Page 79 - Textbook of Pathology, 6th Edition

antigen-presenting cell such as dendritic cell, and the major receptor (TCR) for recognition of MHC molecules. Depending 63
histocompatibilty complex (MHC) in the macrophage, which upon functional activity, T cells have two major subtypes: T
determines whether the invading antigen is to be presented helper cells and T suppressor cells.
to B cells or T cells. Some strong antigens that cannot be dealt T helper cells. Abbreviated as T cells, these cells promote
H
by antibody response from B cells such as certain and enhance the immune reaction and are also termed as
microorganisms (e.g. viruses, mycobacteria M. tuberculosis T-regulatory cells. They carry CD4 molecule on their surface
and M. leprae), cancer cells, tissue transplantation antigen etc, and hence are also called CD4+ cells. CD4+ cells in circulation CHAPTER 4
are presented to T cells. are about twice the number of CD8+ cells (CD4+/CD8 ratio
Features and functions of subtypes of lymphocytes are 2:1). These cells act by elaboration of variety of cytokines.
summed up below and illustrated diagrammatically in Depending upon the type of cytokines elaborated, these
Fig. 4.2:
T cells are further of two subclasses: T 1 and T 2.
H
H
H
B CELLS. These cells are involved in humoral immunity by T 1 cells elaborate IL-2 and interferon (IFN)-γ.
H
inciting antibody response. B cells in circulation comprise T 2 cells elaborate IL-4, IL-5, IL-6, and IL-10.
H
about 10-15% of lymphocytes. On coming in contact with CD4+ cells are predominantly involved in cell-mediated
antigen (e.g. invading microorganims), B cells are activated reactions to viral infections (e.g. in HIV), tissue transplant
to proliferate and transform into plasmacytoid lymphocytes reactions and tumour lysis.
and then into plasma cells. Depending upon the maturation T suppressor cells. Abbreviated as T cells, they suppress
S
stage of B cells, specific CD molecules appear on the cell immune reactions but are cytotoxic and actually destroy the
surface which can be identified by CD markers; common B invading antigen; hence are also termed as cytotoxic T
cell markers include: CD 19, 20, 21, 23. These cells also possess lymphocytes (CTL). These cells carry CD8 molecule on their
B cell receptors (BCR) for surface immunoglobulins (IgM and surface and hence are also called CD8+ cells. CD8+ cells in Immunopathology Including Amyloidosis
IgG) and Fc receptor for attaching to antibody molecule. circulation are about half the number of CD4+ cells.
T cell help is provided to B cells by a subset of T helper cells, Compared to CD4+ cells which act by elaboration of
T 2, by elaborated interleukins (IL-4, IL-5, IL-10, IL-13). cytokines, CD8+ cells are directly cytotoxic to the antigen.
H
T CELLS. These cells are implicated in inciting cell-mediated CD8+ cells are particularly involved in destroying cells
immunity and delayed type of hypersensitivity. T cells in infected with viruses, foreign cells and tumour cells.
circulation comprise 75-80% of lymphocytes. Pan T cell Contrasting features of B and T cells are given in
markers are CD3, CD7and CD2. Besides, T cells also carry Table 4.2.

Figure 4.2 Schematic representation of functions of B and T lymphocytes and NK cells. (BCR = B cell receptor, TCR = T cell receptor).