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What does being immune to an infectious disease mean?

Created by Timothy Paustian on Jun 23, 2020, 8:46 PM

 

There is a ton of talk, justifiably so, about developing immunity to SARS-CoV-2, and whether is it long-lasting? Many reports I see in the news media are focused on the antibody titer, equating antibody presence with immunity. That is a common misconception. The human immune system has many different parts that can be roughly divided into innate immunity, antibody immunity, and cell-mediated immunity. Antibodies are only one part of the immune system.

When your body encounters a virus, your cells have systems that can recognize viral RNA. Infected cells will detect this RNA and raise the alarm, sending out interferons (a type of hormone) and all sorts of other signals that there is a problem. Macrophages and neutrophils will encounter the virus and destroy it. The macrophages then present pieces of the virus to the immune system, specifically B-cells and T-cells. In addition, infected cells will take parts of the virus and display it on their surface. These bits are also detected by T-cells.

B-cells that recognize the virus activate. Once activated, they start dividing and make copies of themselves. Most of these differentiate into plasma cells and start pumping out antibodies that can attack the virus. A subset of these activated B-cells become memory cells and distribute throughout the bodies immune tissues. T-cells also activate. They start dividing, but instead of making antibodies, they become killers. They then roam the body looking for cells displaying viral proteins. The cytotoxic T-cells then bind to the cell and kill it. Thus, these T-cells kill any cells infected with SARS-CoV-2.  A subset of the activated T-cells also become memory cells. In viral infections, T-cells are arguably more important in clearing infections than antibodies.

So, having an antibody titer is not the whole story. Someone could have their antibody titer against SARS-CoV-2 drop to low levels yet still be protected from reinfection. Your body remembers the infection because of the memory cells that were produced.

If you want to know more about immunity check out Chapters 15 and 16 of Through the Microscope.