Antibodies and Vaccination

This document explains how vaccination programmes help control the spread of infectious diseases. It covers the principles of the immune system, the role of antibodies, and the mechanisms by which vaccines provide protection.

The Immune System: An Overview

The human immune system is a complex network of cells, tissues, and organs that defend the body against harmful invaders, such as bacteria, viruses, fungi, and parasites. It has two main branches: the innate immune system and the adaptive immune system.

Innate Immunity

This is the body's first line of defense, providing a rapid, non-specific response to pathogens. It includes physical barriers (e.g., skin, mucous membranes), chemical barriers (e.g., enzymes in tears and saliva), and cellular components (e.g., phagocytes, natural killer cells).

Adaptive Immunity

This is a slower, more specific response that develops over time. It involves lymphocytes (white blood cells) – B cells and T cells – which recognize and target specific pathogens. The adaptive immune system has two main arms: humoral immunity and cell-mediated immunity.

Antibodies: The Key Players

Antibodies (also known as immunoglobulins) are Y-shaped proteins produced by plasma cells, which are differentiated B lymphocytes. They play a crucial role in humoral immunity. Antibodies are highly specific, meaning each antibody is designed to bind to a particular antigen – a molecule on the surface of a pathogen.

The binding of an antibody to its antigen can have several effects:

• Neutralisation: Antibodies can bind to pathogens and block them from infecting host cells.
• Opsonisation: Antibodies can coat pathogens, making them more easily recognised and engulfed by phagocytes.
• Complement Activation: Antibodies can trigger the complement system, a cascade of proteins that can directly kill pathogens or enhance inflammation.

How Vaccines Work

Vaccines work by introducing a weakened or inactive form of a pathogen (or a part of it) into the body. This triggers an immune response without causing the disease.

When a vaccine is administered, the adaptive immune system recognizes the vaccine antigen as foreign. This activates B lymphocytes to produce antibodies specific to that antigen. Memory B cells are also created, which "remember" the antigen and can quickly produce antibodies if the body encounters the live pathogen in the future.

There are different types of vaccines:

• Live-attenuated vaccines: These contain weakened versions of the pathogen. They produce a strong immune response but are not suitable for people with weakened immune systems.
• Inactivated vaccines: These contain killed pathogens. They are safer than live-attenuated vaccines but may not produce as strong an immune response.
• Subunit, recombinant, polysaccharide, and conjugate vaccines: These contain only specific parts of the pathogen, such as proteins or polysaccharides. They are very safe and effective.
• Toxoid vaccines: These contain inactivated toxins produced by the pathogen.
• mRNA vaccines: These contain genetic material (mRNA) that instructs host cells to produce viral proteins, triggering an immune response.

Vaccination Programmes and Disease Control

Vaccination programmes are a cornerstone of public health and have been instrumental in controlling and eradicating many infectious diseases. By vaccinating a large proportion of the population, we can achieve herd immunity.

Herd Immunity

Herd immunity occurs when a sufficiently high percentage of the population is immune to a disease, either through vaccination or previous infection. This makes it difficult for the disease to spread, protecting those who are not immune (e.g., infants, immunocompromised individuals).

The required percentage of the population needed for herd immunity varies depending on the disease's infectiousness. Highly contagious diseases like measles require a very high vaccination coverage (around 95%) to achieve herd immunity.

Table: Examples of Vaccines and Diseases Controlled

Vaccine	Disease Controlled	Type of Vaccine
Measles, Mumps, Rubella (MMR)	Measles, Mumps, Rubella	Live-attenuated
Polio	Polio	Inactivated/Live-attenuated
Hib (Haemophilus influenzae type b)	Pneumonia, Meningitis caused by Hib	Subunit
Varicella (Chickenpox)	Varicella (Chickenpox)	Live-attenuated
COVID-19	COVID-19	mRNA

Vaccination programmes have dramatically reduced the incidence of many infectious diseases, saving millions of lives worldwide. However, vaccine hesitancy remains a challenge, and it is important to promote evidence-based information about the benefits of vaccination.