Innate and acquired arms of immunity work together, as an integrated immune system, to protect us against infections by pathogenic microbes. Interestingly, innate and acquired immune systems have some differences. We shall highlight these differences under the following headings:
- Speed of response and potency of response
- Specificity of response
- Immunological memory
- Cellular mediators
Overview of innate host defense and acquired immunity
An extremely complex relationship exists between innate and acquired immune systems. Moreover, the key to understanding basic immunology is lies on the ability to distinguish the different components of each system together with the functions of each component. To ease your understanding, you can think of innate and acquired immune systems as two distinct parts of the immune system. However, understand that, in the bigger picture, immune reactions to virtually all pathogenic microbes involve both innate and acquired immune systems.
Actually, innate and acquired immune reactions occur neither separately nor in isolation, but cells of the two systems communicate with one another using a wide variety of chemical mediators. This interconnection between cells of the two systems enables the systems to mount a well-coordinated and integrated response against any given pathogen.
Innate host defense is present at birth, even before the individual encounters a pathogen for the first time. In other words, innate immunity is inborn and constitutively present (i.e. it is always present and gets acts activated immediately a pathogen enters the human body).
On the other hand, acquired immunity is absent at the first time an individual is exposed to a pathogen. Acquired immunity against a pathogen develops only after the individual encounters the pathogen.
Speed of response
Immediately a foreign microbe enters the body, components of innate immune system begin to act against the microbe. This is so because components of innate immune system are always available in the body good quantities at all times. For the reason that innate immune system offers the first response to a pathogenic challenge, it is the first line of defense against pathogens.
Conversely, a lag time exists between infection and the time the acquired immune system responds if it is encountering the microbe for the first time. This lag time takes about 6 days, normally. The body uses this time to activate and produce sufficient amounts of lymphocytes, which are specific for the harmful foreign antigen. Moreover, acquired immune system uses this period to generate antibodies or T-cell receptors that are specific for the infecting antigen. This implies that acquired immune system is unable to defend against microbes during the first few days of the infection.
Specificity of response
The defense mechanisms of innate immune system are non-specific in nature. Components of innate defense system recognize pathogens (i.e. can distinguish pathogens from host cells) and can decipher the different classes of pathogens but are unable to make precise and fine distinctions between pathogens of the same class. For instance, eosinophils generally recognize parasites and kill them but are unable to distinguish one parasite from another. In the same way, neutrophils recognize and phagocytose bacteria but are unable to make fine distinctions between different intracellular and extracellular bacteria.
Components of innate immune system recognize pathogens by detecting pathogen-associated molecular patterns (PAMPs); repeating sequence of carbohydrates, lipids, and amino acids common to cell walls of pathogens and not usually found on human cells. Pathogens express PAMPs but host cells do not express PAMPs.
However, acquired immune system recognizes pathogens by detecting specific antigens, which different pathogens express. These antigens are mostly protein molecules: antibodies and T-cell receptors can distinguish between two peptide sequences that differ by only one amino acid residue.
Moreover, acquired immune responses are highly specific to a particular pathogen. For instance, the acquired immune response against the bacteria Streptococcus pyogenes will act against Streptococcus pyogenes but not against any other microbe.
Innate immune system is unable to keep an immunological memory of the antigen that challenges the system, because of this, if a particular pathogen infects the system for a second time, innate immune system will be unable to mount a more effective defense against the antigen because it is unable to “remember” the pathogen so to say.
Acquired immune system is able to keep immunological “memory” of all foreign antigens it has encountered. At the end of an immune response, acquired immune system preserves memory cells: memory cells keep immunological memory of all foreign antigens the body encountered. The next time a pathogen displaying the same antigen infects the body; the antigens will activate memory cells, which transform into effector cells and induce an immune response that is both more rapid and effective in kill the pathogens.
This means that memory cells in a way ‘remembers’ the foreign antigen that induced its formation. Because of the memory, subsequent exposure to the same foreign antigen triggers a faster, stronger, and more effective immune response. A secondary immune response occurs during a second exposure to the pathogen, and is more potent and faster than the initial response. Memory cells are responsible for secondary immune response.
You must understand that memory cells may survive for many years, or even up to several decades. Therefore, without an acquired immune response, the body will be unable to have long-lasting immunity against any foreign microbe. Recall that specificity and memory are the hallmarks of immunity.
Neutrophils, eosinophils, basophils and mast cells, natural killer (NK) cells, tissue macrophages, and dendritic cells all participate in innate host defense. Natural killer cells are large cytotoxic lymphocytes that are not T cells.
B and T-lymphocytes are the cellular mediators of acquired immunity. Finally, certain components of innate immune system activate adaptive immune response. Note that antigen-presenting cells (dendritic cells and macrophages) are cells of innate immune system that digest a pathogenic microbe and display the microbe’s antigen on MHC proteins to alert and lymphocytes to the presence of the infecting pathogen. Alerted and activated lymphocytes then multiply and mount an immune response against the pathogen.
Components of innate host defense and acquired immunity
Components of innate host defense include:
- Anatomical barrier component (skin, and epithelia of respiratory, GI, and genitourinary systems)
- Physiological barrier component ( e.g. stomach acid)
- Phagocytic component (professional and non-professional phagocytes)
- Inflammatory and fever
- Enzymes and proteins of innate immune system (e.g. complement proteins, C-reactive proteins, lipopolysaccharide binding protein and other acute phase proteins). Sometimes, these humoral factors are called natural antibodies because they bind.
Whereas, the components of acquired immunity are:
- Humoral component: B-cells mature into plasma cells and produce antibodies. Antibodies mediate humoral arm of acquired immunity.
- Cellular component: T-cells mature into two effector cell types (Helper T-cells and cytotoxic T-cells).