Immune system: immune response

The nonspecific defenses act fast, but they do not always win. The adaptive immune response targets one particular pathogen, and it has a memory. Learning to recognize a pathogen takes time, but it makes the next meeting with that pathogen much easier.

Every pathogen has marker molecules on its surface called antigens. The immune system's main specialists are lymphocytes, a type of white blood cell made in red marrow. There are two main kinds. B cells make antibodies, Y-shaped proteins that fit one particular antigen like a key fits a lock. Antibodies attach to the pathogen, block it from entering cells, cause pathogens to clump together, and mark them for phagocytes to destroy. T cells mature in the thymus. Helper T cells act as coordinators, activating B cells and other immune cells. Killer T cells find body cells that are infected and destroy them.

During a first infection, it takes several days for the right B and T cells to be found and multiply, which is why you feel sick before you recover. When the infection ends, most of these cells die off, but a few long-lived memory cells remain, sometimes for decades. If the same pathogen returns, memory cells recognize it right away, and the response is faster and stronger. You usually do not even get sick. This is immunity.

Vaccines use this memory. A vaccine contains a harmless version of an antigen, such as a weakened or inactivated pathogen, a piece of one, or instructions for your cells to make a piece of one. The immune system responds and builds memory cells without you having to get the disease. Immunity from your own immune system is called active immunity. Passive immunity is temporary: a baby gets antibodies from the mother before birth and in breast milk, which protect the baby for a few months.

Sometimes the system goes wrong. In an allergy, the immune system overreacts to a harmless substance such as pollen or peanuts. In an autoimmune disease, it mistakenly attacks the body's own cells.

Common mistakes: thinking a vaccine gives you the disease, and thinking antibodies are living cells. Antibodies are proteins made by B cells. Next lesson: the lymphatic system, which carries immune cells around.