When a person acquires one disease soon after another, both innate and adaptive immunity must kick in at the same time.

A new study from the Weizmann Institute of Science Immunology Department, directed by Prof. Ziv Shulman, gives light on how human immune systems select one virus over another. Their discoveries could help in the creation of new medicines for autoimmune disorders including rheumatoid arthritis and lupus, as well as alter the manufacturing of certain vaccinations in the future.

Their study, which was published in the peer-reviewed journal Immunity, examines the two arms of human immunity. Innate immunity is the body’s first line of defense, and it is activated when our immune system detects a viral, bacterial, or other types of invasion.

Adaptive immunity is our second line of defense, and it consists of particular cells and antibodies that can fight off invaders with higher accuracy and provide longer-lasting immunity. Adaptive immunity takes longer to activate than innate immunity and so occurs at a later stage of infection.

When a person contracts one pathogen soon after contracting another, such as by eating salmonella-contaminated food while recovering from the flu, both innate and adaptive immunity must work together to build long-term immunity to the first pathogen – in this case, the flu virus – while fighting off the second pathogen: salmonella.

Experiments with mice headed by PhD student Adi Biram revealed that secondary infection interferes with the generation of antibodies against the first infection. The immune system views the generation of these long-term defenders to be non-essential, preferring to focus on the more immediate threat.

This prioritizing, however, is not caused by the bacteria. The researchers observed that when the bacteria entered the mice’s lymph nodes, an “alarm” went out, alerting innate immune cells called monocytes to leave the bone marrow where they are formed and proceed to the lymph nodes to fight the bacteria.

The activity results in a lack of oxygen in the lymph nodes. While most immune cells can adapt, the subgroup that produces antibodies cannot survive without oxygen, and as a result, the cells die, interrupting the long-term protective process.

The process as “an either/or situation,” explains the author.

“When you are fighting life-threatening bacteria, you can’t be bothered with long-lasting immunity. Destroying the salmonella gets priority because it’s a matter of survival.”

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