**1. Introduction**

The incidence of lung transplantations worldwide has increased annually with chronic obstructive pulmonary disease being the leading cause [1]. From 2009 to June 2016, the median survival of primary lung transplantation was 6.5 years [2]. The frequency of at least one treated acute rejection episode occurring within 1 year posttransplantation is around 27% [2]. Bronchiolitis obliterans syndrome (BOS), a phenotype of chronic lung rejection, is currently one of the most significant long-term complications of lung transplantation with a 5-year follow-up incidence of 41.5% [2].

Primary graft dysfunction (PGD) complicates lung transplant outcomes. PGD is a common early complication of lung transplantation that often occurs in the first

72 h posttransplantation [3]. PGD has also been indicated as a risk factor for the development of BOS [4].

Acute lung rejection (ALR) in lung transplant recipients is a major cause of early complication and death [5]. It is a major risk factor for the development of BOS [6]. BOS is the most common manifestation of chronic lung allograft dysfunction (CLAD) and is characterized by subepithelial fibrosis of small cartilaginous airways leading to partial or total occlusion [7].

PGD, ALR, and CLAD all have been associated with pro- and anti-inflammatory cytokine and chemokine expressions. This review aims to summarize the specific associations between bronchoalveolar lavage (BAL) and plasma cytokine levels and the development of PGD, ALR, and CLAD.
