i) Cytokines (IL-1, IL-2, IL-6, 8, IL-10, IL-12, tumour necrosis HISTOCOMPATIBILITY COMPLEX
factor-α) and prostaglandins (PGE, thromboxane-A, Though not a component of immune system, HLA system is
leukotrienes) which are chemical mediators of inflammation described here as it is considered important in the regulation
and activate other leucocytes.
of the immune system. HLA stands for Human Leucocyte
ii) Secretion of proteins involved in wound healing e.g. Antigens because these antigens or genetic proteins in the
collagenase, elastase, fibroblast growth factor, angiogenesis body which determine one’s own tissue from non-self
factor. (histocompatibility) were first discovered on the surface of
iii) Acute phase reactants e.g. fibronectin, microglobulin, leucocytes. Subsequently, it was found that HLA are actually
complement components. gene complexes of proteins on the surface of all nucleated
4. Antigen presentation. When macrophages are unable to cells of the body and platelets. Since these complexes are of
lyse an antigen or an organism, the next best course adopted immense importance in matching donor and recipient for
by them is to act as antigen-presenting cells for presenting organ transplant, they are called major histocompatibility
to immunocompetent T cells (subtype CD4+ or CD8+ cells), complex (MHC) or HLA complex.