Vaccines

Results of vaccination

Increased antigen‑specific lymphocytes, heightened sensitivity to activation, higher‑affinity antibodies, isotype switching, and mucosal IgA.

Live attenuated vaccine

Live pathogen with reduced virulence; induces strong humoral B-cells(IgG,IgA) and cellular immunity(T cells).

Examples of live attenuated vaccines

MMR, intranasal influenza, Sabin polio, smallpox, yellow fever.

Advantages of live attenuated vaccines

Strong humoral + cellular immunity, long‑lasting, single dose, herd immunity.

Disadvantages of live attenuated vaccines

Risk of reversion, contraindicated in immunocompromised/pregnant, unstable, rare adverse reactions.

Killed (inactivated) vaccine

Pathogen is inactivated but antigenic structure preserved.

Examples of killed vaccines

Injected influenza, Salk polio.

Advantages of killed vaccines

Safe, no reversion, stable.

Disadvantages of killed vaccines

Primarily humoral response(B cells), weaker, boosters required.

Subunit (acellular) vaccine

Contains purified antigen only (protein or polysaccharide).

Examples of subunit vaccines

Pertussis, hepatitis B surface antigen.

Advantages of subunit(acellular) vaccines

Safe, stable, fewer adverse reactions.

Disadvantages of subunit vaccines

Weaker immunity(Humoral), short‑lived, expensive, often require adjuvants.

Toxoid vaccine

Denatured toxin retaining receptor‑binding site to induce antitoxin antibodies.

Examples of toxoid vaccines

Tetanus, diphtheria.

Advantages of toxoid vaccines

Safe, stable, low adverse reactions.

Disadvantages of toxoid vaccines

Short‑lived, require boosters, may need adjuvants.

Conjugate vaccine

Polysaccharide antigen chemically linked to protein carrier to induce T‑dependent response.

Examples of conjugate vaccines

Hib, pneumococcal conjugate, meningococcal conjugate.

Purpose of conjugate vaccines

Convert T‑independent polysaccharide antigens into T‑dependent forms for infants.

Linked recognition

B cell binds polysaccharide; presents carrier protein peptide to CD4 T cells; T cells help B cells produce anti‑polysaccharide antibodies.

Recombinant vaccines

Vaccines using mRNA or viral/bacterial vectors encoding antigen.

Examples of recombinant vaccines

mRNA COVID‑19 vaccines, adenovirus‑based vaccines.

Targeting vaccines: required immune response

Depends on pathogen: neutralizing antibody (polio), Cytotoxic T cells (HIV), mucosal IgA (respiratory/enteric pathogens).

TLR agonists in vaccines

Promote dendritic cell maturation and shift Th1/Th2 balance.

Adjuvant definition

Substances that enhance immune responses by activating innate immunity and dendritic cells.

Purpose of adjuvants

Increase immunogenicity of weak antigens and promote dendritic cell maturation.

Flagellin as adjuvant

TLR5 agonist that enhances antibody responses and protection.