MCAT teaching passage: Innate vs adaptive immunity

Immunologists break our immune system into two main branches: innate (non-specific) and adaptive (acquired) immunity.

Figure 1 White blood cells play major roles in both the innate and adaptive immune responses. Attribution: modified from Mikael Haggstrom CC BY-SA 3.0

Innate immunity is our first defense against invaders. The epithelial and mucosal linings of our respiratory and GI tracts, as well as our skin are major components of the innate immune system. These act as physical and chemical barriers against pathogens. Cells known as macrophages also play a large role in innate immunity. Macrophages recognize bacterial or viral components such as lipopolysaccharide (LPS) or double-stranded RNA (dsRNA), via special receptors known as Toll-like receptors (TLRs). TLR activation cause macrophages to secrete cytokines (small molecules involved in cell signaling and attraction), as well as to phagocytose the infected cells. The innate immune system is required to activate our adaptive immune system.

The adaptive immune response is mediated by immune cells known as lymphocytes. These are B and T cells. B cells secrete antibodies, highly specific protein molecules that bind to a specific pathogen. These antibodies bind specific parts of pathogens known as antigens - either presented extracellularly on infected cells or free-floating in the body. Antibody binding attracts mechanisms that will then attack and destroy the infected cell or pathogen. Some of these B cells become memory cells, which help the body “remember” the disease and prevent re-infection. T cells can either be helper T cells or cytotoxic T cells, and bind pathogens via the T-cell receptor (TCR), which senses specific protein sequences. Helper T cells activate B cells, attract macrophages, and secrete cytokines. Cytotoxic T cells create pores in infected cells through which they introduce chemicals that trigger apoptosis, thus actively killing the cell.