Vaccination

When a vaccine is administered, the immune system detects the antigens in the vaccine as foreign. This triggers an immune response, including activation of lymphocytes and production of antibodies.

The mild fever and soreness are signs that the immune system is active — phagocytes and lymphocytes are responding to the vaccine antigens. This is a normal and expected part of immunisation, not a sign of disease.

Vaccine side effects are caused by the immune system's reaction to antigens, not by active pathogen infection. The pathogens in vaccines are killed, weakened, or represented only by proteins — they cannot cause the full disease.

The correct explanation is that the child's immune system is responding to the vaccine antigens, creating a normal inflammatory reaction. This confirms the vaccine is working.

The question asks for the most accurate explanation of mild fever and soreness after vaccination. We need to distinguish between a normal immune response and other possibilities.

The option stating the child has contracted the disease is incorrect. Vaccines contain dead, weakened, or partial antigens — they cannot cause the full disease in a healthy person.

Even live-attenuated vaccines are too weakened to multiply and cause illness in a healthy immune system. Mild soreness and one-day fever do not indicate uncontrolled pathogen replication.

A true vaccine allergy (anaphylaxis) is extremely rare and would involve symptoms like difficulty breathing or hives — not mild soreness and a brief fever, which are universal mild side effects.

The remaining option — that the immune system is responding to vaccine antigens — correctly describes why mild, short-lived symptoms occur after vaccination.

The secondary immune response is characterised by a shorter lag time and higher peak antibody concentration compared to the primary response. This happens because the immune system has encountered the antigen before.

After the primary response, some B-lymphocytes differentiate into memory cells. These long-lived cells persist in the body and can recognise the same antigen if it appears again.

When the same antigen is encountered a second time, memory cells are rapidly activated and quickly differentiate into antibody-producing plasma cells. This bypasses the slow initial activation steps of the primary response.

Memory cells formed during the primary immune response are responsible for the faster, stronger secondary response.

The question asks which cells specifically cause the faster antibody production seen in the secondary immune response.

Phagocytes (like macrophages) present antigens to lymphocytes but do not store them for future rapid antibody production. This is not their role in the secondary response.

Producing brand new B-lymphocytes would be just as slow as the primary response. The whole point is that pre-existing cells accelerate the response — not newly produced naive cells.

T-lymphocytes coordinate the immune response and destroy infected cells, but they do not produce antibodies. Antibody production is the specific role of B-lymphocytes (plasma cells).

Memory cells formed during the primary immune response are the correct answer — they are long-lived B-lymphocytes that rapidly produce antibodies upon re-exposure to the same antigen.