1. Immunity can be considered to be the result of two different, but related, immune responses, namely the Cell-Mediated Immune Response and the Humoral Immune Response.

The Cell-Mediated Response is controlled by T-Lymphocytes; the Humoral Response is controlled by B-Lymphocytes- white blood cells which are produced in the Thymus Gland and Bone Marrow respectively. T-Lymphocytes do not recognise antigens and thus do not produce antibodies. B-Lymphocytes produce antibodies when foreign antigens are present in the body.

But the T-Lymphocytes do produce Lymphokines and, depending on the type of lymphokine produced which in turn depends on the type of T-Lymphocyte producing it, these may either stimulate or suppress the action of ALL of the cells of the immune system, including both T-Lymphocytes, and B-Lymphocytes.

The B-Lymphocytes then depend on instructions from the T-Lymphocytes- in the form of particular lymphokines- to begin production of antibodies when a foreign antigen is recognised.

The T-Lymphocytes produce lymphokines when they in turn recognise pieces of a foreign body- which may be part of an antigen- but only when presented to them by a type of phagocyte called a Macrophage.

The lymphokines produced may cause multiplication of T-Lymphocytes and inflammation, as well as cause the B-Lymphocytes to produce antibodies. The B-Lymphocytes themselves recognise foreign antigens because specific antibodies form part of their cell surface membrane and allow them to recognise specific antigens which match the receptors. Upon recognition, they bind via the antigen receptors, and the lymphokines produced by the T-Lymphocytes causes them to clone themselves and begin mass-production of antibodies to aid the phagocytes in dealing with the foreigner.

Thus the two immune responses are separate , but they depend on each other, and combine to form the immune response.

2. When a cell has grown to a particular pre-determined size, and is no longer able to efficiently exhibit gaseous exchange at its cell surface membrane because of a small Surface Area:Volume ratio, it divides by Mitosis to form to identical daughter cells.

Mitosis is composed of 5 main stages: Interphase, Prophase, Metaphase, Anaphase, and Telophase.

In Interphase, the cell is still in the process of building up materials for growth and reproduction. This stage is the longest in the cell cycle. The chromosomes are in their long and extended, single-stranded form, and exist as fine threads which can be seen as a mass called chromatin, rather than individually, under the light microscope following staining. During Interphase the chromosomes replicate to form two identical chromatids- joined by a centromere- and the whole structure is known as a chromosome. At this point the centrioles also replicate.

During Prophase the chromosomes become shorter and thicker, so that they can be seen individually under the microscope, and can now each be seen to be composed of two chromatids joined at a centromere. The centrioles begin to migrate towards the poles of the cell, forming spindle fibre between them, and the Nuclear Envelope begins to break down.

During Metaphase, the chromosomes line up at the equator of the cell along the spindle fibres, and at their centromeres. The Nuclear Envelope has now broken down completely by this stage.

The next stage is the shortest stage of Mitosis, Anaphase and during this stage the chromatids are pulled towards opposite poles of the cell, centromere first, by the centrioles which cause the spindle fibres to become shorter to facilitate this.

In Telophase the Nuclear Envelope behinds to reform at the poles, to surround the chromatids. The chromatids themselves become longer and extended forming chromosomes, which once again cannot be seen individually. The two nuclei at opposite poles of the cell contain an identical set of chromosomes; each contains the same number of the same types of chromosomes, and chromosomes of the same type are the same for each nucleus.

Following the division of the nucleus by Mitosis, the cell cytoplasm begins to be divided into two, and the cell becomes invaginated at both side of the cell equator until the opposite sides of the cell surface membrane meet and the cell splits its contents into two daughter cells, by the process of Cytokinesis. Cytokinesis follows division of the nucleus and division of the cytoplasm and its constituents.

Note that Cytokinesis in plants occurs slightly differently, involving the incomplete formation of a dividing cell wall, resulting in the presence of cytoplasmic threads linking daughter cells (plasmodesmata)

Once the cell has divided and two smaller daughter cells are produced, each cell enters the long phase of interphase and begins to build up materials once more for its general metabolism and growth. Each cell thus enters Interphase, and the cell cycle repeats itself.

3. A condom is a sheath which is placed over the man's penis, and prevents the transfer of semen and the exchange of other fluids as the result of sexual intercourse. There is no exchange of fluids because the condom serves as a barrier, and is therefore effective against such Sexually Transmitted Diseases (STDs) as Gonorrhea.

However, Gonorrhea is caused by a bacterium, whereas HIV is a virus. The bacterium is a cell, whereas the virus is TINY as it is simply a strand of RNA, along with enzymes surrounded by a protein coat. The condom thus serves as an effective barrier against bacteria- which are relatively huge, when compared to viruses- but not HIV because there is the slim chance of the virus being able to pass through tiny spaces between the fibres of the condom.

Furthermore, condoms used may be or low quality and this not function as desired, or may not be used properly and so defeat the purpose of using them. In such scenarios any STD could be easily spread, including AIDS, brought about by HIV transission.

Also, condoms are used to prevent HIV infection by sexual intercourse, but to prevent HIV infection on the whole it can never be fully effective because HIV can be transmitted by other means namely blood transfusions, sharing of needles by intravenous drug users, deep kissing, and transmission from mother to foetus.

[Total = 23.5/25; 94%]

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