Antigen

Antigen (from Greek anti — against and gennao — to produce, create) is any foreign substance recognized by immune cell receptors, which initiates a cascade of protective immune reactions (induces an immune response).

In a normal, healthy immune system, antigens are viewed as markers of biological threat and initiates their destruction process.

Etiology and pathophysiology

In terms of chemical structure, antigens are often complex, high-molecular-weight proteins, glycoproteins, or polysaccharides. The immune system does not recognize the entire molecule as a whole. It reacts only to tiny unique segments on its surface, called epitopes.

The process of antigen presentation and triggering an immune response includes the following stages:

1. Uptake of the pathogen — when a bacterium or virus enters the tissue, special patrolling cells (macrophages and dendritic cells) ingest it;
2. Fragmentation and presentation — cells physically break down the pathogen into fragments and display these antigenic epitopes on their own cell membrane via the major histocompatibility complex (MHC);
3. Activation of T-lymphocytes — only then can T-cells ‘see’ the enemy, activate, and command the mass production of specific antibodies.

Clinical significance

The concept of antigens forms the basis for all laboratory diagnostics, transfusiology, and vaccination:

• Vaccination — vaccines contain highly purified or artificially synthesized antigens that safely train the immune system to recognize infections;
• Laboratory diagnostics — rapid tests (for COVID-19, flu, streptococcus) target the physical identification of specific pathogen antigens in patient biomaterial;
• Transfusology — human blood groups (the ABO system and Rh factor) are determined by the presence of specific antigens on the erythrocyte membrane.

A mistaken blood transfusion containing a foreign antigen causes a life-threatening hemolytic crisis.