immune system
Summary
The immune system is the network of cells, tissues, and molecules that detects and destroys pathogens and abnormal cells. It has a fast, nonspecific innate arm and a slower, specific adaptive arm; adaptive memory cells make a second exposure to the same antigen produce a faster, stronger response.
Detail
Meaning: The immune system defends the body against pathogens and abnormal cells. Innate immunity is present from birth and acts fast but nonspecifically: skin and mucous barriers, phagocytes such as neutrophils and macrophages, natural killer cells, complement, and inflammation. Adaptive immunity is specific and takes days to develop: B cells make antibodies (humoral response), and T cells carry out cell-mediated responses. After a first exposure, memory cells persist, so a second exposure to the same antigen produces a faster, larger response. Vaccines exploit this.
On the MCAT: Passages often show antibody concentration over time after a first and second exposure; the secondary curve rises sooner and higher. Expect questions that ask you to predict the effect of a missing cell type: without helper T cells, both humoral and cell-mediated responses are weakened. Others test active versus passive immunity, or MHC I (on nucleated cells, shown to cytotoxic T cells) versus MHC II (on antigen-presenting cells, shown to helper T cells).
Common confusion: Adaptive immunity is not just antibodies; T cells are adaptive too. Passive immunity, such as antibodies passed from mother to child or given by injection, protects immediately but does not produce memory cells.
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.