The first line of defense includes the skin, mucus, sweat, and tears. These defenses are the body’s barriers against threatening factors in the environment. Therefore, A is the correct answer. Red blood cells do not have the ability to fight pathogens.

A. red blood cells

A mast cell triggers the inflammatory response by secreting histamines. Once the histamines are released, the body will suffer from smooth muscle contractions, dilation of blood vessels, an increase of blood flow, and mucus production.

B. Blood vessels near the wound shrink.

Since bacteria and viruses are very sensitive to temperature, raising the body temperature would increase the heart rate. In turn, the white blood cells are carried to the infection at a faster pace. This action will inhibit the replication of bacteria, viruses, and other organisms.

D. It increases the heart rate.

B cells, which are involved in humoral immunity, produce antibodies and they do not kill the viruses directly. Instead, they connect to the antigens on the surface of the virus or bacteria.

C. B cells

Activated B cells develop into plasma cells, which in turn produce antibodies.

C. B cells

An antibody is a special kind of protein that binds to the antigens located on the surface of the virus or bacteria.

A. antibody

Phagocytes are white blood cells that engulf or ingest pathogens and other harmful foreign particles.

E. Phagocytes

The human immunodeficiency virus or HIV is a virus that attacks helper T cells. This is the reason why the T cell concentration in the blood declines as the HIV concentration increases.

B. HIV destroys T cells

C. It steadily increased.

When the pathogen was first introduced into the body, it was recognized by the immune system. In turn, it triggered the body’s defense system or immune response. This is the reason why there was an abrupt decrease in the HIV concentration which was accompanied by an increase in the helper T cell concentration during the first year of the HIV infection.

A. immune response