Antibodies are part of the adaptive immune response, which means that they are formed later in an infection. Because both recognition sites have the same structure, they are responsive to only one type of antigen. They are located on the outside of B cells, and can be released freely into the plasma by plasma cells.

Placental crossing is done mainly by IgG antibodies. These fast-acting antibodies provide temporary immunity for the fetus until it is able to have its own immune system produce sufficient antibody amounts.

The type of blood a person has not only tells us which antibodies they create, but also which blood type can be given in the event of a transfusion. A person with type A blood has A antigens on his red blood cells. As a result, the person does not make A antibodies; these would bind to the A antigens and initiate an autoimmune response.

Because the person does not make the B antigen, there are B antibodies in the body. This means that the person cannot be given type B blood. B type blood cells would be targeted by the antibodies present in the body, causing an autoimmune reaction against the foreign blood. Since type O blood does not have any surface antigens, this blood type can be given to anybody. No antibodies will react with type O blood.

B cells play numerous integral roles in the immune response against foreign pathogens (viruses, bacteria, and fungi), including forming transient microenvironments called germinal centers, where they produce long-lived plasma cells that are high affinity for specific antigen and memory B cells. They also serve as antigen-presenting cells and producers of cytokines and chemokines; However, B cells are not able to produce extracellular traps, which primarily are composed of DNA and work to trap pathogens. Neutrophils produce extracellular traps.

Naive B cells (and most other immune cell subtypes) need more than one signal to become activated. They normally need B cell receptor signaling (signal 1), costimulation by other receptors (signal 2), and cytokines/chemokines (signal 3). This system is necessary in order to prevent aberrant activation of lymphocytes (safeguard against autoimmunity).  

In regards to the other statements, there are numerous autoreactive B cells at any given time due to the stochastic nature of VDJ recombination and germinal center reactions. Therefore, tolerance mechanisms and checkpoints are incredibly important to keep these cells in check; central and peripheral tolerance are equally important. Self-nuclear reactive B cells and T cells are both necessary and critical in autoimmune pathogenesis. Female sex hormones are definitely believed to contribute greatly to autoimmune disease pathogenesis (e.g. estrogen). Over 75% of autoimmune patients are women.

The oropharynx is capable of accepting both air and food from the mouth and nose. In the laryngopharynx, however, a crucial division occurs. Food, solids, and liquids are passed down the esophagus, while air travels through the trachea to the lungs. During swallowing, the entrance to the trachea, the larynx, must be covered so that food does not go through the trachea and enter the lungs. This is accomplished by the epiglottis, an elastic cartilage structure that covers the larynx during swallowing.

The alveoli are lined with a single layer of squamous epithelial cells, which allow for easy diffusion of vital gases. Basal and apical cells refer to cells located at the bottom and top of structures, respectively. Endothelial cells line the circulatory system and blood vessels. There is no formal class of cells known as "respiratory cells."

The spherical or grape-like shape of the alveoli allows for maximum contact between the alveoli and the capillaries that surround them. The alveoli are filled with air that has been taken into the lungs from the environment, so a high surface area allows for maximum contact between air from the environment and capillaries. Oxygen rapidly diffuses through the exceptionally thin alveolar walls to the capillaries, which carry hemoglobin-containing blood cells that bind to the oxygen and shuttle it around the body. Blood cells also release carbon dioxide into the alveoli and lungs, which is why this process is called gas exchange. 

Capillaries are considerably smaller than alveoli; they surround the alveoli like a mesh, and are certainly not the cause of the alveoli's shape. Furthermore, though the cells of the alveoli do secrete extracellular matrix material, the cells affect the structure of the extracellular matrix, rather than the other way around. The shape of the alveoli is crucial to their function in gas exchange and cannot be considered an "accident," or the unexpected result of the shapes of other biological structures. 

Alveoli are the site of gas exchange in the lungs. Because rapid diffusion of gases is necessary between the capillaries and the alveoli, a very thin epithelial layer is needed. As a result, alveoli use simple squamous epithelium so that gases can easily diffuses to and from the bloodstream.

Alveoli are at the end of the respiratory pathway in humans, and act as a site of gas exchange (carbon dioxide and oxygen).

The path of air through the respiratory tract is: trachea, bronchi, bronchioles, alveoli. It is important to note that no gas exchange takes place in the bronchi, but does in the bronchioles, which are passageways that branch off from the main bronchi and eventually lead to alveolar ducts.

Micorvilli are found int he small intestine and act to increase the surface area in order to increase nutrient absorption. Secretory vesicles are used to transport proteins, hormones, and other molecules from a cell into the extracellular space.