CELL SUSTENANCE #6: Innate barriers and Immunoglobulins in Neonatal development

Embryology is a branch of science which deals with the study of the embryo. The embryo is a baby in development. Embryology studies the step by step development of a baby from the point of fertilization. Embryologist are scientists who are tasked with this study.

The developing baby

The development of an offspring starts with fertilization, this is the fusion of the sex gametes of the two parents, the male gamete is the spermatozoa while the female gamete is the ovum, the spermatozoa penetrates the ovum and fuses with its nucleus, this marks the onset of several developments and differentiations which leads to the formation of a fully developed baby; embryology is a painstaking study in the sense that it carefully studies the chronological development of most, if not all parts of the human body.

^{Image source Pixabay. CC0 creative commons license. Author sbtlneet}

Foetal development takes about thirty-nine (39) weeks but in some cases, this may extend to forty-one (41) weeks in occasion of delayed labour. During this duration, the fertilized egg starts developing, first into a blastocyst and at the fourth week of parturition, an embryo is observed. From this point, the embryo exhibits the five levels of complexity of vertebrates, first of all having an outlook of a reptile and subsequently an amphibian (tadpole) at the fifth week of development, this is am evidence of human evolution.

At the tenth week, the embryo has completed the most critical portion of development and has acquired most physiologic identity of a human, this includes gas exchange apparatuses such as lungs; at the fifteenth (15) week, the embryo has light sensing ability amidst closed eyes and it’s digestive system develops fully at the twentieth (20th) week.

The baby’s nervous system matures at the thirty-fourth (34th) week, and hence delivery at this time poses no health problems, the baby will still be normal, however, normal delivery time is at the thirty-ninth (39th) week. This period of development of the embryo is very critical as an impairment of this process might result in a deformity in the overall appearance and physiology of the baby. Hence care should be taken to regulate the activities if the mother in terms of nutrition and exercise so as to ensure normal development.

Developmental immunology

Immunology is another of science that studies the immune system, how the components of this system interacts with one another to protect the body from infection. Developmental immunology studies the development of these components of the immune system and how the body develops the ability to defend itself from certain antigens. This has an application in foetal development and also in the maturation of the baby’s immunity after birth. During the development of the baby in the womb, the baby acquires innate immunity, the skin and the mucous membranes are developed at birth and the baby is thus able to resist infection from certain immunogens.

The immune system is a system of complex cells, organic and inorganic molecules and organs which acts together to protect the body from infection. This system however, is diverse in the sense that different parts of the system develops at different time in an individual’s lifetime.

This distinguishes the immune system into adaptive and innate immune system. The adaptive immune system develops after birth, this system is however more specialized and and versatile than the innate immune system which is present at the time of birth of an individual.

The innate immune system amidst not being a complex system is very important in the sustenance of the body cells till the development of the more complex adaptive immune system. The innate immune system is a non-specific immune system, this implies that the innate immune system reacts the same way to every antigen or immunogen that enters the body, unlike the adaptive immune system which is structured to identify different forms of antigens;

the adaptive immune system detects antigens rang in from a few thousands over one million antigens and react differently to them, the innate immune system lacks this ability. These short comings of the innate immune system does not in any way limit it’s action in the protection of the body, in fact, it is relatively more efficient than you could imagine!

Barriers of the Innate Immune System (IIS)

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The innate immune system forms barriers through several means to block the entry of antigens into the body, it also has mechanisms of removing antigens should they invade the body.

Anatomic barrier

The first thing you’d notice in a new born baby is his/her skin, this allows you to describe the baby as fair, dark, or an intermediate of these two extremities. The skin however serves a more important purpose than this. The intact skin is a potent barrier to many microorganisms and immunogens, the keratin layer of the skin is so intact that it prevents the diffusion of water into the skin, this same ability enables it to keep the body from the invasion of antigens.

In the cross-section of the skin are noticeable sweat glands, sweat pores and sweat ducts, this sweat system produces sweat, a fluid which contains soluble salts and other wastes of the body’s metabolism, this makes the sweat acidic and antimicrobial. The sebaceous glands which can also be found in the skin produces acids which also has antimicrobial activities and are either bacteriostatic or bactericidal, hence a limitation to the invasion of these microorganisms and other antigens.

The body also consists of hollow organs such as the mouth and other digestive tract, the vessels and the respiratory tract. These hollow organs are lined with mucous membranes which resists the entrance of microbes by trapping these antigens in its gummy secretions. The mucous secreted by these membranes contains lytic enzymes such as lysozymes, these enzymes are bactericidal and are able to kill microbes. The lactobacilus on the mucous membranes of the vagina metabolizes carbohydrates anaerobically, producing lactic acid, thus creating an acidic environment which is adverse to the microbes.

Phagocytic barrier

Macrophages and neutrophils are professional phagocytes; these cells are able to detect antigens and engulf them in a bulk transport movement known as phagocytosis. Phagocytosis involves the delivery of the phagocytes to an area of injury in a process known as diapedesis, these cells travel to the location of the antigens with the aid of chemoattractants.

These are molecules released by the mediators of the immune reaction and the antigen itself, on reaching this location, the phagocytes forms pseudopodia around these antigens, the pseudopodia fuses and the antigen is caged on a membrane. Lysosomes from the cytoplasm of the phagocytes attaches to the membrane which encloses the antigen. These lysosomes releases their secretions which are hydrolytic enzymes such as kisses and proteases which kills the antigens and digests them. The phagocytes performs similar action to every other antigens, hence this reaction is non-specific.

Inflammatory barrier

We’ve described inflammation as a localized non-specific immune reaction in which the body undergoes vascular, metabolic and cellular changes to eliminate the cause of the reaction and restore the tissue architecture and physiology. Inflammation is a primary function of the innate immune system; involving the release of certain proteins and cells which functions as already described to remove the immunogen which initiated the reaction.

Physiologic barrier

The internal body system is maintained in an almost constant body temperature of 37oC, this temperature is unconducive for many microorganisms, hence these microorganisms are unable to survive inside the body, the lower respiratory tract which has similar temperature inhibits the multiplication of viruses which are transmitted via aerosols; hence the virus cannot inhabit this zone; this acts as barrier to these immunogens. Other body proteins such as interferon also interferes with virus' multiplication.

The innate immune system ant it’s processes are already developed at birth and thus are termed the ‘first line of defense ’. However, this innate immune system continues its maturation process after birth and the ability of an individual to react to antigens is dependent on the person’s age and maternal factors, the type of antigen and where it is presented also comes into play as some part of the body has stronger immunity than others, the nobody part with a weaker immunity is known as immunologically privileged sites, this includes the kidney and the brain.

Immunoglobulins in neonatal development

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Neonates are physiologically immunodeficient ,this is because both their adaptive and innate immune systems are not completely matured, hence they react with different strengths against different forms of antigens, this is where the antigen type comes into play. Antigens maybe proteins, glycoproteins and polysaccharides, these antigens has different immunogenicity. Proteins are relatively less immunogenic than glycoproteins and polysaccharides. Polysaccharides are the most immunogenic of the three.

Hence a new born baby can respond well to protein antigens but not to glycoproteins and certainly no to polysaccharides, this is due to the following reasons:

• The B-cells are not fully active even though they are developed during early uterine development of the embryo / foetus.
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  ^{Opsonin. Source credit: wikimedia. CC3.0 license. Author GrahamColm}

  Opsonins are substances which coated the antigens and simplifies the engulfment by phagocytes, hence enhancing the action of the adaptive and innate immune system. Opsonization is the coating of the antigens with these opsonins, this action is very limited in the neonates, hence the phagocytic process is dampened.

• The complement system is also poorly developed as the newborn has a decreased level of complement proteins in the plasma.
• The manicures have a reduced ATP production and hence are slow in action, and the T-cell produces a small amount of cytokines (such as Interleukins 1,2 and 4 and interferon) this limits the capability to activate the humoral immune response and also the phagocytic process.

Maternal factors comes into play in the protection of the neonates, this is basically due to the transfer of immunoglobulins from the mother to the baby before and after birth. Immunoglobulin G are the most abundant class of immunoglobulins in the serum.

More relevant to developmental immunology, this Immunoglobulin G has four subtypes, namely; IgG1, IgG2, IgG3, and IgG4. IgG1, IgG2 and IgG4 can readily cross the placenta and are the class of immunoglobulins which protects the foetus during its development in the womb. At birth, this Immunoglobulin is the only Immunoglobulin present in the womb, this is because other subtypes of immunoglobulins cannot cross the placenta.


^{Immunoglobulins. Source Wikimedia. CC4.0 license. Author CNX OpenStax}

Contributing to the defense at the time when the newborn is still developing it’s adaptive immune system are the body fluid such as breast milk which provides passive immunity to the newborn; busy fluids such as saliva and breast milk contains secretory Immunoglobulin A, which are transferred to the baby during normal breastfeeding process. Colostrum, the first milk produced by a nursing mother after birth is rich in Immunoglobulin A.

Secretory Immunoglobulin A is a dimer, consisting of two units of the basic Immunoglobulin structure and are thus tetravalent, having four antigen binding sites. This polymerization is facilitated by the J-chain. In addition to this J-chain, the Immunoglobulin A contains another polypeptide of 70KDa molecular weight, this is known as the secretory component. The function of the secretory component is not actually known, but there are evidence that it aids the passage of immunoglobulins across the epithelial cells, hence it increases the amount of Immunoglobulin A secreted daily.

Immunoglobulin A binds to several antigens which includes viruses such as polio, reoviruses and influenza viruses; bacteria such as vibro cholera, neisseria gonorrhoea and prevents the attacomment of these immunogens to the mucosa cels. This stresses the importance of the breastfeeding exercise for newborn babies.
When breastfeeding is commenced for the newborn, the mother passively transfers some secretory immunoglobulin A to the baby through breast milk which contains high quantity of this Immunoglobulin, this protects the baby for up to seventeen (17) months.

At the sixth month of neonatal development, the baby starts responding to glycoproteins, this increases with age and at about the ninth (9th) month, the baby responds strongly to glycoproteins, but still there is little or no increase in it’s response to polysaccharide antigens, the baby starts responding to polysaccharide antigens at about twelve (12) month of age. Further development of the immune system at the adolescent stage is mediated by sex steroids, testosterone in males and estrogens in females, the first Immunoglobulin synthesized by the neonate is Immunoglobulin M. Autoimmunity may occur during puberty.

REFERENCES

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1. Transmission control protocol -intronetworks
2. Innate immunity -Khanacademy
3. Innate immune system -wikipedia
4. the immunoglobulin development during the first year of life -wiley
5. Antibodies of the IgG, IgM, and IgA classes in newborn and adult sera reactive with gram-negative bacteria -ncbi

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