Adaptive Immune System

When the innate immune response is not sufficient in taking care of the invading pathogens, dendritic cells and macrophages initiate the adaptive immune response. Antigen presenting cells phagocytize pathogens and presents antigens through the exogenous pathway. These now activated antigen presenting cells migrate to secondary lymphatic tissues including lymph nodes and spleen where appropriate adaptive cells are activated. Adaptive immune cells include T lymphocytes and B lymphocytes of various types.

T cells are made in the thymus, activated by antigen presenting cells in the lymph node, and are differentiated into various effector T cells including cytotoxic T cells (CD8+), helper T cells (CD4+), and regulator T cells. Cytotoxic T cells function to recognize and kill specific infected cells by releasing cytotoxic compounds from its granules. These compounds induce apoptosis in infected cells. Helper T cells are mainly divided into Th-1, and Th-2 cells. Th-1 cells release cytokines and activates cytotoxic T cells and induces antibody production by plasma cells. Th-2 cells stimulate plasma cells and produce antibodies. In addition, memory T cells are crucial for long-term host defense against repeat invaders.

B cells called plasma cells are made in the bone marrow and activated in the lymph nodes or spleen. They secrete antibodies to the blood and lymphatic tissues which is part of humoral adaptive immunity.