immune response
Summary
The immune response is the body's coordinated defense against pathogens and other foreign antigens. It combines a fast, broad innate arm with a slower, antigen-specific adaptive arm. Most testable: memory cells make a second exposure to the same antigen faster and stronger.
Detail
Meaning: The immune response is the body's coordinated defense against pathogens and other foreign antigens. The innate arm (barriers, phagocytes, natural killer cells, complement, inflammation) acts within hours and recognizes broad pathogen features. The adaptive arm is slower the first time but specific for an antigen. In humoral immunity, B cells become plasma cells that secrete antibodies against extracellular pathogens. In cell-mediated immunity, helper T cells (CD4) coordinate the response and cytotoxic T cells (CD8) kill infected cells that display antigen on MHC I. Memory B and T cells persist afterward.
On the MCAT: Passages often graph antibody level after two exposures: the primary response has a lag and a modest peak, while the secondary response is faster and higher because of memory cells. Other items remove one cell type, such as B cells, or CD4 T cells as in HIV, and ask which arm fails.
Common confusion: Passive immunity (maternal antibodies, transferred antiserum) gives temporary protection without memory cells, unlike active immunity from infection or vaccination. Also, antibodies mostly mark and neutralize pathogens or activate complement rather than kill them directly.
Sources
- Campbell Biology
- Janeway's Immunobiology
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.