Vaccines

Vaccines are among the most effective tools in preventive medicine, designed to prepare the immune system to recognize and combat infectious agents. By introducing antigens—substances that the immune system identifies as foreign—vaccines stimulate an adaptive immune response that leads to immunological memory. This immunological memory enables the body to mount a faster and more effective response upon future exposures to the actual pathogen.

Vaccines can be categorized based on the type of antigen they deliver. Whole-pathogen vaccines use either live attenuated or inactivated organisms. Live attenuated vaccines, like MMR, contain weakened pathogens that replicate in the host but do not cause illness. They provide lifelong immunity with a single dose. Inactivated vaccines, such as the rabies vaccine, use pathogens killed by heat or chemicals. They cannot replicate but often produce a weaker response, requiring booster shots over time. Subunit vaccines include only specific components of a pathogen—typically proteins or polysaccharides—that are sufficient to elicit an immune response. Modern mRNA vaccines, such as those developed for SARS-CoV-2, deliver synthetic messenger RNA encoding a viral antigen, which is synthesized intracellularly and presented via MHC molecules, triggering an immune response without the need for actual viral particles.

Mechanism of Action

The immunological mechanism of vaccines involves both innate and adaptive immune components. Following administration, antigen-presenting cells (APCs)—primarily dendritic cells—internalize the vaccine antigens. These antigens are processed and presented on the cell surface via major histocompatibility complex (MHC) molecules. MHC class II molecules present extracellular antigens to CD4⁺ helper T cells, while MHC class I molecules present intracellular antigens, such as those generated by mRNA vaccines, to CD8⁺ cytotoxic T cells.

Activated CD4⁺ helper T cells promote the differentiation of B cells into plasma cells, which secrete antigen-specific antibodies. These antibodies can neutralize pathogens, opsonize them for phagocytosis, or activate the complement system. Simultaneously, CD8⁺ T cells eliminate infected cells by inducing apoptosis, particularly important for intracellular pathogens.

A subset of activated B and T cells differentiates into memory cells, which persist long-term in the host. Upon re-exposure to the pathogen, these memory cells rapidly expand and mount a robust immune response, often reducing disease severity or preventing illness. This capacity for immunological memory underpins the protective efficacy of vaccines.