Introduction

The immune response is the body's ability to stay safe by protecting against harmful agents. It involves lines of defense against most microbes and specialized and highly specific responses to a particular offender. This immune response is either innate, nonspecific, adaptive acquired, or highly specific. The innate response, often our first line of defense against anything foreign, defends the body against a pathogen in a similar fashion at all times. These natural mechanisms include the skin barrier, saliva, tears, various cytokines, complement proteins, lysozyme, bacterial flora, and numerous cells, including neutrophils, basophils, eosinophils, monocytes, macrophages, reticuloendothelial system, natural killer (NK) cells, epithelial cells, endothelial cells, red blood cells, and platelets.

The adaptive acquired immune response will utilize the ability of specific lymphocytes and their products (immunoglobulins and cytokines) to generate a response against the invading microbes; its typical features are:

1. Specificity: The triggering mechanism is a particular pathogen, immunogen, or antigen.  
2. Heterogeneity: Signifies the production of millions of different effectors of the immune response (antibodies) against millions of intruders
3. Memory: The immune system has the ability not only to recognize the pathogen on its second contact but to generate a faster and stronger response.[1][2][3]

The inflammatory immune response is an example of innate immunity as it blocks the entry of invading pathogens through the skin, respiratory, or gastrointestinal tract. If pathogens can breach the epithelial surfaces, they encounter macrophages in the subepithelial tissues that will attempt to engulf them and produce cytokines to amplify the inflammatory response.

Active immunity results from the immune system's response to an antigen and, therefore, is acquired. Immunity resulting from the transfer of immune cells or antibodies from an immunized individual is passive immunity. The immune system has evolved to maintain homeostasis, as it can discriminate between foreign antigens and self; however, an autoimmune reaction or disease develops when this specificity is affected.

Issues of Concern

Although the immune system is designed to protect the individual against threats, an exaggerated immune response sometimes generates a reaction against self-antigens, leading to autoimmunity. There are also several reasons that the immune system cannot always defend against all threats, including:

• Transplantation rejections: These are immune-mediated responses that represent a hindrance to transplantation.
• Autoimmune disorders: The etiology of many autoimmune disorders is obscure; the prevalence of these disorders increases and manifests more aggressively.
• Type-I hypersensitivity disorders: These are immune-mediated and include allergic bronchial asthma, food allergy, and anaphylactic shock.
• Immunodeficiency disorders: These are rare, but they affect some children.

Vaccination is required to induce an adequate active immune response to specific pathogens:

• Live attenuated vaccines: Induce both humoral and cellular response; contraindicated in pregnancy and immunocompromised states. Examples include adenovirus, Polio (Sabin), Varicella, Smallpox, Bacillus Calmette Guerin (BCG), yellow fever, influenza (intranasal), MMR, Rotavirus, etc
• Killed or inactivated vaccines: Induce only humoral response. Examples include rabies, influenza (injection), Polio (Salk), Hepatitis A, etc
• Subunit vaccines: Examples include HBV, HPV (types 6,11,16 and 18), acellular pertussis, Neisseria meningitides, Streptococcus pneumoniae, Hemophilus influenza type b, etc
• Toxoid vaccine: Examples include Clostridium tetani, Corynebacterium diphtheria, etc.

Cellular Level

Cells of the innate immunity are:

• Phagocytes (monocytes, macrophages, neutrophils, and dendritic cells)
• Natural killer (NK) cells

Cells of the adaptive response are:

• T Lymphocytes classified as  CD4+T cells and CD8+T cells
• B Lymphocytes differentiate into plasma cells, which produce specific antibodies

Development

Mesoderm cells are induced to form hemangioblasts, a common precursor for vessels and blood cell formation. The aorta-gonad-mesonephros region is the definitive hematopoietic stem cells from the mesoderm surrounding the aorta. These stem cells colonize the liver and are actively produced by the bone marrow by the seventh month of gestation.[4]

Organ Systems Involved

The organ systems involved in the immune response are primarily lymphoid organs, which include the spleen, thymus, bone marrow, lymph nodes, tonsils, and liver. The lymphoid organ system is classified according to the following:

1. Primary lymphoid organs (thymus and bone marrow), where T and B cells first express antigen receptors and become mature functionally.  
2. Secondary lymphoid organs like the spleen, tonsils, lymph nodes, and the cutaneous and mucosal immune system are where B and T lymphocytes recognize foreign antigens and develop appropriate immune responses.

T lymphocytes mature in the thymus, where they reach a stage of functional competence, while B lymphocytes mature in the bone marrow, the site of generation of all circulating blood cells. Excessive release of cytokines stimulated by these organisms can cause tissue damage, such as endotoxin shock syndrome.

Function

The immune system responds variedly to different microorganisms, which are often determined by the features of the microorganism. These are some different ways in which the immune system acts

Immune Response to Bacteria

The response often depends on the pathogenicity of the bacteria[5]:

• Neutralizing antibodies are synthesized if the bacterial pathogenicity is due to a toxin
• Opsonizing antibodies - produced as they are essential in destroying extracellular bacteria
• The complement system is activated especially by gram-negative bacterial lipid layers
• Phagocytes kill most bacteria utilizing positive chemotaxis, attachment, uptake, and finally engulfing the bacteria
•  CD8+ T cells can kill cells infected by bacteria

Immune Response to Fungi [6]

• The innate immunity to fungi includes defensins and phagocytes
•  CD4+ T helper cells are responsible for the adaptive immune response against fungi
• Dendritic cells secrete IL-12 after ingesting fungi, and IL-12 activates the synthesis of gamma interferon, which activates the cell-mediated immunity

Immune Response to Viruses [7]

• Interferon, NK cells, and phagocytes prevent the spread of viruses in the early stage
• Specific antibodies and complement proteins participate in viral neutralization and can limit the spread and reinfection
• The adaptive immunity is of foremost importance in the protection against viruses - these include CD8+ T cells that kill them and CD4+ T cells as the dominant effector cell population in response to many virus infections

Immune response to parasites[8]:

• Parasitic infection stimulates various mechanisms of immunity due to their complex life cycle
• Both CD4+ and CD8+ Cells protect against parasites
• Macrophages, eosinophils, neutrophils, and platelets can kill protozoa and worms by releasing reactive oxygen radicals and nitric oxide
• Increased eosinophil number and the stimulation of IgE by Th-2 CD4+ T cells are necessary for the killing of intestinal worms
• Inflammatory responses also combat parasitic infections

Despite Immune response(s) generated by intact and functional Immune systems, we still fall sick, often due to evasive mechanisms employed by these microbes. Here are some of those. 

Strategies of Viruses to Evade the Immune System

Antigenic variation is a mutation in proteins typically recognized by antibodies and lymphocytes. HIV continually mutates, making it difficult for the immune system to protect against it and hindering the development of a vaccine.

 Virches disrupt the interferon response by disrupting 2',5'-oligoadenylate synthetase activity or by producing soluble interferon receptors.

By several mechanisms, Viruses affect the expression of MHC molecules.   

A virus can infect immune cells: Normal T and B cells are also sites of virus persistence. HIV hides in CD4+T cells, and EBV in B cells.

Strategies of Bacteria to Evade the Immune System

Intracellular pathogens may hide in cells: Bacteria can live inside metabolically damaged host leukocytes and escape from phagolysosomes (Shigella spp).

Other mechanisms:  

• Production of toxins that inhibit the phagocytosis
• They prevent killing by encapsulation
• The release of catalase inactivates hydrogen peroxide
• They infect cells and then cause impaired antigenic presentation
• The organism may kill the phagocyte by apoptosis or necrosis

Strategies of Fungi to Evade the Immune System

• Fungi produce a polysaccharide capsule, which inhibits the process of phagocytosis and overcoming opsonization, complement, and antibodies
• Some fungi inhibit the activities of host T cells from delaying cell-mediated killing
• Other organisms (e.g., Histoplasma capsulatum) evade macrophage killing by entering the cells via CR3 and them escaping from phagosome formation

Strategies of Parasites to Evade the Immune System  

• Parasites can resist destruction by complement
• Intracellular parasites can avoid being killed by lysosomal enzymes and oxygen metabolites
• Parasites disguise themselves as a protection mechanism
• Antigenic variation (e.g., African trypanosome) is an essential mechanism to evade the immune system
• Parasites release molecules that interfere with the immune system's normal function

Mechanism

The most important mechanisms of the immune system by which it generates immune response include:

Macrophages produce lysosomal enzymes and reactive oxygen species to eliminate the ingested pathogens. These cells produce cytokines that attract other leukocytes to the site of infection to protect the body. The innate response to viruses includes synthesizing and releasing interferons and activating natural killer cells that recognize and destroy the virus-infected cells. The innate immunity against bacteria consists of activating neutrophils that ingest pathogens and moving monocytes to the inflamed tissue, where they become macrophages. They can engulf and process the antigen and then present it to a group of specialized cells that have acquired the immune response. Eosinophils protect against parasitic infections by releasing the content of their granules.[9][10]

Antibody-dependent cell-mediated cytotoxicity (ADCC) is a cytotoxic reaction in which Fc-receptor-expressing killer cells recognize target cells via specific antibodies.

Affinity maturation: The increase in average antibody affinity is mostly seen during a secondary immune response.

Complement system: It is a molecular cascade of serum proteins involved in controlling inflammation, lytic attack on cell membranes, and activation of phagocytes. The system can undergo activation by interaction with IgG or IgM (classical pathway) or by involving factors B, D, H, P, I, and C3, which interact closely with an activator surface to generate an alternative pathway, C3 convertase. 

Anergy: It is the failure to induce an immune response following stimulation with a potential immunogen.

Antigen processing: Conversion of an antigen into a form that can be recognized by lymphocytes. It is the initial stimulus for the generation of an immune response.

Antigen presentation: It is a process in which specific cells of the immune system express antigenic peptides in their cell membrane along with alleles of the major histocompatibility complex (MHC) which is recognizable by lymphocytes.

Apoptosis: Programmed cell death involving nuclear fragmentation and the formation of apoptotic bodies.

Chemotaxis: Migration of cells in response to concentration gradients of chemotactic factors.

Hypersensitivity reaction: A robust immune response that causes tissue damage more considerable than the one caused by the antigen or pathogen that induced the response. For instance, allergic bronchial asthma and systemic lupus erythematosus are examples of type I and type III hypersensitivity reactions, respectively.

Inflammation: Certain reactions that attract cells and molecules of the immune system to the site of infection or damage. It featured increased blood supply, vascular permeability, and transendothelial migration of blood cells (leukocytes).

Opsonization: A process of facilitated phagocytosis by depositing opsonins (IgG and C3b) on the antigen.

Phagocytosis: The process by which cells (e.g., macrophages and dendritic cells) take up or engulf an antigenic material or microbe and enclose it within a phagosome in the cytoplasm.

Immunological tolerance: A state of specific immunological unresponsiveness.

Hypersensitivity Reactions

They are overreactive immune responses to antigens that would not normally cause an immune reaction.

Type 1 hypersensitivity reactions: Initial exposure to the antigen stimulates Th2 cells. They release IL-4, leading the B cells to switch their production of IgM to IgE antibodies, which are antigen-specific. The IgE antibodies bind to mast cells and basophils, sensitizing them to the antigen.

When the body is exposed to the allergen again, it cross-links the IgE bound to the sensitized mast cells and basophils, causing the degranulation and release of preformed mediators, including histamine, leukotrienes, and prostaglandins. This causes systemic vasodilation, bronchoconstriction, and increased permeability of vascular endothelium.

The reaction can be divided into two stages – 1) Immediate, in which the release of preformed mediators causes the immune response, and 2) Late-phase response 8-12 hours later, in which the cytokines released in the immediate stage stimulate basophils, eosinophils, and neutrophils even though the allergen is removed.

Type 2 hypersensitivity reactions (Antibody-dependent cytotoxic hypersensitivity): Immune response against the antigens on the cell surface. Antibodies binding to the cell surface, activate the complement system and cause the degranulation of neutrophils and cell destruction. Such reactions can be targeted at self or non-self antigens. ABO blood group incompatibility leading to acute hemolytic transfusion reactions is an example of Type 2 hypersensitivity.

Type 3 hypersensitivity reactions are also mediated by circulating antigen-antibody complex that may be deposited in and damage tissues. Antigens in type 3 relations are soluble instead of cell-bound antigens in type 2. 

Type 4 hypersensitivity reactions (delayed-type hypersensitivity reactions): They are mediated by antigen-specific activated T-cells. When the antigen enters the body, it is processed by antigen-presenting cells and presented with the MHC II to a Th1 cell. Suppose the T-helper cell has already been sensitized to that particular antigen. In that case, it will be stimulated to release chemokines to recruit macrophages and cytokines such as interferon-γ to activate them. This causes local tissue damage. The reaction takes longer than all other types, around 24 to 72 hours. 

Transplant Rejection

• Xenografts are grafts between members of different species that trigger the maximal immune response. Rapid rejection.
• Allografts are grafts between members of the same species.
• Autografts are grafts from one part of the body to another. No rejection.
• Isografts are grafts between genetically identical individuals. No rejection.

Hyperacute Rejection: In hyperacute rejection, the transplanted tissue is rejected within minutes to hours because vascularization is rapidly destroyed. Hyperacute rejection is antibody-mediated and occurs because the recipient has preexisting antibodies against the graft, possibly due to prior blood transfusions, multiple pregnancies, prior transplantation, or xenografts. Activation of the complement system leads to thrombosis in the vessels, preventing the vascularization of the graft. 

Acute Rejection: Develops within weeks to months. Involves the activation of T lymphocytes against donor MHCs. May also involve humoral immune response, which antibodies developing after transplant. It manifests as vasculitis of graft vessels with dense interstitial lymphocytic infiltrate. 

Chronic Rejection: Chronic rejection develops months to years after acute rejection episodes have subsided. Chronic rejections are both antibody- and cell-mediated. The use of immunosuppressive drugs and tissue-typing methods has increased the survival of allografts in the first year, but chronic rejection is not prevented in most cases. It generally presents as fibrosis and scarring. In heart transplants, chronic rejection manifests as accelerated atherosclerosis. In transplanted lungs, it manifests as bronchiolitis obliterans. In liver transplants, it manifests as vanishing bile duct syndrome. In kidney recipients, it manifests as fibrosis and glomerulopathy. 

Graft-versus-host Disease: The onset of the disorder varies. Grafted immunocompetent T cells proliferate in the immunocompromised host and reject host cells which they consider 'nonself' leading to severe organ dysfunction. It is a type 4 hypersensitivity reaction and manifests as maculopapular rash, jaundice, diarrhea, hepatosplenomegaly. Usually occurs in the bone marrow and liver transplants, which are rich in lymphocytes. 

Related Testing

The immunological investigations for the study of innate and adaptive immunity are listed below and include the assessment of immunoglobulins, B and T-lymphocyte counts, lymphocyte stimulation assays, quantification of complement system components, and phagocytic activity.[11][12][13][14][15]

Quantitative Serum Immunoglobulins

• IgG
• IgM
• IgA
• IgE

IgG Sub-Classes

• IgG1
• IgG2
• IgG3
• IgG4

Antibody Activity 

IgG antibodies (post-immunization)

• Tetanus toxoid
• Diphtheria toxoid
• Pneumococcal polysaccharide
• Polio

IgG antibodies (post-exposure)

• Rubella
• Measles
• Varicella Zoster

Detection of isohemagglutinins (IgM)

• Anti-type A blood
• Anti-type B blood

Other assays

• Test for heterophile antibody
• Anti-streptolysin O titer
• Immunodiagnosis of infectious diseases (HIV, hepatitis B and C, HTLV, and dengue)
• Serum protein electrophoresis

Blood Lymphocyte Subpopulations

• Total lymphocyte count
• T lymphocytes (CD3, CD4, and CD8)
• B lymphocytes (CD19 and CD20)
• CD4/CD8 ratio

Lymphocyte Stimulation Assays

• Phorbol ester and ionophore
• Phytohemagglutinin
• Antiserum to CD3

Phagocytic Function  

Nitroblue tetrazolium (NBT) test (before and after stimulation with endotoxin)

• Unstimulated
• Stimulated

Neutrophil mobility

• In medium alone
• In the presence of chemoattractant

Complement System Evaluation

Measurement of individual components by immunoprecipitation tests, ELISA, or Western blotting

• C3 serum levels
• C4 serum levels
• Factor B serum levels 
• C1 inhibitor serum levels

Hemolytic assays

• CH50
• CH100
• AH50

Complement system functional studies

• Classical pathway assay (using IgM on a microtiter plate)
• Alternative pathway assay (using LPS on a microtiter plate)
• Mannose pathway assay (using mannose on a microtiter plate)

Measurement of complement-activating agents

• Circulating immune complexes
• Cold agglutinins 

Assays for complement-binding

• C1q autoantibody ELISA
• C1 inhibitor autoantibody ELISA

Others complement assays

• LPS activation assay
• Specific properdin test
• C1 inhibitor activity test

Autoimmunity Studies

• Anti-nuclear antibodies (ANA)
• Detection of specific auto-immune antibodies for systemic disorders (anti-ds DNA, rheumatoid factor, anti-histones, anti-Smith, anti-(SS-A) and anti-(SS-B)
• Detection of anti-RBC, antiplatelet, and anti-neutrophil
• Testing for organ-specific auto-immune antibodies

Microbiological Studies

• Blood (bacterial culture, HIV by PCR, HTLV testing)
• Urine (testing for cytomegalovirus, sepsis, and proteinuria)
• Nasopharyngeal swab (testing for Rhinovirus)
• Stool (testing for viral, bacterial, or parasitic infection)
• Sputum (bacterial culture and pneumocystis PCR)
• Cerebrospinal fluid (culture, chemistry, and histopathology)

Coagulation Tests 

• Factor V assay
• Fibrinogen level
• Prothrombin time
• Thrombin time
• Bleeding time

Other Investigations  

• Complete blood cell count    
• Tuberculin test
• Bone marrow biopsy
• Histopathological studies
• Liver function test
• Blood chemistry
• Tumoral markers
• Serum levels of cytokines
• Chest X-ray
• Diagnostic ultrasound
• CT scan
• Fluorescent in situ hybridization (FISH)
• DNA testing (for most congenital disorders)

Pathophysiology

The immune system protects the body against many diseases, including recurrent infections, allergies, tumors, and autoimmunity. The consequences of an altered immunity will manifest in the development of many immunological disorders, some of which are listed below:

• X- X-linked agammaglobulinemia (Bruton disease)
• Selective IgA Deficiency
• Selective IgG deficiency
• Congenital thymic aplasia (DiGeorge Syndrome)
• Chronic mucocutaneous candidiasis
• Hyper-IgM syndrome
• Interleukin-12 receptor deficiency
• Severe combined immunodeficiency disease (SCID)
• ZAP-70 deficiency
• Janus kinase 3 deficiency
• RAG1 and RAG2 deficiency
• Wiskott-Aldrich syndrome
• Immunodeficiency with ataxia-telangiectasia
• MHC deficiency (bare leukocyte syndrome)
• Complement system deficiencies 
• Hereditary angioedema
• Chronic granulomatous disease (CGD)
• Leukocyte adhesion deficiency syndrome
• Job syndrome
• Chediak Higashi syndrome
• Acquired immunodeficiency syndrome
• Anaphylaxis 
• Allergic bronchial asthma  
• Allergic rhinitis
• Allergic conjunctivitis
• Food allergy
• Atopic eczema 
• Drug allergy 
• Immune thrombocytopenia 
• Autoimmune hemolytic anemia  
• Autoimmune neutropenia 
• Systemic lupus erythematosus  
• Rheumatoid arthritis 
• Autoimmune hepatitis  
• Hemolytic disease of the fetus and the newborn (erythroblastosis fetalis) 
• Myasthenia gravis 
• Goodpasture syndrome 
• Pemphigus 
• Tuberculosis 
• Contact dermatitis 
• Leprosy 
• Insulin-dependent diabetes mellitus 
• Schistosomiasis 
• Sarcoidosis 
• Crohn disease
• Autoimmune lymphoproliferative syndrome
• X-linked lymphoproliferative disorder
• Common variable immunodeficiency
• B-cell chronic lymphocytic leukemia
• B-cell prolymphocytic leukemia
• Non-Hodgkin lymphoma (including mantle cell lymphoma) in leukemic phase
• Hairy cell leukemia
• Multiple myeloma
• Splenic lymphoma with villous lymphocytes
• Sezary syndrome
• T-cell prolymphocytic leukemia
• Adult T-cell leukemia-lymphoma
• Large granulated lymphocyte leukemia
• Leukocyte adhesion deficiency syndrome      
• Chronic active hepatitis
• Coccidioidomycosis
• Behcet disease
• Aphthous stomatitis
• Familial keratoacanthoma
• Autoimmune polyendocrinopathy candidiasis ectodermal dystrophy   
• Idiopathic CD4+ lymphocytopenia
• Complement system deficiencies
• ADA-SCID  
• Artemis SCID
• Newly diagnosed non-germinal center B-cell subtype of diffuse large B-cell lymphoma
• Melanoma
• Chagas disease

Clinical Significance

Highly specific and discriminatory immunity is of utmost importance for survival. The immune system has evolved as a collection of protective mechanisms to defend the host against a long list of potential invaders that would take advantage in immunodeficiency disorders, inflammatory diseases, cancers, and autoimmunity. This system has to be sophisticated enough to recognize "self" from "non-self" and provide help in infections, malignant tumors, organ transplantations, and various other situations the immune system encounters.