An acquired immune response to a secondary infection by a pathogen results in presentation of antigen to residual memory helper T-cells, memory T-cells, and memory B-cells. Upon recognizing the antigen, memory T-cells can become cytotoxic T-cells and target the infected region. The process of presentation promotes direct expansion of the existing population of immune cells already capable of responding to the pathogen. Immunological memory provides a more rapid response time to combat the pathogen during the second exposure.

Innate defenses, such as skin and macrophages, are a primary defense against all diseases, but adaptive immunity is related to exposure to a specific disease. The ability for the body to produce specific antibodies quickly provides adaptive immunity. While some antibodies may remain in the blood after initial exposure, this small amount does not provide sufficient immunity. 

The specific lymphocytes that produce antibodies during a second exposure are called memory B-cells. When an antigen is presented to a memory B-cell that produces the appropriate antibody, the cell divides and differentiates into plasma cells. Plasma cells are then responsible for producing large amounts of antibodies against the specific antigen. Antibodies are cell-surface markers that attach to pathogens, signaling effector cells to rapidly destroy the pathogen. The rapid multiplication of B-cells to generate antibody to a specific threat is known as clonal selection.

The two types of immune system responses are innate (nonspecific) and adaptive (specific). Innate responses are those that act on many pathogens in the same general way. For example, the skin and the mucus in the nasal cavity both physically block the entry of pathogens into the body, but they do not specifically target certain antigens. Other examples of the innate response are inflammation and the general activity of phagocytes. On the other hand, the adaptive immune system provides a second line of defense against certain, previously encountered pathogens. Here, the only answer choice that deals with the recognition of specific antigens is the production of memory B-lymphocytes. The adaptive immune response generally involves T-lymphocytes, B-lymphocytes, antigens, and antibodies.

The differentiation of many acquired immune cells is largely dependent on helper T-cells. In the presence of a matching antigen, B-lymphocytes can be differentiated into plasma cells and memory B-cells with the assistance of helper T-cells. Memory B-cells are easily triggered if an antigen is presented during a second infection, while plasma cells are the final differentiated form of a B-cell responsible for mass-producing antibodies. Helper T-cells are also important in the activation of cytotoxic T-cells, which detect antibody-antigen complexes on cell membranes and help to destroy these tagged cells.

Acquired, or adaptive, immunity is a second immune defense system and develops slowly after exposure to an initial infection. As a result, the immune system is "trained" and capable of recognizing many specific components or antigens from the pathogen. Acquired immunity stores the information from an initial infection in preparation for reintroduction of the pathogen; it does not immediately respond to the initial infection.

Upon reinfection or exposure to a pathogen, the acquired immunity is better able to detect and defend the body. This includes the generation of antibodies that can bind a pathogen and cytotoxic T-lymphocytes that can detect and eliminate infected cells.

Acquired immunity is balanced by innate immunity, which responds indiscriminately to all pathogens. Innate immunity is essential for fighting and preventing initial infections, before adaptive immunity has learned to recognize the specific pathogen present.

Since the genes in chromosome 6 is codominance, each gene will be expressed. With nondisjunction, there will be an extra chromosome. This will result in all three genes being expressed. The expression of all three genes will increase the diversity of the MHC I pool. Recall MHC I is responsible for notifying which cells are related to the body to prevent autoimmunity.