The HLA system is the major tissue tolerance system in humans, located on chromosome 6, on its shorter arm, and is made up of class I, II, and III antigens. Its role is to distinguish its own cells from other people's and when the organism comes into contact with foreign antigens to activate the immune response. Some of the most important features of this system are pronounced polymorphism and association with diseases. It is important in the transplantation of hematopoietic stem cells and solid organs, as well as in the diagnosis and treatment of autoimmune diseases. The aim of this study was to point out the peculiarities of the HLA system related to diseases, especially psoriasis, dermatitis and psoriatic arthritis, and to describe the advantages and disadvantages of serological versus molecular method of tissue typing. Two methods, serological and molecular PCR-SSO, were used in the determination of HLA antigens and genes in this study. I determined the alleles of class I loci (HLA-A and HLA-B) by the serological method, and the presence of HLA-Cw6 antigen by the molecular method, which cannot be determined using exclusively the serological method. The importance of using two different methods for determining antigens and alleles of the HLA system loci was demonstrated on the example of 70 subjects with reference diagnoses of psoriasis, dermatitis, psoriatic arthritis and other rheumatic diseases from the Tissue Typing Laboratory. In conclusion, without tissue typing, the process of diagnosing the autoimmune diseases and their treatment would be significantly more difficult. Its implementation makes it easier for clinicians to choose the right therapy.