Immunization is a successful use of immunotherapy to treat many infectious diseases by stimulating the immune system to produce specific antibodies or specific lymphocytes to fight off pathogens and more recent to protect against malignant tumors.  This immunotherapy creates an immunological memory that can be long-lasting. The current immunizations protect against diphtheria, tetanus, pertussis, poliomyelitis, measles, mumps, rubella, pneumococcal pneumonia, smallpox, sepsis, meningitis, hepatitis B, varicella-zoster, tuberculosis, cholera, diarrhea caused by rotavirus, salmonellosis, and dengue. However, the development of vaccine technology in recent years, the emergence of HIV, SARS, avian influenza, Ebola, and Zika emphasizes the need for global preparedness for a pandemic.[1]

Live vaccines are most effective than killed vaccines because they retain more antigens of the microbes. However, toxoids, including those that cause tetanus and diphtheria, are the most effective bacterial vaccines of all because it bases on inactivated exotoxins that stimulate strong antibody production. Subunit vaccines, including hepatitis B, meningococcal, and Hemophilus influenzae B vaccines are effective when conjugated to carrier proteins such as tetanus toxoid. Vaccinologists produce subunit vaccines either by recombinant DNA technology or by antigen purification from different bacterial strains.[2]

Vaccines contain one or various immunogens (peptides), which antigen-presenting cells can engulf, process, and present along with MCH antigens to CD4+ T cells. These lymphocytes can synthesize cytokines that activate humoral and cellular responses, including antibody production, activation of CD8+ T cells, macrophage stimulation, and other functions.[3] Memory cells can develop in this process. They can proliferate more quickly in further encounters with the antigen. 

B cells can recognize vaccines made of carbohydrates and other compounds except for proteins. Subsequently, B lymphocytes can differentiate into plasma cells that produce specific antibodies to protect against infectious diseases caused by bacteria (including meningitis caused by N. meningitidis and pneumonia caused by S. pneumoniae). This immune response against a non-peptidic antigen does not involve T-cell presentation, class switching, affinity maturation, or generation of memory T cells.[4]

Using adjuvants enhances antibody synthesis and T-cell responses.[5] Certain compounds, including aluminum salts added to immunogens, stimulate immune responses. This effect can mediate by two essential functions: cytokine induction that regulate T and B cell functions and increased antigen presentation in sites where lymphocytes can concentrate. Many bacterial substances can activate pattern recognition receptors[6] that activate cytokine production by antigen-presenting cells. 

Immunological studies for testing the humoral and cellular immunity after immunizing a host: 

Quantitative Serum Immunoglobulins

• IgG
• IgM
• IgA
• IgE

IgG Sub-Classes

• IgG1
• IgG2
• IgG3
• IgG4

Antibody Activity 

IgG antibodies (post-immunization)[7]

• Tetanus toxoid
• Diphtheria toxoid
• Pneumococcal polysaccharide
• Polio

IgG antibodies (post-exposure)

• Rubella
• Measles
• Varicella-zoster

Blood lymphocyte subpopulations

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

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)

Most human vaccines are administered by injection, although this approach is risky in the developing world where the use of injections can transmit diseases such as HIV infections.[8] Live vaccines can be given orally but not killed vaccines. Alternatively, the use of the oral route and other mucosal surfaces have been explored as immunization route. For example, polio vaccination underwent a successful implementation via the oral route.

Attenuated vaccines have several potential safety issues, including:

• Hypersensitivity to viral antigens (measles)
• Hypersensitivity to egg antigens (mumps)
• Persistent infection (varicella-zoster)
• In an immunodeficient patient, it may cause severe disease (BCG)

Killed vaccine safety issues include:

• Yeast contaminant (hepatitis B)
• Contamination with animal viruses (polio)
• Endotoxin contamination (pertussis)

All vaccines have as contraindications severe allergic reactions (e.g., anaphylactic reaction) after a previous dose or to a vaccine component. DTaP should contraindicate if the child develops encephalopathy within seven days of administration of a prior dose of DTP or DTaP and after ruled out other causes of brain illness. Hepatitis B vaccine contraindicates in patients with hypersensitivity to yeast. Hib vaccine is contraindicated in infants aged less than six weeks.

MMR vaccine is avoided in those with a known severe immunodeficiency due to lymphoid malignancies, congenital cause, chemotherapy, family history of immunosuppression, and in patients with HIV/AIDS. Rotavirus vaccine must contraindicate in children with a history of intussusception, and it should use with precaution in altered immunocompetence, other than severe combined-immunodeficiency disorder. Both varicella and zoster vaccines contraindicate in immunocompromised host and pregnancy. Live-attenuated influenza virus vaccine should be avoided when in the previous 48 hours, a patient has taken influenza antiviral medication; dosing should proceed with caution in patients who developed Guillain-Barre syndrome within six weeks after a prior dose of influenza vaccine and in patients who have asthma.