Acute respiratory infections kill approximately 3.5 million people each year.

The infection of a virally-infected cell prevents the virus from utilising that cell for replication.

Innervation of the penis comes from the pudendal nerve, which is made up of the 2nd, 3rd and 4th sacral nerves. The penile nerve (also known as the dorsal nerve of the penis) is the deepest branch of the pudendal nerve.

There are receptors for C3b on the surface of phagocytic cell.

The superantigens combine with the framework regions of certain TCR V beta families irrespective of their complementarity determining regions.

Capsules do not adhere readily to phagocytic cells and may cover carbohydrate molecules, which could be recognised by phagocyte receptors.

Antibody may block the binding to a target cell and would increase the rate of removal of a toxin from circulation by immune complex formation.

The membrane attack complex containing C8 and C9 is important for destroying the Neisseria bacterium.

T-cells are important for immunity to M. tuberculosis through the provision of Th1 cytokines,  such as IFN-gamma to activate macrophages containing the bacilli and perhaps also the participation of cytotoxic T-cells. The latter may kill macrophages,  which cannot be rescued by cytokines thereby releasing the bacteria to be taken up by fresh young macrophages. They can be activated to become killers.

The ability to interchange large segments of the genome with a viral reservoir in other animal species is the basis of antigenic shift leading to radical change in antigen, particularly crucial antigens giving protective immunity such as the viral hemagglutinin.

Poor T-cell responses specific for the leprosy bacillus, prevent the host from mounting a killing response and the bacilli grow profusely.

Interferon inhibits the intracellular replication of viruses.

It thereby prevents effective presentation of antigen peptides to the T-cell receptor.

Primary haemostasis initially involves vasoconstriction in response to vascular damage, which limits blood loss, and subsequent platelet adhesion to the damaged area and activation due to exposure of subendothelial matrix.

The internal pudendal arteries serve as the penile blood supply by entering the penis dorsally and running deep into the tissues of the corpora cavernosa. The internal pudendal artery becomes the penile artery.