Page 160 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

142 Veterinary Histology of Domestic Mammals and Birds

Morphologically, lymphocytes are characterised by a T 1 cells have an important role in infection, synthesis-
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VetBooks.ir round, heterochromatic nucleus (occasionally with a ing interferon, interleukin-2 and tumour necrosis factor-α.
nucleolus), a narrow rim of cytoplasm and, typically, few They promote activation of cytotoxic T cells and destruc-
tion of pathogens by macrophages.
organelles. Due to a high density of ribosomes, polyribo-
somes and rough ER, the cytoplasm is basophilic (Figure T 2 cells secrete numerous immunomodulators
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7.8). including interleukin-3, -4, -5 and -6. Their main functions
Lymphocytes exhibit a broad range of functional prop- include regulation of humoral immunity and activation of
erties. Capable of amoeboid movement, they move from B lymphocytes, limitation of the inflammatory response
the blood and lymph vessels into the interstitial tissue and and activation of allergic reactions. These cells regulate the
can reach the paracellular spaces between epithelial cells transformation of B cells into plasma cells.
(intra-epithelial lymphocytes). CD8 T lymphocytes: The role of these lymphocytes
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Lymphocytes have limited phagocytic activity but can is primarily cytotoxic. They also produce cytokines and
influence the activity of other cells including phagocytes. perform immunoregulatory functions.
Lymphokines (chemical mediators) produced by T lym- Cytotoxic T lymphocytes destroy the target cell (e.g.
phocytes are non-antigen-specific proteins that affect the cells containing bacteria or viruses) by secreting perforins,
biological activity of macrophages and granulocytes, as lytic enzymes and cytokines (e.g. interferon or tumour
well as that of other T and B lymphocytes. Lymphokines necrosis factor) that result in lysis of the plasmalemma.
can either inhibit or activate macrophage function. Cytotoxic lymphocytes may also be involved in lysis of
Based on differences in their development and function, antibody-coated cells through Fc receptor interactions, and
lymphocytes are divided into T lymphocytes (T cells), B apoptosis via proteases.
lymphocytes (B cells) and natural killer cells (NK cells). T-cell receptor (TCR): Found on the surface of T cells,
Descriptions of T and B cells are provided below. the T-cell receptor (TCR) is a molecule that recognises
antigens. It consists of an antigen-binding heterodimer
T LYMPHOCYTES (T CELLS) comprising highly variable αβ (TCRab) or γδ (TCRgd)
T lymphocytes leave the bone marrow relatively early in chains. TCRs regulate differentiation of T lymphocytes
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their development from haemopoietic stem cells, passing into CD4 or CD8 T cells. Cells that present antigens to
to the cortex of the thymus. Under the influence of cellular T cells include interdigitating dendritic cells (‘professional’
and humoral factors, they mature into immunocompetent antigen-presenting cells) and macrophages.
T cells and migrate from the medulla of the thymus to the
peripheral lymphoid organs, where they become activated B LYMPHOCYTES
upon contact with antigens. T lymphocytes are involved in B lymphocytes are responsible for humoral immune
cell-mediated immunity. Following contact with an anti- responses. Induced particularly by foreign proteins,
gen, T cells are transformed into effector cells or memory viruses or toxins, these result in the production of specific
cells. Based on function, T cells can be divided into: protective products (immunoglobulins, antibodies). In
birds, B lymphocytes mature in the bursa of Fabricius,
· cytotoxic T cells, hence their designation as B cells. The bone marrow and
· helper T cells, gut (Peyer’s patches) are considered the equivalent sites
· suppressor (regulatory) T cells, in mammals, though further differentiation also occurs
· memory cells and in the secondary lymphoid organs. On their surface, B
· natural killer T cells (NKT cells). cells express many specific receptors (membrane-bound
immunoglobulins) that function as the site of antigen
Based on differences in their surface molecules, T cells are binding. Antigen–antibody complexes are endocytosed by
classified as: the B cell. Under the influence of activated helper T cells,
activated B cells (immunoblasts) are transformed into
· CD4 T lymphocytes and plasmablasts (short-lived antibody-secreting cells), long-
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· CD8 T lymphocytes. lived antibody-secreting plasma cells and memory B cells
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(Figures 7.8 and 7.11; see Figure 8.2). Memory B cells can
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CD4 T lymphocytes: CD4 lymphocytes are helper T cells generate antibodies more quickly when next exposed to
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that recognise antigens bound to MHC II molecules and the same antigen.
stimulate differentiation of B lymphocytes. This group can Follicular dendritic cells and helper T cells play an
be further divided into two major subtypes: important part in antigen presentation during B-cell activa-
tion. Non-activated B cells have a lifespan of approximately
· T 1 cells and 2 weeks (see also Chapter 8, ‘Immune system and lym-
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· T 2 cells. phatic organs’).
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