**3.1 What are human leucocyte antigens (HLA)?**

HLA are glycoproteins expressed on the cell surface that differentiate between 'self' and 'non-self' antigens. The primary purpose of HLA is to combat pathogens [5], but it poses a crucial barrier to transplantation.

The human major histocompatibility complex (MHC) on the short arm of chromosome 6 encodes HLA. They are the most polymorphic genes in the human genome. There are currently 37,068 HLA and related alleles in the human genome, and it is


**Table 1.**

*Association of cPRA level and waiting time to renal transplantation.*

growing [6]. Antigen presenting cells present foreign antigens *via* MHC proteins to the T cells, which switch on the host immune system.

MHC encodes two classes of antigens, i.e., class I, which includes HLA-A, B, and C antigenic clusters and class II antigens comprising HLA-DQ, DP, and DR clusters [7]. Class I antigens are expressed on nucleated cells, platelets, and red blood cells (in ∼15% of individuals). Class II antigens are expressed on antigen-presenting cells, e.g., macrophages, B cells, and dendritic cells. However, other cells, e.g., epithelial and endothelial cells, can express class II antigens during inflammatory states, e.g., infections [8].
