Classification by Source
Immunomodulating agents are globally divided into two large groups based on their origin:
- Exogenous immunomodulators. These are substances that are initially foreign to the body. Their nature is extremely diverse, including natural and synthetic chemical compounds as well as various microbial components (adjuvants, chemical formulations). Furthermore, physical factors such as ionizing radiation or climatic conditions are also considered exogenous factors capable of modulating immunity.
- Endogenous immunomodulators. These compounds are normally synthesized by the body itself, particularly by immunocompetent cells. Chemically, they are predominantly oligopeptides. Their primary role is immune activation by enhancing the proliferation and functional activity of immune cells. This broad group includes interferons ($\alpha$, $\beta$, $\gamma$), various interleukins, thymic peptides, tumor necrosis factors (TNF), chemokines, myelopeptides, colony-stimulating factors (CSF), and transforming growth factor (TGF).
Classification by Direction of Action
Regardless of their origin, any of these substances can exert a dual effect on the immune system. Based on the direction of the effect, they are classified into immunostimulants (activate defense mechanisms) and immunosuppressors (suppress immune responses).
- Immunostimulants. These include adjuvants: polymers, sorbents, polysaccharides, and lipopolysaccharides. This category also encompasses BCG-based complexes (known as Freund's adjuvant), bacterial components (muramyl dipeptide, prodigiosan, and salmazan), chemical compounds (e.g., levamisole), and endogenous molecules (cytokines, thymic and bone marrow peptides).
- Immunosuppressors. This group includes cytotoxic drugs (amino acid, enzyme, and purine antagonists such as 6-mercaptopurine), hormonal agents (glucocorticoids), and biological agents. Biological agents include monoclonal antibodies targeted at immunocompetent cell receptors and antilymphocyte serum. Physical factors such as X-ray or $\gamma$-irradiation also possess potent suppressive effects.
Main Pharmacological Groups
Modern medicine utilizes numerous drugs developed from immunomodulating substances:
- Recombinant cytokine drugs. These include interferon inducers, interferons themselves, and interleukins (IL-1$\beta$ or beta-leukin, IL-2, IL-3, IL-6).
- Thymic (thymus gland) preparations. Obtained either via recombinant DNA technology or as bovine thymus extracts. Well-known representatives include tactivir, thymosin, titulin, and thymopoietin.
- Bone marrow preparations. These include naturally derived myelopeptides and their recombinant analogs.
- Microbial-derived preparations. This is a vast group of exogenous modulators. Many are based on gram-negative bacterial lipopolysaccharides: pyrogenal is synthesized from Pseudomonas aeruginosa, prodigiosan from B. prodigiosum, and salmazan from Salmonella species. Modified bacterial cell wall components (e.g., lycopid, a modified muramyl dipeptide), ribosomal preparations (ribomunyl), VP-4 vaccine, and sodium nucleinate (low-molecular-weight yeast RNA salt) are also used. Lysates and components of Klebsiella, streptococci, and pneumococci containing membrane proteoglycans are widely applied.
Clinical Applications
Immunomodulating agents are widely used in the treatment and prevention of numerous pathological conditions. Primarily, they are prescribed for primary and secondary immunodeficiencies to restore host defense functions.
They are critically important in oncology and transplantology, where immunosuppressors are used to prevent the rejection of transplanted organs and tissues. Additionally, immunomodulators are actively employed in the management of immunopathological and allergic disorders. Microbial and other preparations have proven effective in treating infectious diseases as agents for immunocorrection and immunoprophylaxis, as well as in non-infectious diseases involving the immune system in their pathogenesis.