Immune System

The immune system safeguards our health by warding off harmful invaders like bacteria, viruses, parasites, and fungi (pathogens), using a complex network of organs, cells, proteins, and chemicals. Without this protection, infections would rapidly overcome us, leading to severe illness and death.

Your blood constantly circulates billions of immune cells that search for pathogens. About half of the blood consists of plasma, while the other half contains various cells. Most of these are red blood cells, or erythrocytes, responsible for oxygen transport. However, a smaller portion consists of white blood cells known as leukocytes, which function as the body’s defense troops.

The immune system generates multiple white blood cell types, each having a unique function. For example, neutrophils are the body’s primary defenders. They constitute the largest portion of white blood cells and work by identifying and eliminating pathogens such as bacteria and fungi. Eosinophils mainly combat parasitic infections and can also kill cancer cells. Basophils release chemicals like histamine that modulate allergic reactions, while monocytes clear away dead cells.

Once produced in the bone marrow, these immune cells enter the bloodstream. Many move to tissues, like those lining the skin, lungs, and gut, poised to combat intruders. Others continue to travel across the body, seeking out any germs. These frontline defenders, known as the “innate” immune system, are generalists; they defend against all infections.

The skin serves as the body’s principal barrier, warding off germs. Moreover, openings such as the mouth and eyes are safeguarded by saliva, mucus, and tears, which contain enzymes that degrade bacterial cell walls. If a pathogen penetrates the body, secondary defenses, like an inflammatory response, activate: blood vessels in the affected area enlarge, white blood cells, or phagocytes, migrate from vessels to the infected tissue to consume and neutralize bacteria, often resulting in redness, swelling, and pain.

Simultaneously, the immune system might release chemicals to induce a fever, enhancing body temperature to potentially inhibit pathogen growth and expedite immune responses.

If these responses prove insufficient to control an infection, the adaptive immune system activates. This mechanism tailors its response to each specific invader. Adaptive immune cells, or lymphocytes, undergo rigorous training, with T cells maturing in the thymus, an organ near the heart, and B cells remaining in the bone marrow.

The training involves two critical tests. First, ensuring the cells can respond to their specific invader to avoid wasting energy on non-responsive cells. Secondly, verifying that the cells won’t attack the body itself, preventing potential autoimmune diseases like diabetes and multiple sclerosis.

After training, these cells roam the body, inspecting lymph nodes for invaders. Hundreds of lymph nodes are scattered throughout the body, including in the neck, armpits, and groin, and swelling in these nodes often signals an active fight against infection. Each adaptive immune cell is specialized to combat one type of invader, necessitating the body to create a vast array of these cells to deal with various potential threats.

Upon recognizing an invader, these cells multiply and launch their attack. This process can take a week, explaining the delay in recovering from illnesses. After defeating an infection, the involved immune cells remain active, allowing for quicker responses to future encounters with the same threat. This memory function underpins the effectiveness of vaccinations.

Lastly, immunodeficiency, which emerges when the immune system’s strength is compromised, results in persistent and potentially fatal infections. This condition can be genetic, as with severe combined immunodeficiency, acquired, as with HIV/AIDS, or induced via immunosuppressive medication.