Pathogens (viruses, bacteria) carry antigens

Antigens are recognizable markers that trigger immunity

Barriers and rapid defenses (skin, mucus, inflammation)

Phagocytes (macrophages, neutrophils) engulf invaders

Dendritic cells capture antigens and initiate adaptive immunity

B cells

T cells

Neutralization: block pathogen entry into cells

Opsonization: tag pathogens for destruction

Complement activation: enhance pathogen clearance

Use weakened forms of the pathogen

Strong, long-lasting immune response (antibody + T-cell)

Typically fewer doses needed

Considerations

Use pathogen that cannot replicate

Primarily induces antibody responses

Often needs boosters or adjuvants for durability

Use specific parts of the pathogen (proteins, sugars)

Focused immune response; good safety profile

Often requires multiple doses or boosters

Link polysaccharide antigens to a protein carrier

Improves immune response in infants and young children

Builds stronger immune memory to bacterial capsules

Use inactivated toxins produced by bacteria

Protect against toxin-mediated disease (not necessarily infection itself)

Relies on antibody neutralization of toxins

Use a harmless virus to deliver genetic instructions for an antigen

Strong cellular and antibody responses possible

Non-replicating vs replicating vectors (different immune profiles)

Deliver mRNA instructions to make an antigen protein

Cells produce antigen briefly; immune system learns to recognize it

Strong antibody and T-cell responses; no live virus involved

Deliver DNA encoding antigen (often via special delivery methods)

Cells produce antigen; stimulates adaptive immunity

Common routes: intramuscular, subcutaneous, oral, intranasal

Route influences type and location of immune response (systemic vs mucosal)

Local immune cells detect antigen and danger signals

Inflammation recruits more immune cells

Dendritic cells process antigen and present it on MHC molecules

Migration to lymph nodes to meet naive T cells

Recognize antigen presented by dendritic cells

Provide signals (cytokines) that guide B-cell and CD8+ T-cell responses

B cells bind antigen via B-cell receptor

With T-cell help, B cells undergo:

Formation of plasma cells (antibody factories)

Learn to recognize infected cells presenting antigen

Develop ability to kill infected cells quickly upon real infection

Memory B cells and long-lived plasma cells persist

Memory T cells persist and respond rapidly on re-exposure

Faster, stronger secondary response prevents illness or reduces severity

Establishes initial immunity and immune memory

Restore or amplify waning immunity

Broaden response and improve protection against variants in some cases

Some antigens are less immunogenic

Immunity can decline over time

Different age groups respond differently

Prevents infection entirely (not always achievable)

Prevents severe outcomes (hospitalization, death)

May still allow mild or asymptomatic infection

Lower viral or bacterial load and shorter infectious period in many cases

How contagious the pathogen is (R0)

Vaccine effectiveness

Duration of immunity and population mixing

Soreness, redness, swelling at injection site

Fever, fatigue, aches

Reflect immune activation and inflammation

Monitored through clinical trials and post-licensure surveillance

Risk-benefit assessment guides recommendations

Immune markers that predict real-world protection

Infants: immune system still developing; conjugates often needed

Older adults: immune responses may be weaker; high-dose or adjuvanted vaccines may help

May need specific vaccine types and schedules

Live attenuated vaccines may be contraindicated in some cases

Some vaccines protect both mother and newborn via transferred antibodies

Recommendations depend on vaccine type and timing