Contents
The body must distinguish "self" from "non-self"—and do so with agents it has never encountered. The evolutionary solution is two complementary systems.
First line: innate immunity
Responds within minutes to hours, without needing to know exactly who the invader is.
Cells such as macrophages and dendritic cells carry pattern recognition receptors (PRRs). They do not recognize specific viruses but rather identify general molecular features of pathogens—for example, double-stranded RNA, which almost only appears when a virus is replicating inside a cell.
When PRRs are activated, cells secrete interferons and cytokines, causing local inflammation and alerting neighboring cells to switch to an antiviral state.
Second line: adaptive immunity
Slower—taking 5–10 days for the first encounter—but specific and possessing memory.
B lymphocytes produce antibodies that bind to specific regions on the virus surface (epitopes), neutralizing it or marking it for destruction by other cells.
T lymphocytes are divided into two main roles: helper T cells coordinate the overall response, while cytotoxic T cells destroy infected cells.
Why it is so specific
Each B and T cell carries a unique receptor, created by the random recombination of gene segments (V(D)J mechanism). This process generates about 10¹¹ different variants—enough so that, for almost any foreign molecule, a cell with a matching receptor already exists.
When a cell encounters its specific target, it multiplies rapidly. This is called clonal selection: the body does not design antibodies on demand; it selects an existing suitable one and amplifies it.
Immune memory
After an infection ends, most effector cells die off, but a group of memory B and T cells persists—potentially for decades. Upon a second encounter, the response occurs within 1–3 days instead of 5–10 days and is much stronger.
Vaccines exploit this exact mechanism: introducing a non-pathogenic part of the pathogen (surface proteins, inactivated viruses, or mRNA encoding that protein) so the immune system creates memory without the person having to go through the actual illness.
Why we still get the flu every year
The influenza virus constantly changes its surface proteins (antigenic drift). Antibodies created last year may no longer match this year's strain well—this is why the flu vaccine is updated annually, unlike the measles vaccine which is effective for nearly a lifetime.
This article provides scientific information for reference purposes and is not a substitute for advice, diagnosis, or treatment by a medical professional. If you have specific health concerns, please consult a doctor.
Further reading
References
- [1]Janeway's Immunobiology, 10th edition — W. W. Norton (2022)
- [2]Immune memory: understanding long-term protection — Nature Reviews Immunology (2021)
Image: NIAID/NIH — Wikimedia Commons, Public domain.
