T cells do not. It is also worth noting that there are a number of different kinds of T
cells including T helper cells, natural killer cells, T suppressor cells and cytotoxic killer
cells.
T and B cells operate very differently when attacking infectious agents. The former
bring about cell-mediated immunity while the latter bring about antibody-mediated
immunity (see Table 2.1 for an overview of the roles of T and B cells). In the former
case, when an infectious agent enters the body, it is recognized by a type of monocyte
called a macrophage, which presents the infectious agent to a T helper cell and releases
interleukin-1 (IL-1; a type of cytokine released from cells to influence the activity of
other cells), this in turn stimulates T-helper cell activity. As a result, the T helper cells
then release interleukin-2 (another cytokine), which triggers the proliferation of T cells
and eventually the release of cytotoxic killer cells which attack and destroy the
infectious agent.
In antibody-mediated immunity, the initial stages are similar, such that there is
collaboration between macrophages and T helper cells. However, in this case, the T
helper cells stimulate the proliferation of B cells leading to the secretion of antibodies,
which identify and bind to specific features of the infectious agent. The antibodies
then immobilize and destroy the pathogen.
STRESS AND THE IMMUNE SYSTEM
Can stress alter immune functioning? There is evidence to show that stress can suppress
cell-mediated immunity, although the data relating to the antibody response and B
cell function in particular are less clear (cf., Rabin, 1999; Cohen, Miller and Rabin,
2001). For example, many studies have shown that increased secretion of stress
hormones such as cortisol can alter the production of cytokines. As we already know,
cytokines are important in the activation of T cells as well as in mediating the pro-
inflammatory response (this process is explained further later in this chapter). Therefore,
stress-induced changes in the production of cytokines may represent an important
mechanism through which stress compromises the body’s response to infectious
illness. An important study by Kunz-Ebrecht et al.(2003) showed that cortisol responses
to psychological stress were inversely associated with the production of two cytokines
(IL-6 and IL-1ra), indicating that psychological factors can influence important com -
ponents of immune functioning. Moreover, there is emerging evidence to suggest that
BIOPSYCHOSOCIAL PATHWAYS 33TABLE 2.1Comparison of the roles of T and B cells
Cell-mediated immunity: T cells Antibody-mediated immunity: B cells
Work directly at cell level Work via the bloodstream
A type of lymphocyte (white blood cell) A type of lymphocyte (white blood cell)
Formed in the bone marrow, but matured Formed and matured in the bone (B) marrow
in thymus (T)
Attack and destroy infectious agents by Attack and destroy infectious agents by
triggering release of cytotoxic killer cells stimulating the release of antibodies