Defending against pathogens
Once a pathogen enters the body, the immune system destroys it. Two kinds of white blood cell are central. Phagocytes carry out phagocytosis: they engulf and digest pathogens. Lymphocytes produce antibodies — proteins with a specific shape that fit onto antigens (marker molecules) on the surface of a particular pathogen. Each antibody is specific to one type of antigen, like a key to a lock.
Key idea
Antigens are markers on a pathogen's surface. Antibodies are specific proteins made by lymphocytes that bind to a particular antigen, clumping pathogens together and marking them for destruction.
Active and passive immunity
Active immunity is protection produced by the body making its own antibodies after infection or vaccination. It is long-lasting because memory cells remain and respond quickly if the same pathogen returns. Passive immunity is short-term protection from antibodies given from another source, for example antibodies passing across the placenta or in breast milk to a baby; no memory cells are formed.
Vaccination
A vaccine contains a weakened, dead or modified form of a pathogen, or its antigens. It stimulates lymphocytes to make antibodies and memory cells without causing the disease. If the real pathogen later enters the body, memory cells produce antibodies rapidly, preventing illness.
Remember
- Phagocytes engulf; lymphocytes make antibodies.
- Each antibody is specific to one antigen.
- Active immunity makes memory cells; passive immunity does not.
- Vaccines create immunity without causing the disease.