White blood cells emerge from the bloodstream through capillaries and engage harmful molecules by either engulfing them or poisoning them. There are different types of white blood cells, which divide between them various infection-fighting activities, from detection to elimination.
White blood cells constitute the body's defense mechanism against illness. They fight against bacteria, viruses and other harmful agents. White blood cells are made continually in the bone marrow, as they only live for 1 to 3 days. White blood cells join the blood stream, which also contains red blood cells, platelets and plasma.
White blood cells use thin walls of capillaries to exit the blood stream and travel to the tissue that needs help. Macrophages are the type of white blood cells that "eat" harmful molecules by engulfing them. Macrophages detect the invading substances by special antigens that harmful microbes carry on their surface. Macrophages destroy most of the microbe, except for these antigens, which they deliver to lymph nodes. There, other white cells are introduced to the antigens.
Lymphocytes of type T can be offensive and kill infected human cells with chemicals, or they can be defensive and produce chemical signals that direct other parts of the immune system in fighting the disease. B-type lymphocytes secrete antibodies, each of which is tailored to a particular antigen. Antibodies bind to a corresponding antigen and subsequently engulf the microbe. Vaccines work by mimicking infection and encouraging the immune system to produce T cells and B cells that will recognize the illness in the future.