Introduction to the Pharmacology of Immunity
The human immune system performs a vital protective function. Its main physiological task is the continuous recognition and subsequent elimination (destruction) of any foreign structures. Such potentially dangerous objects primarily include pathogenic microorganisms entering the body from the outside, 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 adjustment. It is for these purposes that immunotropic agents are used—drugs capable of pharmacologically regulating the intensity of the immune response. The classification of this extensive group of drugs traditionally begins with a detailed examination of immunosuppressive agents (immunosuppressants), whose action is aimed at suppressing excessive immune reactions.
The Role of Antigen-Presenting Cells (APCs)
For the immune system to react to a threat, the foreign agent must be correctly «presented» to key immune cells. Specialized antigen-presenting cells (APCs) perform this crucial task. The main pools of APCs include:
- Macrophages;
- Dendritic cells;
- B lymphocytes.
The work of antigen-presenting cells consists of two sequential and strictly regulated stages:
- Antigen processing. Upon contact with a foreign structure, the APC uses its internal arsenal of enzymes. These enzymes break down the large antigen to 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 demonstrated not in isolation, but in strict association with a special molecule — MHC (Major Histocompatibility Complex).
Mechanism of Interaction and Recognition
The mere fact that an antigen is displayed on the APC surface is only preparation for the main event. The key moment in triggering an immune response is the physical interaction of cells.
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 membrane of the T lymphocyte. Only after the T lymphocyte receptor successfully binds to the presented complex does the immune system receive an unambiguous signal to initiate active actions against the specific foreign structure.
Pathways of the Immune Response Based on MHC Class
The subsequent scenario of the immune reaction is not universal. The direction the immune response takes 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+ T killers. Activation of CD8+ T killers triggers an aggressive cellular response, the ultimate goal of which is the direct destruction of the body's cells bearing this 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 helpers. 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.
Briefly: answers to topical questions
What are the main pharmacological groups into which immunotropic agents are divided?
- Immunostimulants (immunomodulators) — pharmacological agents acting on the immune system.
- Immunosuppressants — medicinal products that suppress unwanted immune reactions.
Which groups of drugs belong to immunosuppressive agents (immunosuppressants)?
- Cytostatics — include antimetabolites and alkylating agents.
- Glucocorticoids — such as prednisolone and dexamethasone.
- Selective inhibitors of cytokine synthesis and action — calcineurin inhibitors and mTOR inhibitors.
- Monoclonal antibody (mAb) drugs — agents against TNF-alpha, the IL-2 receptor, and miscellaneous agents.
What is the name of the specific receptor on the T lymphocyte membrane that recognizes the «MHC + antigenic peptide» complex?
- T-cell receptor (TCR) — recognizes and binds the «MHC + antigen» complex.
Which specific intracellular enzymes carry out antigen processing (cleavage) in APCs?
- Antigen-presenting cell (APC) enzymes — with their help, APCs form the antigenic peptide.
For quick memorization of response pathways, use the association with cell functions: MHC I works with CD8+ T killers (number I — one-on-one, direct destruction of the target cell). MHC II works with CD4+ T helpers (number II — paired work: the helper stimulates the B cell to synthesize antibodies).
Class and exam questions
What is the main function of the immune system according to physiological mechanisms?
The main function consists of the timely recognition and subsequent elimination of any foreign structures, which include pathogenic microorganisms and tumor cells.
Which cells perform the function of antigen presentation?
The role of antigen-presenting cells (APCs) in the body is performed by macrophages, dendritic cells, and B lymphocytes.
What does the antigen processing process entail?
Processing is an intracellular stage in which, with the help of specific APC enzymes, foreign material is cleaved and an antigenic peptide is formed from it.
How does the type of MHC molecule affect the outcome of the immune response?
The type of molecule determines the response vector: MHC class I activates CD8+ T killers to destroy infected cells, while MHC class II stimulates CD4+ T helpers, ultimately leading to antibody synthesis by B cells.
What else to study on this topic
- Features of the first classification group — immunosuppressive agents.
- Detailed work of antigen-presenting cell enzymes during processing.
- Structure of the «MHC + antigenic peptide» complex on the membrane surface.
- The process of complex binding to the T-lymphocyte receptor.
- Mechanisms of destroying antigen-bearing cells using CD8+ T killers.
- The cascade of reactions from CD4+ T helper division to antibody synthesis by B cells.
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