Memory: A Concern of Older Persons. Yes, but Two Types!

First published Aug. 15, 2021

Memory has been of central interest to philosophers, psychologists and neuroscientists for millennia. How do we remember what we have experienced? Why do we forget what we experience in the first several years of life, so-called childhood amnesia. Lapses of memory range from the last place we left the car keys to dementia and everything in between. Initially, it was thought memory resided in some special part of the brain. Experimental ablation of parts of animal brains have been attempted to find “memory centers”. Mice trained to run through a maze have had one or another part of their brain ablated and required to run the maze again. In every case, whichever part of the brain was irreversibly damaged, they remembered how to get through the maze. Given the importance of memory to function and survival, it is not surprising we have evolved so as not to let such a vital function reside in one critical place. We now know at least four areas of the brain participate in memory. Also, neurotransmitters, chemicals that play a role in communicating among neurons, and certain hormones play a vital role in imprinting passed events and experiences on our brain to allow recall.

But the point of my discussing memory is to point out a type of memory most persons do not think about explicitly, immunologic memory. Yet, it is as important to life as is neurological memory. The immune system responds to invading infectious organisms with two arms: the innate system and the adaptive system. The former is the earliest immune mechanism that evolved and it is shared by species of plants and primitive animal forms and all multicellular organisms in one form or another. After the naïve host encounters a new pathogenic organism, innate immunity acts immediately and fills a time gap since days or weeks are required for an effective adaptive immune response. The innate immune system can identify microbes and destroy them by antimicrobial chemicals and by direct cell killing. Natural killer cells (NK cells) are a part of the innate immune system and do not require antibodies to permit them to attack microbes (including SARS-CoV-2). The two systems also cooperate; cells in the innate system ingest and kill microorganisms coated with antibody made by the adaptive system. The innate immune system, also, includes the barriers to infection, such as the skin and mucous membranes that prevent infection, and gastric acid that can kill many microorganisms that are swallowed. Although a system that developed early in the evolution of living things, it is diverse and complex.

The adaptive immune system is what most of us think about when we consider gaining immunity to a bacterium, virus or another microbe. Unlike the innate immune system, which provides a rapid response, but its potency does not increase after exposure to a pathogen, the adaptive immune response is a more effective response to an infectious organism. It imparts “memory” to the host so on re-exposure to the same organism, it can respond rapidly and, usually, effectively. That reaction is the basis of active immunity with a vaccine. The vaccine acts as the natural microorganism, but is either only a key part of the organism or an inactivated microorganism, incapable of infection, but capable of stimulating the adaptive immune system to generate antibodies to the organism and, also, to induce memory so when the organism is encountered again, it acts rapidly to kill or inactivate the microbe. That is why the measles vaccine provides virtually life-long immunity. So, immunological memory is critical to human survival, given the constant exposure and challenge to innumerable viruses, bacteria, fungi and other pathogens. Ignoring immunological memory is the fallacy that occurs when persons measure antibody levels, alone, after infection. When, over time, they are very low or undetectable, they conclude, we may be unprotected. Plasma antibodies decrease over time. But, two types of immunological memory cells are waiting to provide (i) a rapid and robust new antibody response on re-exposure to the microbe and, often, (ii) a microbe-inactivating cellular response on a second microbial exposure, which are the bases for prolonged natural immunity and the efficacy of vaccine therapy. Older persons may have gaps in both neurological and immunological memory. Thus, passive immunity (especially monoclonal antibodies against SARS-CoV-2) is an important modality for us.

Written August 2020