This study is a phase I, randomised, double blind (sponsor unblinded), placebo-controlled, single ascending dose study GSK2831781 administered by IV. GSK2831781 is a humanized Antibody Dependent Cell Cytotoxicity (ADCC) enhanced monoclonal afucosylated antibody that is specific to the Lymphocyte Activation Gene-3 (LAG-3) protein. This is the first administration of GSK2831781 in humans and will evaluate in two parts the safety, tolerability, pharmacokinetics (PK), pharmacodynamics (PD) and immunogenicity of single IV doses of GSK2831781 administered to healthy subjects previously vaccinated with Bacillus Calmette Guérin (BCG) (Part A delayed type hypersensitivity [DTH] cohorts) and patients with plaque psoriasis (Part B). The inclusion of DTH and psoriasis subjects to explore the mechanism in biopsies and clinical response endpoints in these populations, as well as investigate systemic biomarkers will provide useful information prior to conducting studies in other immune-inflammatory disease which will involve more invasive tissue biopsies. Measuring the pharmacology of GSK2831781 using the depletion of LAG-3+ T-cells in skin biopsies from Tuberculin Purified Protein Derivative (PPD) skin challenge and lesional skin biopsies from patients with psoriasis, will be helpful in understanding of the dose response relationship, which will be important for designing future studies in immuno-inflammatory diseases, including psoriasis. Approximately 67 subjects will be enrolled to complete dosing and critical assessments. The subject numbers will be split to approximately 40 healthy subjects (Part A) and 27 patients with psoriasis (Part B).