immunity
Summary
Immunity is the body's resistance to pathogens and foreign material, built from fast, nonspecific innate defenses and slower, antigen-specific adaptive responses. The most testable point is that memory, meaning a faster and stronger second response, comes from adaptive immunity (B and T cells).
Detail
Meaning: Innate immunity is present from birth and acts within minutes to hours. It includes skin and mucous barriers, phagocytes (neutrophils, macrophages), natural killer cells, complement and inflammation. It recognizes broad pathogen features, not one specific antigen. Adaptive immunity is antigen-specific and takes days to develop the first time. Humoral immunity uses B cells and antibodies against extracellular pathogens. Cell-mediated immunity uses T cells: helper T cells (CD4) coordinate the response, and cytotoxic T cells (CD8) kill infected cells. Clonal selection expands the lymphocytes that match an antigen, and memory cells remain afterward.
Active immunity means your own immune system makes the response, after infection or vaccination, and it produces memory. Passive immunity is the transfer of ready-made antibodies, such as maternal IgG across the placenta or IgA in breast milk. It acts immediately, fades, and does not produce memory.
On the MCAT: A common figure plots antibody level against time after a first and second exposure. Predict that the second exposure has a shorter lag and a higher peak. Other questions ask you to classify a case as active or passive, or to match a pathogen to the humoral or cell-mediated arm.
Common confusion: Students call a vaccine passive immunity. A vaccine is active, because the person makes their own antibodies and memory cells. Passive immunity is borrowed and temporary.
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.