Cd4, encoding the CD4 protein, is a crucial molecule in the immune system. CD4 is a co-receptor on T helper cells, playing a vital role in the immune response by assisting the T-cell receptor in recognizing antigens presented by major histocompatibility complex class II molecules. It is involved in pathways that direct CD4+ T-cell differentiation, which is essential for an effective immune response against pathogens and in maintaining immune homeostasis [1,2,3,4,5,6,7,8,9].

CD4+ T cells can differentiate into various subsets such as Th1, Th2, Th17, T follicular helper cells, and T effector cells, each with distinct functions [1]. Cytotoxic CD4+ T cells (CD4 CTLs) have also been identified, especially during viral infections, where they can control viral replication and play a role in anti-tumor activity, though they may also be involved in immunopathology in autoimmune diseases [2,3,4,5]. The differentiation of CD4+ T cells is regulated by multiple factors including cytokine production by antigen-presenting cells, signals downstream of the T-cell receptor, and long non-coding RNAs (lncRNAs) [1,7]. Additionally, glutaminolysis has been shown to play a role in the differentiation of CD4+ T cells, and abnormal differentiation of peripheral CD4+ T cells is associated with diseases like autoimmune diseases, transplantation rejection, and irritability [8].

In conclusion, Cd4 is essential for the proper functioning of the immune system, particularly in the differentiation and function of CD4+ T cells. The study of Cd4, especially through genetic models like KO/CKO mouse models (although not specifically detailed in the provided references), could potentially provide more insights into its role in immune-related diseases such as viral infections, cancer, and autoimmune diseases, which are areas where CD4+ T cells have been shown to be involved [2,3,4,5,8,9].