ABSTRACT
The immune system has two main functions; to recognize invading pathogens and then to trigger pathways that will destroy them. The key cells responsible for immunity in vertebrates are white blood cells called lymphocytes which arise from precursor cells in the bone marrow. There are two main parts of the immune system which interact to provide overall protection for the animal: the humoral immune response and the cellular immune response. The humoral immune system relies on B lymphocytes to produce soluble antibodies that will bind the foreign antigens. The cellular immune system uses killer T lymphocytes that recognize and destroy invading cells directly. The primary immune response occurs on initial contact with a foreign antigen and results in production of immunoglobulin M (IgM) and then immunoglobulin G (IgG). One of the most important features of the immune system is that, once an animal has encountered a particular pathogen, the system confers protection against future infection.
