*2.2.2. T lymphocytes*

Mature T lymphocytes express T-cell receptors on their surface. They are able to recognize only those antigens which are associated with the protein termed major histocompatibility complex (MHC): in humans the MHC is known as human leukocyte antigen (HLA) [2] that are presented to them on a cell surface by antigen presenting cells (APCs) [2]. When an APC (e.g. macrophage) encounter an antigen or pathogen, it will engulf, process and present the pathogen to the T cell [2].

T lymphocytes can be further subdivided into CD4+ T cells (Helper T cells) and CD8+ T cells (Cytotoxic T cells). CD4+ T cells are further subclassified into Th1 (T helper 1) and Th2 (T helper 2). Th1 is important for eliminating intracellular pathogen whereas Th2 is important for immunity against extracellular pathogen [2].

The regulation of the immune response is shown in Figure 1. The Th1 cells are involved in cellmediated immunity against intracellular pathogens. They produce cytokines such as inter‐ leukin-2 (IL-2), interferon-gamma (IFN-gamma) and interleukin-10 (IL-10) [2]. IL-2 promotes proliferation of T lymphocytes [2] while IFN-gamma activates the cells involved in the elimination of pathogens and cells with tumour properties (e.g. monocytes, macrophages, cytotoxic T lymphocytes, natural killer cells) [1]. By contrast, the Th2 cells are mainly involved in humoral immunity and immunity against helminthic infection [2]. They produce cytokines such as IL-4, IL-5, IL-10 and IL-13. It is known that IL-4 stimulates proliferation of B lympho‐ cytes while IL-5 promotes activation of eosinophils [2]. IL-10 has inhibitory effect while IL-13 involves in allergic reaction and helminthic infection [2]. Thus, both Th1 and Th2 cells and their cytokines counteract with each other to regulate the immune response [1].

The communication within the acquired immune system and between innate and acquired immunity involves cell surface proteins (e.g. adhesion molecules) and the soluble molecules which can produce signals from one cell to another, that is called cytokines [1].
