Chapter 37, Problem 18A

Summary Introduction

Introduction: 

When pathogens get past the non-specific defense mechanisms, the second line of defense, specific immune response, attack the invading pathogens. The lymphatic system filters the lymph and removes the foreign particles. The lymphocytes present in the lymphatic system act on these invaders and eliminate them from the body. 

Answer to Problem 18A

Correct answer: 

Option (C) a B cell.

Explanation of Solution

Explanation/justification for the correct answer:

Option (C) a B cell. The diagram is depicting antibody-mediated response. When a macrophage comes in contact with an antigen or a foreign particle, it engulfs it through phagocytosis. A portion of the antigen is held outside of the cell by a receptor. The antigen is then presented to the helper T cells by binding of macrophage to the receptor present on the T cell. This binding helps in the division of T cells. The helper T cells get activated and present the antigen to the B cells. B cells divide, and some of the daughter cells produce antibodies, while some of them become memory B cells.

The explanation for the incorrect answer: 

Option (A) a pathogen. The helper T cell presents the antigen to the B cells. The pathogen produces antigens that are detected by macrophages. Hence, it is an incorrect answer.

Option (B) bone-marrow. Bone marrow is responsible for only the production of lymphocytes that further mature in the B and T cell. It plays no role in presenting the antigen during a specific immune response. Hence, it is an incorrect answer.

Option (D) the thymus gland. The thymus gland is the site of T cell maturation. It does not play any role in presenting the antigen during a specific immune response. Hence, it is an incorrect answer.