**1. Introduction**

Lead (Pb) is a heavy metal with versatile properties, including malleability, ductility, poor conductivity, softness, and corrosion resistance, and it has thus been used for several thousands of years, causing widespread distribution in the environment worldwide [1,2]. Due to its non-biodegradability, Pb accumulates in the environment and hazards increase over time, evident from numerous reports of Pb contamination in the environment by industrial, mining, and agricultural activities [1–3]. Pb has no known biological role, but it can accumulate in the body, causing toxicity in many tissues and organs, immune system included [1–12]. It is of concern that there is increasing evidence for the negative effects of chronic exposure to high-level Pb on cancer risk and mortality that has recently emerged from prospective cohort studies of Pb-exposed workers [13,14].

The immune system provides important defense mechanisms that reduce the potential adverse effects of exposure to harmful biological agents, mutant cells, and certain chemicals [15]. Studies in humans and experimental animals have demonstrated the adverse e ffects of Pb exposure on the body's immune system [16–21]. In one study, Pb-poisoned children were found to have lower numbers of CD4+CD8+ helper T cells (Th), while their CD3+CD8+ cytotoxic T cell (Tc) levels were higher than those of a control group [16]. Workers in a factory manufacturing lead stearate were found to have a decreased number of CD45RO+ memory T cells [17]. A decrease in lymphocyte proliferation was noted in another study of occupationally exposed workers [18]. A reduction in humoral immunity by 53.6% was observed in mice receiving Pb in drinking water [19]. In other studies, altered subpopulations of circulating CD4+ T cells (T cells) and CD8+ T cells (T cytotoxic cells) were noted, together with evidence for Th1 up-regulation occurring simultaneously with Th2 down-regulation [20,21].

Nakhon Si Thammarat Province, located in the southern region of Thailand, is not an industrial estate area. However, there are many shipyards in operation, especially in coastal areas such as the districts of Pak Phanang, Mueng, Thasala, and Si-Chon. Many occupations are associated with the shipyards, including boat caulkers and fishing net workers, using lead bars [22]. In a previous report published in 2007, 48% to 67% of caulkers, painters, and mechanics working in the shipyards of Pak Phanang and Thasala districts were found to have blood Pb concentrations >40 μg/dL [23]. The present study was undertaken to assess Pb exposure experienced by current boatyard workers by measuring their blood Pb concentrations. In addition, it examined the adverse e ffects of such exposure, with a focus on the function of immune cells, which included phagocytic activity, proliferation, and cytokine production, and T cell subpopulation profiles.

#### **2. Materials and Methods**
