Introduction to Immune Pharmacology
The human immune system performs a vital protective function. Its primary physiological task is the continuous recognition and subsequent elimination (destruction) of any foreign structures. Such potentially dangerous targets include pathogenic microorganisms entering the body from the exterior, as well as the body's own cells that have undergone malignant transformation (tumor cells).
In medical practice, situations frequently arise where the natural strength of the immune response requires pharmacological correction. Immunotropic drugs are used precisely for these purposes—medications capable of pharmacologically regulating the magnitude of the immune response. The classification of this extensive group of drugs traditionally begins with a detailed examination of immunosuppressive drugs (immunosuppressants), whose action aims to suppress excessive immune reactions.
Role of Antigen-Presenting Cells (APCs)
For the immune system to respond to a threat, a foreign agent must be properly "presented" to key immune cells. Specialized antigen-presenting cells (APCs) perform this vital task. The main pools of APCs include:
- Macrophages;
- Dendritic cells;
- B lymphocytes.
The work of antigen-presenting cells consists of two consecutive and strictly regulated stages:
- Antigen processing. Following contact with a foreign structure, the APC utilizes its internal enzymatic arsenal. These enzymes break down the large antigen and form a small, highly specific antigenic peptide.
- Antigen presentation. The generated antigenic peptide is transported and displayed on the outer surface of the APC membrane. The peptide is presented not in isolation, but in a strict complex with a special molecule—MHC (Major Histocompatibility Complex).
Interaction and Recognition Mechanism
The mere fact that an antigen is displayed on the APC surface serves only as preparation for the main event. The key trigger for launching an immune reaction is physical cell-to-cell contact.
The "MHC + antigenic peptide" complex formed on the surface of the antigen-presenting cell must be recognized. This recognition is carried out by a specific receptor located on the T lymphocyte membrane. Only after the T lymphocyte receptor successfully binds to the presented complex does the immune system receive an unambiguous signal to initiate active operations against the specific foreign structure.
Immune Response Pathways Based on MHC Class
The subsequent scenario of the immune response is not universal. The direction of the immune response is strictly determined by which specific type of MHC molecule participates in the presentation of the antigenic peptide. Physiologically, two main pathways are distinguished:
- MHC class I pathway. If the antigen is presented in a complex with an MHC class I molecule, this signal is read by a specific group of cells—CD8+ cytotoxic T cells (T-killers). Activation of CD8+ T cells triggers an aggressive cellular response whose ultimate goal is the direct destruction of body cells bearing the foreign antigen.
- MHC class II pathway. This pathway specializes in the body's reaction to foreign proteins. If the peptide is displayed in a complex with an MHC class II molecule, another population of lymphocytes is stimulated—CD4+ T helper cells. Upon receiving the signal, CD4+ T helpers begin active division. The proliferated helpers, in turn, stimulate B cells. The final stage of this cascade is the large-scale synthesis of specific antibodies by B cells directed against the original antigen.