Innate immunity: the oldest baseline
Innate defense was formed at the earliest stages of the development of multicellular organisms and is present in all taxa, from sponges to mammals. The efficiency of this system is so high that it reliably protected even large protostomes and lower deuterostomes.
The basis of the system is the homeostatic principle of "self-nonself" recognition. Defense cells act based on evolutionary experience: everything foreign almost always causes harm. The third fundamental step in the evolution of immunity (immediately after the isolation of the internal environment and the appearance of specialized phagocytic cells) was the emergence of pattern-recognition receptors.
- These molecules react not to a specific microbial strain, but to entire groups of similar markers — pathogen-associated molecular patterns (PAMPs) (for example, bacterial cell wall fragments or endotoxins).
- Receptors can be located on the surface of immunocytes, inside their granules, or float freely in biological fluids.
- This approach is genetically extremely efficient: the organism requires very few genes to encode receptors capable of "detecting" almost all possible pathogens.
Mechanisms of alien elimination
The main task of the innate response is the detection and immediate destruction of a threat. The ancestors of modern effector cells were motile amoebocytes of mesenchymal origin. Their profile task was the capture of suspicious objects.
Recognition of pathogen patterns inevitably activates immunocytes. The destruction of the aggressor can proceed via several pathways (intracellular, extracellular, or contact cytolysis). Humoral factors also contribute: soluble molecules can kill pathogens themselves or merely mark them for subsequent engulfment. Regardless of how the agent was neutralized, the finale of the process is always phagocytosis — enzymatic digestion of the remnants.
Adaptive immunity and the genetic paradox
Unlike innate immunity, adaptive (acquired) immunity is an evolutionarily late innovation. It abruptly appeared in chordates about 500 million years ago, and its active development started in gnathostomes (cartilaginous fish) and continued up to mammals. Interesting fact: agnathans (jawless fish) have been found to possess a unique, highly primitive pathway for forming acquired defense.
The adaptive system is closely intertwined with the innate one, but differs cardinally in its object of attack. Instead of general patterns, it recognizes antigens — individual molecular structures of microorganisms. Molecules of the immunoglobulin superfamily are used for this purpose.
During evolution, a problem arose: the number of potential antigens exceeds $10^6$, but the genome of multicellular organisms is too small to store a separate gene for each receptor. The solution was the clonal selection theory:
- During lymphocyte maturation, random rearrangement of gene segments occurs.
- Each cell forms a receptor with a unique specificity.
- Upon contact with an antigen, only the matching cell is activated, which begins to divide, forming a massive clone of identical cells.
Key differences between defense systems
Despite close interaction, the two branches of immunity rely on different principles. It is convenient to compare them by their main parameters.
| Characteristic | Innate defense | Adaptive defense |
|---|---|---|
| Stimulus for development | Established during ontogenesis, always ready | Develops only in response to a specific antigen |
| Effector cells | Predominantly myeloid | Exclusively lymphoid |
| Type of reaction | The entire population reacts synchronously | A specific trained clone responds |
| Risk of autoimmunity | Minimized | Real risk exists |
| Immunological memory | Completely absent | Forms after primary contact |
Briefly: answers to topical questions
Which modern blood and tissue cells belong to the myeloid lineage?
Modern cells of the myeloid lineage include blood leukocytes and their tissue forms.
- Basophils are blood cells.
- Eosinophils are blood cells.
- Neutrophils are blood cells.
- Mast cells are tissue cells.
- Monocytes are blood cells from which the following differentiate in tissues:
- Macrophages are tissue forms of monocytes.
- Dendritic cells are tissue forms of monocytes.
What are the main families of pattern-recognition receptors (PRRs)?
The main families of pattern-recognition receptors are divided into membrane, intracellular, and soluble.
- Toll-like receptors (TLRs) are membrane receptors that activate innate immunity cells.
- C-type lectins are membrane receptors that mediate internalization.
- Scavenger receptors (Scavenger) are membrane receptors for internalization.
- Integrins are membrane receptors for adhesion and motility.
- NOD-like receptors (NLRs) are intracellular receptors that recognize peptidoglycans.
- RIG-like receptors (RLRs) are intracellular receptors that recognize RNA.
- DAI are intracellular receptors that recognize DNA.
- Pentraxins, collectins, ficolins, and complement system components are soluble (secreted) receptors.
Through what molecular mechanism does the somatic rearrangement of lymphocyte receptor genes occur?
Somatic rearrangement of lymphocyte receptor genes occurs via receptor gene rearrangement (recombination) in maturing lymphocytes.
- For heavy chain genes, D-J and then V-DJ recombination are performed.
- For immunoglobulin light chain genes, V and J segments join directly, without a D segment.
- RAG1 and RAG2 participate in the rearrangement; TdT provides non-templated nucleotide addition.
- Allelic exclusion blocks the rearrangement of the second allele after a successful rearrangement and ensures receptor monospecificity.
What specific humoral factors are involved in the innate immune response?
The innate immune response involves the following humoral factors:
- Complement system — provides opsonization, cytolysis, and chemotaxis.
- Acute-phase proteins, including C-reactive protein, — participate in opsonization and replicate certain antibody effects.
- Cytokines, including proinflammatory cytokines and type I interferons.
- Natural antibodies — products of B1 cells that serve as an additional stimulus for complement activation.
- Bactericidal factors, including lysozyme, polyamines, and defensins.
To remember what the systems look for: innate immunity reacts to the "UNIFORM" (common pathogen patterns shared across entire classes of microbes), while adaptive immunity checks the "PASSPORT" (the unique antigen of a specific agent).
Class and exam questions
Why does the immune system attack even those foreign molecules that are not toxic?
During evolution, a «presumption of guilt» was formed. The immune system does not wait for aggression to manifest, because historically most foreign objects posed a threat to the organism's survival.
In which organisms did adaptive immunity first appear?
Its origin is associated with chordates (approximately 500 million years ago). Full development via the classical pathway started in gnathostomes, beginning with cartilaginous fishes.
Why do lymphocytes need somatic gene rearrangement?
This is the answer to the genetic paradox. A limited number of basic genes create millions of receptor variations through combinations. This makes it possible to recognize any of a multitude of antigens without overloading the organism's DNA.
What else to study on this topic
- Mechanisms of intracellular, extracellular, and contact-mediated cytolysis
- Properties and functions of immunoglobulin superfamily molecules
- Alternative pathway of immune system development in cyclostomes
- The process of immunological memory formation by lymphocyte clones
- The role of humoral factors in preparing the pathogen for phagocytosis
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