Vaccines are built on one of biology’s most remarkable features: immune memory. Expose the immune system to a safe version or a fragment of a pathogen, and it learns. When the real pathogen shows up later, the immune response is faster, stronger and more precise. “Vaccines take advantage of the immune system’s capacity for memory,” says Dr. Matthew Tunis, executive secretary to Canada’s National Advisory Committee on Immunization (NACI). “You prime the system in advance so that, at first exposure to the pathogen in the real world, the response is already geared up.” That priming happens through a coordinated network of cells and signals. B cells produce antibodies; proteins that circulate in blood and lymph and can neutralize viruses and toxins before they enter cells. T cells deliver “cell-mediated” immunity, helping orchestrate responses and destroying infected cells directly. Depending on the disease, vaccines are designed to emphasize the arm of immunity that matters most. One of the most remarkable aspects of the immune system, Dr. Tunis says, is how it engineers its own diversity. Immune cells rearrange segments of their own DNA to create an almost limitless range of receptors, each one specialized to recognize a different potential threat. “It’s incredible,” he says. Vaccines at Work Three leading Canadian scientists explain how vaccines prevent disease, protect health care workers and point the way toward a healthier future. “Your immune cells cut and re-stitch parts of their genome to make receptors that all look a little different, each one ‘sticky’ for a particular protein or antigen.” Most of these cells will never encounter the target they’re built for, but that’s the trade-off for flexibility. The result is an immune system capable of responding to nearly anything it might meet over a lifetime, a living library of possibilities, ready to activate when the right match appears. “Different pathogens cause disease in different ways,” Dr. Tunis explains. “Some are best handled by neutralizing antibodies that ‘soak up’ the invader; others require robust T-cell responses. Vaccine design leans into the strategy most likely to overcome the specific pathogen.” Vaccines try to simulate what a natural infection would do, without the risk of the disease. Sometimes we use a weakened version of the virus. 20 | 2026 | Issue 2
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