T cell activation is tightly controlled and generally requires a very strong MHC/antigen activation signal, or additional activation signals provided by "helper" T cells (see below). Helper T cells
Helper T cells regulate both the innate and adaptive immune responses and help determine which immune responses the body makes to a particular pathogen. These cells have no cytotoxic activity and do not kill infected cells or clear pathogens directly. They instead control the immune response by directing other cells to perform these tasks.Helper T cells express T cell receptors that recognize antigen bound to Class II MHC molecules. The MHC:antigen complex is also recognized by the helper cells CD4 co-receptor, which recruits molecules inside the T cell (such as Lck) that are responsible for the T cells activation. Helper T cells have a weaker association with the MHC:antigen complex than observed for killer T cells, meaning many receptors (around 200–300) on the helper T cell must be bound by an MHC:antigen to activate the helper cell, while killer T cells can be activated by engagement of a single MHC:antigen molecule. Helper T cell activation also requires longer duration of engagement with an antigen-presenting cell. The activation of a resting helper T cell causes it to release cytokines that influence the activity of many cell types. Cytokine signals produced by helper T cells enhance the microbicidal function of macrophages and the activity of killer T cells.
An international meta-analysis over 34 jurisdictions quantified the effect size of gender as small, with most effect in Asia, lesser in Europe and Africa, and null in North America, and further finding that economic factors, internet availability, social norms and addiction-related health factors mediate the effect of gender, with nations with a greater GDP per capita having less differences in video game addiction between genders.Comorbid psychiatric disorders act as both risk factors and consequences. Indeed, there is a strong association between video game addiction and anxiety, depression, ADHD, social phobia, and poor psycho-social support. ADHD and its symptoms, such as impulsivity and conduct problems, also increase risks of developing video game disorder. Although internet gaming disorder has a strong relationship with obsessive-compulsive disorder, it is not specific and internet gaming disorder is both phenomenologically and neurobiologically distinct, which indicates that internet gaming disorder is more characterized by impulsivity than compulsivity. Familial factors appear to play an important role, although not well understood.Some personality traits, such as high neuroticism, high impulsivity, and high aggressiveness are consistently significant predictors of internet gaming disorder, and combination of personality traits seem to play a pivotal role in the acquisition, maintenance and development of the disorder. Mechanisms
Although there is much research since the 1980s on problematic online gaming use, the mechanisms are not well understood, due to inconsistent definitions used in studies. Video game structure
Some theories focus on the presumed built-in reward systems of video games, such as compulsion loops, to explain their potentially addictive nature.