Localized pleural thickening and adhesions on the left side are usually caused by various causes of exudative effusion in the pleural cavity, and the deposition of fibrin in the left pleural location, which will produce an activation reaction over time, i.e., calcification of granulation tissue, thickening of the local pleura, and eventually thickening and adhesion of the adjacent diaphragmatic pleura and wall pleura. Pleural effusion is often caused by inflammation, pneumonia, pleurisy or tuberculosis, because pleural effusion usually exudes more obvious, and the protein content of the exudate is higher, and once the protein is generated on the pleura, it will cause protein denaturation, i.e., radicalization, which will stimulate the proliferation of granulation tissue and gradually thicken the pleura and eventually make adhesions. In case of limited thickening and adhesion of the left pleura, often seen in left pleurisy or bleeding after trauma on the left side, the protein content of the blood is relatively high, which also causes the so-called fibrin deposition on the chest wall, causing an activation reaction, promoting the proliferation of granulation tissue, and finally leading to local thickening and adhesion of the left pleura.