How the body defends itself: innate and adaptive immunity

Your defenses work in two overlapping layers. The innate immune response is always ready and reacts quickly, but it treats every invader in roughly the same way. The adaptive immune response takes days to build the first time, but it targets one specific invader and remembers it.

The two branches cooperate. During the first several days of a new infection, innate defenses slow the invader down while the adaptive response gears up.

Barrier defenses do not wait for an infection; they work all the time. Intact skin is the main one: its dry outer layer of dead cells is a poor place for bacteria to grow, and shedding cells carry microbes away.

Mucus in the airways and gut traps microbes, and cilia in the airways sweep that mucus up to be swallowed. Saliva contains lysozyme, an enzyme that breaks down bacterial cell walls, and the acid in the stomach kills many swallowed pathogens.

If something gets past the barriers, or tissue is simply injured, inflammation follows. Its classic signs are heat, redness, swelling, and pain.

1. Damaged cells trigger mast cells to release histamine and other chemical signals.
2. Histamine widens local blood vessels (vasodilation), bringing more blood. This causes the heat and redness.
3. Capillaries become leaky, so fluid seeps into the tissue. This causes the swelling.
4. Chemical signals draw phagocytes, cells that engulf and digest invaders, to the site. Neutrophils pour in early from the blood, and more macrophages follow to clean up debris. Pus is largely dead neutrophils.

Other innate tools:

• Natural killer (NK) cells destroy virus-infected cells.
• Virus-infected cells release interferons that help protect neighboring cells.
• Complement proteins can tag bacteria for phagocytes or punch holes in them.

All blood cells, including lymphocytes, come from stem cells in the red bone marrow. B cells finish maturing in the bone marrow, while T cells travel to the thymus, a gland above the heart, to mature. These are the primary lymphoid organs.

Mature lymphocytes then gather in secondary lymphoid organs, where responses are launched. Lymph nodes filter lymph, the fluid that the lymphatic vessels collect from tissues and return to the blood. The spleen filters blood and removes old red blood cells. Tonsils and patches of lymphoid tissue in the gut lining guard the entry points of the digestive and respiratory tracts.

An antigen is a molecule the immune system recognizes as foreign. Antigen-presenting cells, such as dendritic cells, macrophages, and B cells, display pieces of antigen to T cells. Dendritic cells carry antigen to nearby lymph nodes, where most T cell responses begin.

• Helper T cells (CD4) release chemical signals called cytokines that switch on other immune cells. They are needed for B cells to make antibodies against most protein antigens.
• Cytotoxic T cells (CD8) find body cells infected with a virus and make them self-destruct.
• B cells that bind their matching antigen multiply and become plasma cells, which secrete antibodies. Antibodies can coat a pathogen so it cannot attach to cells and can tag it for destruction.

Some activated B and T cells become long-lived memory cells. On a second exposure, the secondary response is faster and much stronger, often clearing the pathogen before you feel sick.

Active immunity means your own immune system made the response, so memory cells form. Getting an infection gives naturally acquired active immunity. A vaccine, made of a killed or weakened pathogen or its parts, gives artificially acquired active immunity without causing the disease.

Passive immunity means you received ready-made antibodies from someone else. A fetus gets IgG antibodies across the placenta, and a nursing infant gets IgA antibodies in breast milk (natural). An injection of immune globulin gives fast, temporary protection (artificial). Passive protection fades as the antibodies break down, because no memory cells were made.