Why Is the Common Cold So Common? The Role of Serotypes and Immune Specificity

Most people get two to four colds per year. Some get more. And yet, after each one, the immune system does exactly what it is supposed to do; it makes antibodies, clears the infection, and holds onto the memory of that pathogen. So why does the next cold arrive anyway?

The common cold is not one disease. It is the collective name for upper respiratory infections caused by at least six different virus families: rhinoviruses account for roughly 25–30% of cases, followed by coronaviruses (common-cold strains, not SARS-CoV-2) at around 10%, with parainfluenza viruses, respiratory syncytial virus, adenoviruses, and influenza viruses making up most of the remainder. Each of these families contains multiple serotypes (distinct antigenic variants that the immune system treats as separate pathogens). Antibodies made against one serotype of rhinovirus provide little or no protection against a different serotype of the same virus, let alone against a completely different virus family.

This is the core reason the common cold recurs throughout a person's lifetime: there are simply more variants circulating than the immune system can accumulate memory for.

You must be aware of how common it is. Everyone has some understanding or experience of the common cold. You might be suffering from a common cold right now or might have suffered earlier. This is the evidence to prove that the common cold is so common; the question arises, why is the common cold so common?

To answer this question, you need some background knowledge about host-pathogen interaction.

Figure: common cold so common

Each infectious disease results from a long battle between your immune system and the virulence factor (disease-causing capacity of) pathogens. When those pathogens overwhelm your immune system, you suffer from a disease, but if your immune system controls the pathogens, you become immune, or the resolution of the disease occurs. To win this battle (host-pathogen interaction), your body uses various weapons known as immune cells and their mediators, such as T Cells, B Cells, antibodies, natural killer cells, interferons, cytokines, macrophages, etc. Similarly, pathogens also employ various virulence factors that either destroy these weapons (e.g., IgA protease), help them to hide from immune cells, or inactivate the effect of these cells.

As the common cold has multiple etiological agents, primary infection (an infection occurring for the first time) by each of these pathogens is possible. But antibodies and memory cells (formed after primary infection) are expected to protect us from these pathogens’ secondary infection. One of the immune system’s notable characteristics is that the antibodies and memory cells are type-specific; i.e., the antibodies or memory cells formed against X pathogen may protect us from that X pathogen but not others.

Figure: colds are viral

In the case of a common cold caused by rhinovirus, our body synthesizes antibodies and neutralizes or inactivates that virus whenever we get infected. Some antibodies are stored in our body to kill/neutralize if the same rhinovirus attacks our body again. But if the new rhinovirus (with a changed antigenic structure) attacks our body, already synthesized antibodies and memory cells may not recognize and remove/neutralize this virus. So we again suffer from a common cold. Now, our body synthesizes new antibodies for this new type of rhinovirus, and the process continues.

Virologists have identified more than 160 serotypes of rhinovirus, classified across three species: rhinovirus A, B, and C. This property; multiple antigenically distinct variants of the same virus; is called serotyping, and rhinovirus's 160+ serotypes are a key reason the common cold cannot be prevented by a single vaccine.

Another virus with a related but even more dramatic strategy is the influenza virus.