Also called antibodies, Immunoglobulins (Ig) are the soluble form of B cell receptors (BCR) released by plasma cells after they have been activated. Immunoglobulins have to bind to a number of different antigens in a variety of environments and as such there are several different immunoglobulin classes. Each class has an optimum environment of action.

The term 'implantation' is often used to describe the attachment of the placental membranes to the endometrium in most animals. True implantation is a phenomenon in rodents and humans in which the conceptus 'buries' itself in the uterine endometrium. The conceptus temporarily disappears beneath the surface. In most other species, the conceptus does not truly implant, but attaches to the endometrial surface and remains in the luminal compartment.

In many epidemiological studies, it is not possible to include every individual in a population. Rather, a sample of individuals is collected. This may be take the form of a survey, a cross-sectional study, a randomised controlled trial, and so on. The important issue is that not every individual in the source population is included, which means that random, or sampling, error and biases may be introduced. These affect our ability to extrapolate our results (whether descriptive or analytic in nature) to the source population. However, the aim of most studies is to draw some conclusion about the source population, using the results obtained from the sample. This requires the use of statistical methodology in a process known as inferential statistical analysis, and is commonly used in epidemiological investigations.

Both the innate and adaptive immune systems use receptors to recognise foreign organisms. The innate immune system uses pattern recognition receptors which acts as an early warning system. The adaptive immune response is highly specific for each organism, as B and T cells have specialist surface immunoglobulin receptors which detect specific antigens on foreign pathogens. The Innate immune system is the body's first barrier of defence to infection. It relies on an older, more generic, and faster acting set of tools than the adaptive system. While the adaptive system is essential for a specific response to infection, it is ultimately the innate system that conquers foreign attackers through means of phagocytosis.

The simplest way to avoid infection is to prevent microorganisms gaining access to the body. The skin has an external coating of dead cells (cuticle) that, when intact, is impermeable to most infectious agents as very few pathogens are capable of penetrating the thick stratified squamous epithelium of the skin (and lower urinary tract).

Pathogens can invade the body if a breach occurs in the barriers formed by the skin and mucus membranes, for example a wound, they must be detected and destroyed by cellular and humoral means. The cells involved in the cellular response to a wound are mast cells, macrophages, granulocytes, and monocytes.

The innate response to bacterial infection lies in its first-response role of detection of a foreign organism. By using the tools of Pattern-Recognition Receptors (PRRs), the innate response flags up problems while the adaptive response gets itself organized.

Because viruses invade host cells to take over a host's cellular machinery, the innate system has a more difficult time detecting viruses as foreign agents. However, there is a give-away element of the viral attack that the innate system can recognize: the double-stranded RNA (dsRNA) produced by a virus in its replication phase. Because mammalian cells only ever produce single-stranded RNA, the presence of dsRNA signals a foreign intruder. dsRNA can be detected by TLR-3R on the cell surface or intracellularly by the presence of dsRNA-dependent protein kinase.

The integumentary system is an organ system that forms the protective covering of an animal and comprises the skin (including glands and their products), haircoat or feathers, scales, nails, hooves and horns. The integumentary system has a variety of functions; in animals, it serves to waterproof, cushion and protect the deeper tissues, excrete waste, regulate temperature and is the location of sensory receptors for pain, pressure and temperature. Generally mammalian skin is covered with hair and is termed hirsute skin. Where hair is absent, it is termed glabrous skin.

The intermediate mesoderm exists as a strip of tissue between the lateral plate mesoderm and somites. It gives rise to the urinary system and some parts of the reproductive system. Kidney development includes three forms: Pronephros, Mesonephros, and Metanephros. Mammals develop all three, and continue to use the metanephros in adult life. More primitive animals have only the first one or two.

Sexual differentiaton by default follows a pathways for development of female internal and external genitalia, requiring no active intervention. Endocrine activity of the testes, production of Androgens by Leydig cells and Mullerian Inhibiting hormone (MIH) by Sertoli cells, is required for sexual differentiation to be diverted down the male genitalia development pathway.

The jejunum continues from the duodenum and leads into the ileum. It is the longest part of the small intestine and is highly coiled. It has digestive and absorptive functions.

The kidneys are formed from the intermediate mesoderm. Firstly it begins to thicken longitudinally in its caudal domain. This is called the nephrogenic thickening. From this thickening three so called attempts at producing excretory origins arise. All the attempts have at the histological level a series of excretory tubules however only one of these attempts forms the kidney in adult life.

The large intestine extends from the ileum of the small intestine to the anus. Water, electrolytes and nutrients are absorbed which concentrates the ingesta into faeces. Faeces are stored prior to defeacation. There is no secretion of enzymes and any digestion that takes place is carried out by microbes. All species have a large microbial population living in the large intestine, which is of particular importance to the hindgut fermenters. For this reason, hindgut fermenters have a more complex large intestine with highly specialised regions for fermentation.

The larynx is situated below where the pharynx divides into the trachea and the oesophagus. It is contained partly within the rami of the mandible and extends caudally into the neck. Vocal folds and vestibular folds are present in the larynx and due to this, it is more commonly known as the voice box.

Leukopoiesis is the process of formation of leukocytes (white blood cells) from stem cells in haematopoietic organs. Leukocytes develop from either multipotential myeloid stem cells (CFU-GEMM) or multipotential lymphoid stem cells (CFU-L).

The limbs develop from the lateral plate mesoderm. Limb development is highly conserved; in all land vertebrates there are only four limbs and they are always opposite each other with respect to the midline of the body. All vertebrate limbs have the same patterning of; stylopod - proximal part of the limb which produces the humerus or femur; zeugopod - intermediate part of the limb which produces the radius and ulna or tibia and fibula; autopod - distal part of the limb that produces the carpals and metacarpals or tarsals and metatarsals. Other animals also follow this limb pattern including the greatly modified bird's wing.

The liver (hepar) is an extremely important organ in the body of mammals and vertebrates as it provides functions essential for life. It is the largest internal organ and has numerous functions including production of bile and protein, fat and carbohydrate metabolism. During foetal development, the liver has an important haemopoetic function, producing red and white blood cells from tissue between the hepatic cells and vessel walls.