Red blood cells are made in the bone marrow, and live for about 3-4 months. They are enucleated (lacking a nucleus), which makes more space available for hemoglobin molecules, which function to carry oxygen to the tissues. HIV infects T-helper cells, which are white blood cells, not red blood cells. Thus, the name makes sense since the virus infects cells of the immune system (white blood cells) and causes immunodeficiency. The liver and spleen play roles in recycling the red blood cells once they have carried out their function for about 120 days.

Keratinocytes are not immune cells. Rather, they secrete a protein called keratin which is a large proportion of the extracellular matrix and makes up hair, nails, skin, and other parts of the body. All other cells are immune cells. 

B-cells produce antibodies (a subset of adaptive immunity called humoral immunity). The usual sequence involves B-cell activation via interleukins from helper T-lymphocytes, which cause the B-cell to undergo mitosis, creating numerous clones that will differentiate into plasma cells (rapid antibody producers) or memory B-cells. 

Antibodies (or immunoglobulins) are produced by B-leukocytes and plasma cells, and are a key part of humoral (having to do with bodily fluids) immunity. As such, they can be found in several of the fluids circulating or exiting the body. Some types of immunoglobulins can even cross the placenta or be secreted in breast milk to pass immunity from a mother to her child.

All of the given answer options describe mechanisms for immunoglobulin function.

Neutralization occurs when the antibodies simply cover the biologically active portion of the pathogen, rendering it harmless. Complement fixation refers to the antibodies binding to the pathogen and facilitating the activation of the complement system (a series of plasma proteins that activate other immune processes). Precipitation is when antibodies link the antigens on many pathogens together, creating an insoluble clump ready for removal.

The immune system is very adaptive. The body has many antibodies that will each recognize different antigens. If an antibody binds to an antigen, the antibody will be copied so that the body can quickly recognize the threat if it is exposed to the antigen a second time. This process is known as the adaptive immune response.

When an antigen is presented for a second time, antibodies to the antigen are released. These antibodies bind to the antigen, labelling it for attack by immune cells and preventing it from interacting the membrane proteins on the host cells.

Major histocompatibility complex (MHC) class I molecules are found on virtually all cells in the body. They function in routine immune monitoring through presentation of short peptide fragments derived from degradation of intracellular proteins contained within the cell. The T-cell receptor on cytotoxic T-cells interacts with MHC class I, and if a foreign pathogen or peptide is presented, the cytotoxic T-cell becomes activated to kill infected cells. The same system also functions for detection of potential cancer cells.

Antibodies are proteins created by the immune system in order to neutralize foreign objects. An antibody would not be classified as an enzyme because it does not catalyze chemical reactions. When a foreign pathogen enters the body, it will have foreign receptors on its surface. These foreign receptors are known as antigens. When a pathogen is destroyed, immune cells can carry a sample of the antigen to the T-cells for identification. The T-cells help activate B-cells that will synthesize an antibody against the particular antigen. The selected B-cells differentiate into plasma cells and secrete antibody proteins into the blood, which bind the antigens and label the pathogen as foreign. This label attracts other immune cells to attack and destroy the pathogen.

Antibodies are continuously made in the body in different shapes and forms. They are then sent into the blood stream to test for the presence of compatible antigens. Each antibody can only bind to one antigen, and each antigen can only bind to one antibody. Think of them like a codon-anticodon pair; there is only one possibility for them to form a perfectly complementary pair. Once the correct antibody binds to an antigen, they are tagged and used to stimulate production of more antibodies. The antibodies are only capable of binding and tagging the antigens. Cytotoxic T-cells are then able to recognize antibody binding patterns and actually destroy the infected cell.

According to the passage, after the MHC II molecules are fully synthesized in the endoplasmic reticulum, they are transported to the endosome. Foreign molecules are transported into the cell where they are degraded in the endosome as well. From there, the degraded pathogen interact with the MHC II molecule.