What is the Immune system?

The immune system is the complex network of organs, cells, and proteins that protects the body against infections. This defense system proceeds via the recognition and processing of foreign substances. The network consists of the lymphatic system, complement system, spleen, thymus, bone marrow, and cells (WBCs, B cells, T cells).

What is innate immunity?

The nonspecific arm of the immune system with which an individual is born is termed innate immunity (also called natural immunity). The innate immunity acts as a barrier against foreign invading materials. These barriers form the first line of defense and aid in the activation and modulation of the adaptive immune response. The immune system of the body proceeds by recognition, presentation, and processing of foreign invaders, followed by their clearance from the body.

The components of the innate immune system based on innate immunity development include physiological barriers, anatomical barriers, phagocytosis, and inflammatory responses. 

Physiological barriers: These components include the epithelial lining of the skin, mucous membrane, and physical parameters like temperature, pH, and barriers like enzymes, antimicrobial peptides, cytokines, etc.

Anatomical barriers: Being non-specific, many components have pattern recognition in which receptors referred to as pattern recognition receptors (PRRs) can recognize unique molecules present within microbes called pathogen-associated molecular patterns (PAMPs). 

Phagocytosis: It is an endocytosis process in which the foreign body is ingested by specialized cells called phagocytes. Phagosomes are formed when these foreign bodies are ingested, which is a vacuole surrounding these bodies. Later on, the lysosome present within the phagocytes fuses with the phagosome, forming a phagolysosome. The release of lytic enzymes present in lysosome eventually lyses the infectious agent hence clearing the infection.

Inflammatory Reactions: Inflammatory reactions are induced with four cardinal features when host tissue gets damaged. These features include calor (temperature rise), dolor (pain), rubor (redness), and tumor (swelling). Histamines, proinflammatory cytokines, defensins, and kinins are mediators of inflammatory reactions.

Types of innate immunity

It includes individual, racial, and species immunity. 

Individual immunity: Some individuals of the same race and same species can have varied experiences with certain infections. For example, children are more susceptible to viral fever than adults.

Racial immunity: Individuals of different races within the same species have varied susceptibility or resistance toward infection caused by the same etiological agent. For example, races with sickle cell anemia prevalent in Africans on the Mediterranean coast are resistant to malaria caused by Plasmodium falciparum. This is because sickle cell anemia causes an alteration of the shape of the erythrocyte, which prevents its parasitization.

Species immunity: Individuals from different species have different susceptibilities toward any infection. For example, humans are not affected by chicken cholera, infectious horse anemia, etc., while animals are resistant to many human diseases like syphilis, gonorrhea, measles, etc. 

Innate immunity in an individual is also influenced by other factors, such as:

Age: Very old people and young ones are more susceptible to infections when compared to adults. 

Hormonal level: Any individual under corticosteroid hormone treatment is more susceptible to infection. Likewise, hormonal disorders like hypothyroidism, diabetes mellitus, etc., can make the individual more prone to infections. 

Nutritional status: Nutritional status of the host, like deficiency of vitamins and proteins, makes an individual more susceptible to infections.

Significance of innate immunity

1. Physical and chemical barriers prevent the entry of foreign materials.
2. If the infection is established, a cascade of complement reactions and phagocytosis helps clearance of the infecting agents.
3. It activates the adaptive immune system by the release of cytokines and antigen presentation. 

 Innate immunity examples

1. Gastric acidity, whose high pH prevents the entry of pathogens inside the body by microbicidal effect.
2. Skin that acts as a barrier against invasion of foreign bodies inside the body.
3. Phagocytosis of Mycobacterium tuberculosis by antigen-presenting cells.

What is Acquired (Adaptive) Immunity?

The specific arm of the immune system possessing exquisite specificity of antigen recognition and is acquired by an individual during his lifetime is termed acquired immunity. The mediators of acquired infection include:

• Humoral immunity: Serum proteins called antibodies produced by B cells mediate this type of immunity. The humoral immune response protects the body against extracellular infections. Activation and differentiation of B cells into antibody-secreting plasma cells occur when these extracellular antigens are recognized by body cells, especially helper T cells.
• Cellular immunity: The immune response in cellular immunity is mediated by T cells. Cellular immunity protects the body against intracellular infections. The initial step in cellular immune response involves recognizing the antigen by phagocytes, then sensitization of cytotoxic T cells, and, finally, releasing cytokines in response to that antigen.

In case of a repeated encounter with the same antigen, memory T cells made during initial exposure act by a faster and more potent response.

Types of Acquired (Adaptive) Immunity

When a pathogen enters the body, acquired immunity can be induced by the host body or by the artificial transfer of antibodies/ lymphocytes inside the host body.

• Active immunity: This is the type of acquired immunity developed in the host body itself because of exposure to the pathogen. When the antigen is recognized by the host cell, an immune response develops, forming antibodies/ helper T cells or cytotoxic T cells. Active immunity, when developed by natural infections, is called natural active immunity, whereas when developed upon exposure to preformed vaccines is called artificial active immunity.
• Passive immunity: When the host body acquires immunity in the form of preformed serum or lymphocytes, it is called passive immunity. Natural passive immunity is acquired when a mother passes IgG to the fetus during pregnancy, and artificial passive immunity is acquired by the administration of preformed antibodies that help in the neutralization of the antigen.

 Significance of Acquired (Adaptive) Immunity

1. When foreign antigens are presented by professional antigen-presenting cells (APCs) or upon release of cytokines from cells, adaptive immunity comes into the act.
2. Antigen-specific antibodies help to neutralize the infection.
3. Immunological memory is developed in the form of memory B cells which respond immediately upon the re-exposure to the specific infectious agent.
4. T helper cells help in the activation of other immune cells.
5. Cytotoxic T cells destroy cells infected with viruses and tumor cells.
6. Regulatory T cells suppress immune response when required, preventing body cells from unwanted self-attack. 

 Acquired (Adaptive) Immunity Examples

1. When phagocytes cannot clear Mycobacterium tuberculosis bacilli alone, the release of cytokines activates cytotoxic T cells. 
2. After primary infection with COVID-19, secondary infection is less severe as memory has developed during primary infection.
3. Infections due to the Ebola virus and Cytomegalovirus also have the potency to develop memory cells making reinfection less severe. 

Innate Immunity vs. Acquired Immunity (6 Differences)