The human immune system is a complex network of organs,

cells,

proteins,

and chemicals that work together to protect the body from harmful invaders such as bacteria,

viruses,

fungi,

parasites,

and other foreign substances.

Its primary function is to prevent or limit infection by distinguishing between the body's own healthy cells and unhealthy or foreign ones,

often through recognizing "danger" cues like danger-associated molecular patterns (DAMPs).

This system operates continuously,

patrolling the body via the bloodstream and lymphatic system,

and responds by keeping invaders out,

destroying them,

or limiting their harm.

Without a functioning immune system,

even minor infections could become life-threatening

The immune system has evolved over time,

with basic mechanisms like phagocytosis (cell engulfment) and antimicrobial peptides (defensins) present in ancient organisms,

while more advanced features,

such as adaptive recognition,

developed in jawed vertebrates like humans.

It is pervasive,

with cells circulating throughout the body or residing in specific tissues,

each performing unique roles in detection,

communication,

and response.

Key Components of the Immune System

The immune system comprises various organs,

tissues,

cells,

and molecules that interact seamlessly:

Organs and Tissues:

Primary lymphoid organs include the bone marrow (where all immune cells originate) and the thymus (where T-cells mature).

Secondary organs,

such as the spleen,

tonsils,

and lymph nodes,

filter blood and lymph,

trap invaders,

and serve as sites for immune cell activation.

The lymphatic system,

a network of vessels carrying lymph (a fluid containing immune cells),

connects these organs and facilitates cell travel between tissues and the bloodstream.

Mucous membranes in areas like the respiratory and digestive tracts also act as barriers.

Cells:

White blood cells (leukocytes) are central players,

produced in the bone marrow and circulating through blood and tissues to detect and attack microbes.

Key types include:

Phagocytes (e.g., macrophages, neutrophils):

These engulf and digest invaders.

Lymphocytes:

B-cells produce antibodies,

T-cells coordinate attacks or kill infected cells,

and natural killer (NK) cells destroy virus-infected or cancerous cells.

Other cells:

Dendritic cells present antigens to activate adaptive responses,

while mast cells and basophils release chemicals causing inflammation.

Proteins and Chemicals:

Antibodies (produced by B-cells) bind to antigens (markers on invaders) to neutralize them or mark them for destruction.

The complement system,

a group of proteins,

enhances antibody actions by puncturing invader membranes or promoting inflammation.

Cytokines are signaling molecules that coordinate immune responses,

such as triggering fever or inflammation.