Innate and Adaptive Immunity

Innate immunity: the first line

MedlinePlus describes innate immunity as the defense system you are born with, and says it is nonspecific. [1] It includes barriers such as skin, mucus, stomach acid, the cough reflex, and enzymes in tears and skin oils. [1] It also has protein-based parts, such as the complement system and interferon. [1]

Innate immunity is also what starts inflammation. When tissue is injured, damaged cells release chemicals that make blood vessels leak fluid and draw in white blood cells called phagocytes that engulf germs and dead cells. [1] See acute vs chronic inflammation.

Adaptive immunity: the targeted, remembering arm

Adaptive (acquired) immunity develops with exposure to particular antigens, which are substances, usually proteins, that the immune system can recognize. [1] Its main cells are lymphocytes. B lymphocytes become plasma cells that make antibodies, and T lymphocytes attack antigens directly, help control the response and release cytokines. [1] After an exposure, a few of these cells multiply and provide "memory", which lets the body respond faster the next time. [1] Vaccines work by triggering this process without needing the infection first. [1]

A review of the adaptive response to SARS-CoV-2 gives a good example of how the parts divide the work: B cells (the source of antibodies), CD4+ T cells and CD8+ T cells each have different roles, and all three contributed to controlling the virus in both non-hospitalized and hospitalized patients. [2]

How the two arms work together

They are not separate systems. An immunology review describes how the innate system detects microbes in order to give immediate defense and also to generate long-lasting adaptive immunity, with specialized dendritic cells translating that detection into the kind of response needed. [3] In that design, the cells that sense an infection release one set of cytokines, which tell lymphocytes to release another set. [3] See cytokines explained.

Immune memory is described as a defining feature of the acquired system, but a review argues innate activation can also enhance responsiveness to later triggers. [4] That idea is called trained immunity.

When the system goes wrong

MedlinePlus explains that too much, too little or the wrong immune response causes immune system disorders. An overactive response can lead to autoimmune diseases, a missing response causes immunodeficiency with more infections, and allergies are responses to substances most people's bodies treat as harmless. [1] Those conditions need medical care. See immunodeficiency and autoimmune conditions.

Why "boosting" does not follow from this

Because the system has many cell types, signals and safeguards, a single number (for example, one cell count or one cytokine) cannot describe overall protection. More activity is not automatically better: the same MedlinePlus page lists overreactions, such as allergy and autoimmune disease, as immune problems. [1] Read the claim analyses on boosting immunity and one biomarker as a complete immune assessment before accepting an "immune score".

Evidence label for this page: the descriptions above are established textbook-level immunology. Evidence labels used elsewhere in this library are explained in how we label evidence.