How does the immune system work?

4 min read

The immune system is the body's natural defense system. It does not reside in a single part of the body; rather, it is a network of cells, molecules, tissues, and organs that, together, protect it. Each of these elements plays a key role in the functioning of the overall immune system [2].

The immune system is divided into two branches: the innate immune system and the acquired (or adaptive) immune system. The innate immune system acts as a general defense against common pathogens (any bacteria, virus, or other disease-causing microorganism), which is why it is also known as the non-specific immune system. The acquired immune system acts against specific threats and learns to activate precise responses against the viruses or bacteria that the body has already encountered. The different components of the immune system provide both types of protection [2][3].

Physical and chemical barriers: the first line of defense

Before an invader enters, the body attempts to stop it with barriers such as the skin, mucous membranes, and chemical substances (tears, saliva, stomach acid). The skin is the largest organ of the body and serves as a protective barrier against pathogens and toxins. It also possesses its own immune cells and lymphatic vessels [2].

Innate immunity: rapid and general response

If a germ manages to enter, innate immunity acts within minutes or hours. It does not distinguish between a virus and a bacterium; it simply attacks everything that appears foreign. It includes phagocytic cells such as macrophages and neutrophils, inflammation, and fever. Macrophages are known as the "big eaters" of the immune system because they engulf and destroy bacteria and other harmful cells. Like dendritic cells, they present antigens to other immune system cells for identification and destruction [2].

Adaptive immunity: slow and specialized response

This part takes days to activate the first time, but it is very precise and has memory. B lymphocytes release antibodies to defend against invasive harmful cells. Each B cell is programmed to produce a specific type of antibody. CD4+ helper T lymphocytes send help signals to other immune cells (such as CD8+ effector T lymphocytes) to better direct their response. CD8+ cytotoxic T lymphocytes destroy thousands of virus-infected cells every day. These cells can also directly attack and destroy cancer cells [2][1].

How the body recognizes a pathogen: antigens and antibodies

An antigen is any substance that the immune system considers foreign, generally proteins or sugars on the surface of pathogens. Antibodies are proteins that bind to specific markers (antigens) on harmful invaders such as germs, viruses, or tumor cells. Antibodies also mark these harmful cells for attack and destruction [2][1].

White blood cells (leukocytes) are the star cells. Neutrophils and macrophages are part of the innate immunity and phagocytose invaders. Lymphocytes belong to the adaptive immunity: T lymphocytes coordinate or destroy infected cells, while B lymphocytes produce antibodies. Cytokines are messenger molecules that help immune cells coordinate the correct immune response to any invader [2][1].

Inflammation and autoimmune diseases

Inflammation is the visible response of innate immunity: redness, heat, swelling, and pain to bring white blood cells to the damaged site. When the immune system does not function properly, such that it confuses self with non-self, it can attack the body's own tissues and cause an autoimmune disorder, such as rheumatoid arthritis, Hashimoto's thyroiditis, or systemic lupus erythematosus. Regulatory T lymphocytes are the check and balance to ensure that the immune system does not overreact. Chronic immune reaction is known as autoimmune disease [2, 4].

Allergies: a misidentification error

An allergy is an error in judgment: the system produces IgE against harmless substances (pollen, food), activating mast cells that release histamine and cause inflammation. This is related to the hygiene hypothesis: in environments that are too clean, the branch against parasites turns against innocent substances [3].

Vaccines and immunological memory

Vaccines present antigens without causing severe illness, activating B and T lymphocytes that generate memory cells. If the actual pathogen arrives, the response is fast and strong thanks to that memory [2][1].

Influence of sleep, diet, and stress

70-80% of defenses live in the gut. Deep sleep increases cytokines and T lymphocyte activity; lack of sleep reduces the response to infections. A diet rich in fiber, vitamins (C, D, zinc), and probiotics strengthens the microbiota and reduces chronic inflammation. Chronic stress raises cortisol, which suppresses lymphocytes and weakens defenses [5].

Key tissues and organs include bone marrow (produces cells), lymph nodes (filter and educate lymphocytes), spleen, thymus, and appendix [2]. The immune system also protects against cancer through immunotherapy that leverages its power [2].

In summary, the immune system integrates antigen recognition and a series of steps to destroy or neutralize foreign substances, working in a coordinated manner between innate and adaptive immunity [1][2].