معقد التوافق النسيجي الكبير
Major histocompatibility complex class II proteins are important because under normal function they have important responsibility in the human body's immune system response. MHC II proteins present exogenous antigens that activate CD4+ T-lymphocytes, immune cells that are responsible for activating other immune cells like CD8+ T-lymphocytes and macrophages. MHC II proteins are also important for positive and negative selection in the thymus because they present antigens to immature T-cells, allowing the T cells to differentiate into proper functioning CD4+ T-lymphocytes. This may explain why patients with BLS II display decreased levels of CD4+ T-lymphocytes in their blood.